question_id int64 25 74.7M | answer_id int64 332 74.7M | title stringlengths 20 150 | question stringlengths 23 4.1k | answer stringlengths 20 4.1k |
|---|---|---|---|---|
70,755,986 | 70,756,027 | Why is character array processed as true by if statement? | I tried the code given below and found that it actually prints "yes", which means that the character array is taken as true in if statement. But i wonder what is the reason. I mean its an array so did it returned the whole "string". Or it returned its first element that is "s", or it returned its memory location which ... | if (a)
In this context, a reference to an array decays to a pointer to the first value of the array. The same thing would happen if a were to get passed as a function parameter.
So, here, a is just a pointer. And since it's not a null pointer this always evaluates to true.
char a[6] = "string";
This is not really rel... |
70,756,433 | 70,756,662 | Can I assign and return without a utility routine? | Is there a way to "assign and return" without creating a utility routine? Here's my code:
static int& do_work_(const int* p, int& result)
{
result = (*p) * 10;
return result;
}
template<typename T>
T& assign(const T* p, T& result)
{
result = *p;
return result;
}
int& do_work(const int* p, bool cond, int&... | If you want to return the result of an assignment operation you can (in most cases) do so without a 'utility' function like your assign, because the assignment expression itself has a value; from cppreference (bolding mine):
The direct assignment operator expects a modifiable lvalue as its left
operand and an rvalue e... |
70,756,996 | 70,757,224 | Default parameter in a function wouldnt compile/link without static inline | i was writing a BST class in C++ and wanted to add a default parameter to a function but, when i tried to the visual studio compiler gave compile and linking errors, so after a bit of googling, i made the root variable static inline and it worked, i want to know what does static inline change in this specific situation... | This is because
The keyword this shall not be used in a default argument of a member function
and non static member of a class is bound to this (i.e. root is equivalent to this->root). More about default parameters restrictions here.
In your case you could have the following workaround:
node<T>* findmin(node<T>* temp... |
70,757,091 | 70,757,150 | Why does my code do not run the switch statement? (Linked List) | I am a beginner and still learning how to use a switch statement. My professor asked us to make a linked list using a switch statement. I don't really know why the switch statement is not working as intended to be. Hoping someone can help me.
OUTPUT
The output of the program should look like this
Menu
[1] Inserting val... | menu does not return any value.
You probably want to do this:
int menu()
{
char ah;
int ch;
cout << "\n----------------------------------";
cout << "\nMenu";
cout << "\n----------------------------------";
cout << "\n[1] Inserting value in the link";
cout << "\n[2] Deleting value in the link... |
70,757,795 | 70,757,959 | Why does rvalue object does not get moved to function with rvalue parameter? | I was wondering why std::move to a function with an rvalue parameter was not actually moving anything, but passing by reference instead?
Especially when I know it works for constructors.
I was running the following code:
#include <memory>
#include <iostream>
void consume_ptr(std::shared_ptr<int> && ptr) {
std::cou... | std::move by itself does not actually move anything. That is to say, std::move alone does not invoke any move constructors or move-assignment operators. What it actually does is effectively cast its argument into an rvalue as if by static_cast<typename std::remove_reference<T>::type&&>(t), according to cppreference.
In... |
70,757,922 | 70,758,141 | C++ copy assignment operator behaviour | I used the code below to test the behaviour of copy assignment operator:
#include <iostream>
using namespace std;
int group_number = 10; // Global
class Player {
public:
explicit Player(const int &g): group(g)
{
}
Player& operator=(const Player& rhs)
{
cout << "Copy assignment operator called " << en... | You have not copied the data member stamina inside the copy assignment operator. So just add stamina = rhs.stamina; inside the copy assignment operator and you'll get your expected output. So the modified definition of operator= looks like:
Player& operator=(const Player& rhs)
{
cout << "Copy assignment operato... |
70,758,414 | 70,758,709 | How to find certain substring in string and then go back to certain character? | I save messages in string and I need to make filter function that finds user specified word in those messages. I've split each message by '\n' so the example of one chat would be:
user1:Hey, man\nuser2:Hey\nuser1:What's up?\nuser2:Nothing, wbu?\n etc.
Now user could ask to search for word up and I've implemented a sear... | Since you said you split the strings, I image you have a vector of strings where you want to find up for example. You would do something like this
for (const auto& my_string: vector_of_strings){
if (my_string.find("up") != string::npos) {
// message containing up is my_string
}
}
In case you haven't s... |
70,759,474 | 70,759,579 | What would happen if I used a cout statement as the condition for an if statement? | So, my programming instructor asked me to try putting a cout-statement as the condition for an if-statement and see what happens. I tried it (just made a random code) and didn't notice anythimg special. Here's the code.
#include<iostream>
using namespace std;
int main()
{
int x=1;
int y=2022;
if(cout<<"Covi... | Prior to C++11, when you write if(cout << "Covid"), there is an implicit conversion to void*. This value is unspecified by the C++ standard, other than if the stream is in an error state, then nullptr is returned.
From C++11, the implicit conversion is to bool. false denotes the stream is in an error state, true otherw... |
70,759,629 | 70,759,671 | Is there a type in the standard for storing the begin- and end-iterators of a container? | My question is really simple: Is there a type in the standard whose purpose is to store the begin-terator and the end-iterator for a container?
I want to return both iterators from a single function. I'm aware I could use std::pair for this but it feels like there would be some type in the standard for this exact purpo... |
Is there a type in the standard for storing the begin- and end-iterators of a container?
Your description matches closely with the concept of a range.
std::ranges::subrange can be created from a pair of iterators. That will be templated based on the iterator type.
If you need to hide the iterator type for runtime pol... |
70,760,036 | 70,761,803 | How to develop a constexpr implementation of boost::pfr::for_each? | I am using boost::pfr and it's quite impressive the way it uses reflection without any neccesity of macro registering of variables.
But the issue I wonder is why boost::pfr::for_each was not declared as constexpr, avoiding in current situation the use of PFR for_each within any constexpr function declared by users.
Is ... | This looks like an oversight in the implementation, given that it could easily be constexpr, at least for the C++17 implementation - godbolt example
In the meantime you can build your own constexpr one using boost::pfr::structure_tie (which is constexpr) & std::apply:
godbolt example
template<class T, class F>
constexp... |
70,760,158 | 70,760,388 | What is the definition of "single statement" in C++? | Identify a statement that is false for the given snippet:
int n = 1 + 2;
A. 1 and 2 are operands.
B. + is operator.
C. ; marks the end of a statement in C++.
D. It is a single statement.
| The answer is C.
The grammar production for a for statement is
for ( init-statement conditionopt ; expressionopt ) statement
So, in
for (int i = 0; i < 10; ++i)
the first ; ends the "init-statement". The second ; is part of the for loop grammar, and does not end a statement.
|
70,760,458 | 70,760,634 | Why do I keep getting this error: exception Unhandled : Unhandled exception thrown: read access violation. this was 0x4 | I am currently working on the BlackJack project, but there is an error showing "exception Unhandled: Unhandled exception thrown: read access violation. this was 0x4.". I am not quite sure which part I did wrong, and the program sometimes runs normally sometimes shows that exception. In draw_card function, it returns a... | You generate
suite = rand() % 4 + 1;
This is a random number between 1 and 4 inclusive.
You then call
getSuit(suite);
But getSuit only has switch branches for values between 0 and 3 inclusive:
switch (suit)
{
case 0:
return "spades";
break;
case 1:
return "clubs";
break;
case 2:
return "diamonds";... |
70,761,598 | 70,761,684 | Target-typing in modern C++ | When I say target-typing I mean using the type of the receiver variable or parameter as information to infer parts of the code I'm assigning to it. Like for example, in C# you'd write something like this to pass a nullable value or a null (empty) if necessary:
void f(int? i) {}
void caller(bool b) =>
f(b ? 5 : nul... | In C++, expressions have types.
{} is not, however, an expression. There are a handful of restricted contexts where {} could be used when you'd expect an expression should go there. This doesn't make {} into an expression.
?: is an operation, and its arguments must have types. The type of ?: is derived from the type... |
70,761,711 | 70,761,785 | std::variant template deduction when isolating containing type | I have been making a serializer today and am having trouble getting it to work with variants.
I have used this process before for various other things where I keep trimming off the type till I get to the one type I need. However, it seems VC++ can't deduce it this time.
Here is a complete example of the error.
#includ... | The branch with sizeof...(Rest) >= 1 must also be valid when the condition is false, but it isn't. You can get it discarded when the condition is false via if constexpr. Moreoever you were trying to return something but the return type is void.
namespace detail
{
template<typename VariantT, typename T1, typename... |
70,762,310 | 70,806,752 | I'm getting undefined reference to `i2c_smbus_read_word_data and 'extern "C"' doesn't fix it | I'm getting the error undefined reference to i2c_smbus_read_word_data(int, unsigned char)`
I've tried wrapping a few of my libraries in extern "C" but I get the same error. I tried this after seeing this answer to a similar problem.
Regardless of whether I wrap some or all of these include #include <linux/i2c-dev.h>, #... | I did a sudo apt-get update and the problem went away.
I also ran a few commands to update the compiler, though it still says my g++ version is 7.5, so that might have also contributed.
|
70,763,494 | 70,763,573 | What are the downsides to accessing a std::map<std::string, int> using string literals? | I have written a ResourceManager class in C++ that contains a std::map<std::string, Resource>. My current code then accesses these methods using string literals, e.g:
// ResourceManager.h
class ResourceManager {
private:
std::map<std::string, Resource>
public:
void loadResource(std::string_view... | As with any other situation where magic constants are used, it can lead to brittle code, i.e., code that is likely to become broken. In particular, consider what happens if a "background image" resource is loaded once and retrieved from multiple different locations, possibly spanning many source files. If the resource ... |
70,763,549 | 70,772,972 | ROS cpp callback without message type | I want to create a ROS node that is able to listen to a topic without knowing its message type. In python, this is possible as can be seen here:
https://schulz-m.github.io/2016/07/18/rospy-subscribe-to-any-msg-type/
I tried that and it works like a charm. However, I want to avoid copying the whole message; thus, i need... | By nature, this sort of problem is very pythonic and much harder to work around in c++. That being said, there is one (sort of) solution out there already. Take a look at the ros_msg_parser.
The syntax isn't pretty, and I'm not even sure if it's a good idea, but it will let you generate generic subscribers. Example fro... |
70,764,818 | 70,765,683 | member "Node::next" (of a friend class) is inaccessible | I am building my own LinkedList, because I need a special Node that holds more data. I specify that my Node class is a friend of the LinkedList class, but it doesn't seem to allow me to access the private variables in the Node class from the LinkedList class.
Node.h
class Node {
private:
Node* next;
...... | A friend declaration grants access of the specified class to the private members of the declaring class (ie, "that class X over there is a friend of mine, he has permission to use my private stuff").
So, by declaring Node as a friend inside of LinkedList, you are granting Node access to LinkedList's private members, no... |
70,764,819 | 70,764,987 | How can i find the integers that can be written as a sum of a power of 2, a power of 3 and a power of 5? C++ | So the program must compute all the numbers that can be written as a sum of a power of 2, a power of 3 and a power of 5 below 5.000.000.
For example 42 = 16 + 1 + 25 = 2^4 + 3^0 + 5^2. Any idea how can I do this?
| you can get all powers of 2 and all powers of 3 and all powers of 5 under 5.000.000. first Then you can try all combinations
vector<int> solve(){
const int M = 5000000;
vector<int> p_2={1},p_3={1},p_5={1};
while(p_2.back()*2<M)p_2.push_back(p_2.back()*2);
while(p_3.back()*3<M)p_3.push_back(p_3.back()*3);
while(p_... |
70,765,063 | 70,770,238 | V8 Embedding. Cannot print out the `v8::Local` object | In Short
I was trying to print out the v8::Local object content, with either of v8/tools/gdbinit and v8/tools/lldb_commands.py helper scripts, I got Empty Line OR Syntax Error messages. Is there anything I've missed? So my question is how can we print the v8::Local object content?
Most of configurations are from offic... | For debugging, try using a debug build: use gn args <your_output_dir> to set is_debug = true, then recompile.
If you insist on debugging release-mode binaries, you can enable jlh and friends with the v8_enable_object_print = true GN arg, but your experience will likely be weird in other ways (e.g. stepping and breakpoi... |
70,765,272 | 70,766,140 | Does std::format() accept different string types for format and arguments? | With printf I can do something like this
wprintf(L"%hs", "abc");
where the format is wchar_t * but argument is char *. Is this doable with std::format(), or do I have to convert one to the other prior?
When I do
std::format(L"{:hs}", "abc");
I get compile error about type mismatch.
| It cannot.
As per the documentation, the format string matches Python’s format specification, which does not have any magic for directly dealing with different string types or encodings. (Python does everything behind the scenes with CESU-8, IIRC.)
This is one of those things where I wish the C++ Standard would just Do... |
70,765,285 | 70,765,307 | How to use <stacktrace> in GCC trunk? | From https://github.com/gcc-mirror/gcc/commit/3acb929cc0beb79e6f4005eb22ee88b45e1cbc1d commit, C++ standard header <stacktrace> exists things such as std::stacktrace_entry but is not declared since _GLIBCXX_HAVE_STACKTRACE is not defined neither.
I have tried this on https://godbolt.org/z/b9TvEMYnh but linker error is ... | Part of building GCC from source is to run the configure shell script and pass it a bunch of arguments to tell it how to behave. That commit message is telling you that in order to enable this feature, you must add the following configure argument:
--enable-libstdcxx-backtrace=yes
|
70,765,507 | 70,765,625 | Initialize a std::vector of structures to zero | I am guaranteed to initialize a vector of struct with each element initialized to zero with that code ?
#include <vector>
struct A {
int a, b;
float f;
};
int main()
{
// 100 A's initialized to zero (a=0, b=0, f=0.0)
// std::vector<A> my_vector(100, {}); does not compile
std::vector<A> my_vector(1... | Yes it is guaranteed since A{} is value initialization and from cppreference:
Zero initialization is performed in the following situations:
As part of value-initialization sequence for non-class types and for members of value-initialized class types that have no constructors, including value initialization of element... |
70,765,553 | 70,766,925 | CUDA : Shared memory alignement in documentation | We can see on the official Nvidia website that to use multiple arrays of shared memory of unknow size, we can use that code in the kernel:
__global__ void myKernel() {
extern __shared__ int s[];
int *integerData = s; // nI ints
float *floatData = (float*)&integerData[nI]; // nF floats
cha... | Yes, you have to worry about alignment.
The easiest way to get around this is to sort arrays by alignment from largest to smallest, meaning double + long long -> float + int -> …
Since all integral types have sizes and alignments that are powers of 2, you won't waste any space when you pack your arrays like that. Extra... |
70,765,743 | 70,781,619 | Return multiple mock objects from a mocked factory function which returns std::unique_ptr | How to return multiple mock objects from a mocked factory function which returns std::unique_ptr?
Return(ByMove(...)) cannot be used to return multiple times.
Trying to work from this answer:
https://stackoverflow.com/a/70751684/3545094
I came up with this:
class MyType {
public:
virtual ~MyType() {}
};
class MyTyp... | Stepping gtest/gmock to 1.10 solves this issue.
|
70,766,319 | 70,766,404 | Get the total number of prime numbers from 1 to 1million | Env: Microsoft Visual Studio. Project type: Run code in a Windows terminal. Print "Hello World" by default.
Running ok while get the total number of prime number from 2 to 100000.
While not able to get the total number of prime number from 2 to 1000000.
It will just return
Hello World!
Hello World from function().
The... | Your program is probably hanging due too long timeout.
As suggested in comments by @NathanPierson it is enough to check primality until i * i <= number which will speedup code greatly.
As commented by @Quimby if you use GCC or CLang compiler then it is worth adding -O3 command line compile option, which does all possib... |
70,766,398 | 70,766,758 | C++ judgment conditional formatting question | #include <iostream>
using namespace std;
int foo(int x,int n);
int main() {
string s = "hello";
int k = 0;
if((k - s.size()) < 0) {
cout << "yes1";
}
int temp = k - s.size();
if((temp) < 0) {
cout << "yes2";
}
}
Anyone can tell me why the output is yes2?
Where is the yes1???... | If you look at if((k-s.size())<0), there was no type casting datatype.
When you write int temp = k-s.size();, integer datatype convert result as integer value(-5). Then your result "yes2".
Change k-s.size() to int(k-s.size()), you will get "yes1" and also "yes2".
|
70,766,614 | 70,766,732 | C++ - Why does aggregate initialization not work with template struct | This code works, without having to specify a constructor:
struct Foo
{
int a;
int b;
};
//...
int a1, b1;
Foo foo = {a1, b1};
If I make Foo a template, it doesn't work.
template<typename T1, typename T2>
struct Foo
{
T1 a;
T2 b;
};
//...
int a1, b1;
Foo foo = {a1, b1};
It says de... | Since C++20 aggregates have implicitly-generated deduction guide so class template argument deduction works for aggregates too.
int a1, b1;
Foo foo = {a1, b1}; // works since C++20; T1 and T2 are deduced as int
Before C++20, you need to add user-defined deduction guide, e.g.
template<typename T1, typename T2>
struct F... |
70,766,639 | 70,766,703 | opengl window initialization issue | I'm trying to encapsulate GLFW in a class to make my code cleaner.
So in my class, I've made the basic window creation but when I execute the program, it always enter in the if statement where I check if the initialization went right or not and I don't know why.
This is what I have
#include "include/Window.h"
#include ... | Make sure to glfwInit().
Also good luck in your jorney learning GL and C++!
|
70,766,824 | 70,767,015 | How I can run two threads parallelly one by one? | In C++ how i can write two parallel threads which will work one by one.For example in below code it need to print 0 t 100 sequentially.In below code the numbers are printing ,but all are not sequential.I need to print like 1,2,3,4,5,6.....99.If any body know , try to add with sample code also.
#pragma once
#includ... | You can use ready variable as a condition for condition variable.
#include <mutex>
#include <iostream>
#include <vector>
#include <thread>
#include <condition_variable>
using namespace std;
class CuncurrentThread
{
public:
mutex mtx;
condition_variable cv;
static bool ready;
//static bool processed;
... |
70,767,210 | 70,767,557 | How to solve undefined reference in main function? | Below is my header file , which declares the function set_params
#ifndef PID_H
#define PID_H
class PID
{
float _kp,_ki,_kd,_error,_previous_error,_dt;
public:
void set_params(float kp, float ki, float kd,float error,float previous_error,float dt);
};
#endif
The following is my pid.cpp, it contains fun... | After you comment, even if including the cpp files is a workaround, please do not do that!
You have two acceptable ways.
define the method in the class itself in the include file
Some IDE by default create an include file containing only the method declarations and put the definitions in an implementation cpp file. Bu... |
70,768,157 | 70,768,389 | Variable is not a type | #include<iostream>
#include<string>
#include<vector>
using namespace std;
class Course
{
public:
string name;
string instructorInCharge;
int numberOfStudents;
int totalMarks;
Course(string u, string v, int p, int q){
this->name=u;
this->instructorInCha... | If you want in-class initialization, you may do this:
vector<int> studentMarks = vector<int>(numberOfStudents);
Now the default constructor will initialize studentMarks with vector<int>(numberOfStudents) when an instance of Course is created.
However, this will cause undefined behavior since numberOfStudents is not in... |
70,768,599 | 70,768,679 | Pointer to an unspecified variable | The following code gets compiled
int v[] = { 0,1,2,3,4,5,6,7,8,9 };
std::cout << v[10] << std::endl;
int x;
int* ptr = &x;
std::cout << *ptr << std::endl;
I would like to understand why it was possible to display v[10] and *ptr and also why v[10] and *ptr have the same value. Could you please provide an explanation?
| It is undefined behavior but for some of compilers it is how the variables are places in memory. In C/C++ you have direct access to memory (as a huge space with some addressation). x is just placed just after v[].
v[10] means 10th object from start (v is pointing to beginning). If it exceeds the size of container, it w... |
70,769,044 | 70,771,086 | How to build cv::Mat from coloured rectangles | My goal is to create yield maps using OpenCV. These yield maps need to be built with coloured rectangles to indicate yield. An example of a Mat built by rectangles here.
So is it possible to create a cv::Mat with coloured rectangles? The amount of rectangles isn't constant, thus changes with every use.
To make the ques... | OpenCV has cv::hconcat and cv::vconcat. Use them like numpy's hstack/vstack.
make sure your parts have the same type (and number of channels).
The documentation has a code example.
cv::Mat matArray[] = { cv::Mat(4, 1, CV_8UC1, cv::Scalar(1)),
cv::Mat(4, 1, CV_8UC1, cv::Scalar(2)),
... |
70,769,192 | 70,776,764 | Error using Thrust Zip iterator with device functor | I'm working in a Linux environnement with Cuda and Thrust
I got a standard code and i need to transform it Because of this i try to use use zip_iterator for the first time and i have some issues with it.
error: function "gridTransform::operator()" cannot be called with the given argument list
Does someone know how to r... | gridTransform::operator() needs to take a const Float3 &. I guess Thrust doesn't implement a cast from their internal thrust::detail::tuple_of_iterator_references<float &, float &, float &> to the non-const tuple.
The reason for this design decision is probably that for a non-const reference the behavior would be somew... |
70,769,321 | 70,769,505 | Can a variable and a method have the same name in C++? | class Controller {
bool isBackendActive();
void anotherMethodDoingSomething();
}
Can I declare a boolean variable with the same name as a method so that I can use it like this:
void Controller::anotherMethodDoingSomething() {
bool isBackendActive = isBackendActive();
if (aBoolean && isBackendActive) {
... | Without discussing merits (or lack thereof), yes, you can disambiguate a member from a local variable rather easily.
bool const isBackendActive = this->isBackendActive();
or
bool const isBackendActive = Controller::isBackendActive();
Will work (and might as well be const correct).
|
70,769,513 | 70,769,576 | Rotate through a zero based range for positive and negative numbers | Say I have a zero-based range like
[0, 1, 2, 3, 4]
these are essentially the indexes of an array with size = 5. Is it possible to use a signed integer variable to continuously loop up or down in that range?
By up or down, I mean that the variable is permitted to perform arbitrarily many increment or decrement operatio... | C made a really poor decision for % to round towards zero, rather than be a straightforward floor, so the math's clunkier than it has to be. Basically, though, you want (index % size + size) % size. That code starts by chopping off the negative or positive multiples to bring the index into the range (-size, size), adds... |
70,769,545 | 70,770,090 | Why in range for loop do begin/end need to be copyable? | When looking at what range-based for loops actually do here: https://en.cppreference.com/w/cpp/language/range-for#Explanation, I see that the begin_expr and end_expr are taken by value, i.e. using copy constructor:
auto __begin = begin_expr ;
auto __end = end_expr ;
Is there a reason the begin/end are taken like that,... |
Is there a reason the begin/end are taken like that, or is it an oversight? Wouldn't it be better to either move them:
In general no, because such specification could potentially prohibit copy elision.
Why in range for loop do begin/end need to be copyable?
The initialisation that you quote doesn't generally necess... |
70,770,596 | 70,770,758 | C++ Arduino ESP8266 fucntion gives an exception, but running the same directly in loop works fine | I wrote function which should make a url request.
function:
int updateX(float x)
{
if(WiFi.status() == WL_CONNECTED)
{
WiFiClient client;
HTTPClient http;
String sendStr = "192.168.1.1/updateXdeg?xDegVal="+String(x);
http.begin(client, sendStr);
int httpCode = http.GET();
http.end();
}
... | My function was declared to return an integer, but I didn't return anything.
|
70,770,614 | 70,770,642 | C++ camera component can't look up or down | I replaced the standard blue print camera with a c++ version so it works with my other functions that have been converted to c++
m_camera = CreateDefaultSubobject<UCameraComponent>(TEXT("camera"));
but for some reason even though the m_camera node is connected to everything the blueprint node was I can’t look up or dow... | Check your values for UseControllerRotation
To see details for the camera it has to have a metatag EditAnywhere in the .h
UPROPERTY(EditAnywhere)
UCameraComponent* m_Camera
|
70,770,677 | 70,770,729 | why C++ recognizes an uninitialized raw-pointer as true? | Why the following code produces a seg-fault
//somewhere in main
...
int *pointer;
if(pointer)
cout << *pointer;
...
But slightly changed following code doesn't
//somewhere in main
...
int *pointer = nullptr;
if(pointer)
cout << *pointer;
...
the question is what in C++ ... |
Why C++ Recognizes an uninitialized Raw Pointer (or say a daemon) as true?
The behaviour may appear to be so, because the behaviour of the program is undefined.
Why the following code produces a SEGMENTATION FAULT!!!
Because the behaviour of the program is undefined, and that is one of the possible behaviours.
Bu... |
70,770,818 | 70,772,283 | Need help to store the record of the current student in the students array | I have to develop a C++ program that reads the student’s name and scores from a file and store them in an array of Student struct objects, then calculate and display each student’s final grade based on the following criteria: midterm exam is counted for 25% of the final grade, final exam is counted for 25% of the final... | You should use the C++ goodies, and learn to be more conservative with the idioms.
main should be int main() or int main(int argc, char *argv[]). On some environments you can also use int main(int argc, char *argv[], char **environ) but never use the ugly void main(). Despite being equivalent in C++ int main(void) will... |
70,771,600 | 70,779,270 | How do I compare single multibyte character constants cross-platform? | As I am writing a cross-platform application, I want my code to work on both platforms.
In Windows, I use this code to compare 2 single characters.
for( int i = 0; i < str.size(); i++ )
if( str.substr( i, 1 ) == std::string( "¶" ) )
printf( "Found!\n" );
Now, in Linux, the character is not found. It is f... | Solved using str.compare() and size() of character string:
std::string str = "Some string ¶ some text ¶ to see";
std::string char_to_compare = "¶";
for( int i = 0; i < str.size(); i++ )
if( str.compare( i, char_to_compare.size(), char_to_compare ) == 0 )
printf( "Found!\n" );
|
70,772,159 | 70,775,635 | Executing C functions from C++ file results in linker error: "conflicting declaration with 'C' linkage" and "previous declaration 'C++' linkage | I am attempting to execute a C function from within a C++ file in the Arduino framework. The function I am trying to run, GuiLib_ShowScreen, appears in GuiLib.h and GuiLib.c as follows (the files are massive so for convenience and relevance sake I included only definitions):
extern void GuiLib_ShowScreen(
const G... | After trying suggestions from the comments, this cleared out the error. I included the library outside the scope after extern "C" and changed the parameters of GuiLib_ShowScreen() to their native types, defined in the library as #define GuiConst_INT16U unsigned short. There was a compatibility issue placing the includ... |
70,772,186 | 70,772,722 | Is there an easy way to copy an instance of a class in C++? | I'm trying to make a step based puzzle game (kind of like Baba Is You) and I need to store the state of the level at each step. So I'm using classes for the level, entities and some other things, and I was wondering if I can use default constructors like "Level(Level &&)" or "Level(const Level &)" I saw proposed by aut... |
use default constructors like Level(Level &&) or Level(const Level &)
Level(Level &&) is the move constructor. You don't want that, because you don't want to damage the original object
Level(Level const &) is the copy constructor, which is for making a duplicate of an existing object without altering the original.
... |
70,772,241 | 70,772,600 | Xml without indent char but with new line | Is there a way to write xml using boost::property_tree, without indent characters but with new line?
Consider this code
using namespace boost::property_tree;
int main() {
ptree tree;
ptree* root = &tree.add("root", "");
root->add("something", "yes");
root->add("something2", "no");
write_xml("test.xm... | It seems like you cannot do this.
Take a look at: https://github.com/boostorg/property_tree/blob/d30ff9404bd6af5cc8922a177865e566f4846b19/include/boost/property_tree/detail/xml_parser_write.hpp#L76
bool want_pretty = settings.indent_count > 0;
and then, at 101 line:
if (want_pretty)
stream << Ch('\n');
So, if yo... |
70,772,672 | 70,772,833 | Order of memory locations | What is the order of memory locations allocated by new operator?
For the context, if we are implementing a linked list, then does the new nodes created successively have increasing memory locations?
|
What is the order of memory locations allocated by new operator?
The order is unspecified by the language. The order is what-ever the language implementation chooses it to be.
if we are implementing a linked list, then does the new nodes created successively have increasing memory locations?
If you use the default ... |
70,772,818 | 70,773,460 | how to prevent priority queue from having duplicate values at c++ | now I'm trying to use priority queue in c++ like this
struct compare
{
bool operator()(const int &a, const int &b)
{
return weight[a] < weight[b];
}
};
priority_queue<int, vector<int>, compare> q;
but I want it to ignore any duplicate values
Is it possible using any technique to do this?
or should ... | The answer is creating a custom container for the std::priority_queue:
template <typename T>
class Vector : public std::vector<T>
{
public:
using std::vector<T>::vector;
void push_back(const T &value)
{
if (std::find(this->begin(), this->end(), value) == this->end()) std::vector<T>::push_back(value... |
70,773,087 | 70,773,293 | C++ operator overloading [] and return types | I'm just revisiting C++, and I have a question about overloading of the [] operator, and more specifically why my program doesn't work.
Given the following code in vec.cpp:
double Vec::operator[](unsigned int i) const {
return this->values[i];
}
double & Vec::operator[](unsigned int i) {
return this->values[i]... | In this declaration
Vec *a = new Vec(2, avals);
there is declared a pointer of the type Vec *. So an expression with the dereferenced pointer has the type Vec.
So in this statement
cout << a[0] << endl;
the expression a[0] has the type Vec.
It seems you mean
( *a )[0]
or
a[0][0]
|
70,773,117 | 70,773,336 | A letter in a byte? C# | I have an imported C++ method that receives a byte parameter, but according to the documentation, I can send a letter to that parameter, this is the C++ and C# method:
int WINAPI Sys_InitType(HID_DEVICE device, BYTE type)
public static extern int Sys_InitType(IntPtr device, byte type);
This causes me a syntax error i... | Convert.ToByte(string); doesn't do what you think it does, according to the documentation
Converts the specified string representation of a number to an equivalent 8-bit unsigned integer.
This would work byte random = Conver.ToByte("52"); which will return the byte 52.
See here:
https://learn.microsoft.com/en-us/dotn... |
70,773,154 | 70,773,428 | Why can I use scalar type as the return type of operator->? | From cppreference:
The overload of operator -> must either return a raw pointer, or return an object (by reference or by value) for which operator -> is in turn overloaded.
However, I tested the following example, and it is accepted by GCC, Clang and MSVC:
struct A {
int operator->();
};
According to cppreference,... | While the standard doesn't directly place restrictions on the return type of operator->, it does describe what happens when the overloaded -> operator is used:
12.6.5 Class member access [over.ref]
A class member access operator function is a function named operator-> that is a non-static member function taking no par... |
70,773,702 | 70,773,810 | How to limit the template parameter to only floating point type | Is there a way to limit a template parameter T to a specific type or category?
The below code works but I want to make it simpler:
#include <iostream>
#include <type_traits>
template <typename T>
constexpr auto func( const T num ) -> T
{
static_assert( std::is_floating_point_v<T>, "Floating point required." );
... |
The below code works but I want to make it simpler:
You may combine abbreviated function templates and the std::floating_point concept for a condensed constrained function template definition:
constexpr auto func(std::floating_point auto num) {
return num * 123;
}
Note that this does not include an explicitly spec... |
70,774,255 | 70,774,469 | How can I use Unique_ptrs with an class object having std::function as argument in the constructor | Problem Description
We have 2 classes A and B. B has the following constructor:
B(std::function<std::unique_ptr<A>()> a)
I am trying to create a unique_ptr as in std::unique_ptr<B> aPtr = std::unique_ptr<B>(new B(std::function<std::unique_ptr<A>())); I can't figure out how to do it. It's not compiling and I can't real... | auto bPtr = std::unique_ptr<B>(new B( std::function< std::unique_ptr<A>() > () ));
// need to close the template ^
// need to construct an instance of ^^
You get the same a bit simpler by using std::make_unique:
auto bPtr = std::make_u... |
70,774,419 | 70,774,606 | OpenGL depth testing and blending not working simultaniously | I'm currently writing a gravity-simulation and I have a small problem displaying the particles with OpenGL.
To get "round" particles, I create a small float-array like this:
for (int n = 0; n < 16; n++)
for (int m = 0; m < 16; m++)
{
AlphaData[n * 16 + m] = ((n - 8) * (n - 8) + (... | You're in a special case where your fragments are either fully opaque or fully transparent, so it's possible to get depth-testing and blending to work at the same time. The actual problem is, that for depth testing even a fully transparent fragment will store it's depth value. You can prevent the writing by explicitly ... |
70,774,468 | 70,775,393 | Using a union to prevent destruction? | Related: How does =delete on destructor prevent stack allocation?
First, I realize this sort of thing is playing with fire, tempting UB. I'm asking to get a better understanding of corner-cases of unions.
As I understand it, if I have a class with a c'tor and d'tor and put it in a union, the compiler will force me to t... | [class.dtor]/15 says that a program is ill-formed if a potentially invoked destructor is defined as deleted.
[class.base.init]/12 says that a destructor of a potentially constructed subobject of a class is potentially invoked in a non-delegating constructor.
[special]/7 says that all non-static data members are potenti... |
70,774,914 | 70,791,799 | ld: can't open output file for writing: execs/aligns, errno=2 for architecture arm64 | I created a small program to learn some concepts of STL, but when I compile, I get following error:
ld: can't open output file for writing: execs/aligns, errno=2 for architecture arm64
clang: error: linker command failed with exit code 1 (use -v to see invocation)
Compiled with:
g++ --std=c++17 aligns.cpp -o execs/ali... | I have finally found the solution!
Folder 'execs' simply didn't exist)))
|
70,775,025 | 70,775,136 | c++ Two Classes Refering To Each Other | i'm working with c++.
I need to create two classes that refering to each other.
Something like this:
class A {
private:
B b;
//etc...
};
class B {
private:
A a;
//etc...
};
How can i do that?
Sorry for my bad English and thanks you for helping me :)
| You can't do that even in principle, because a single A object would contain a B which contains another A, which contains another B and another A ...
If you just want a reference, you can simply do
class B; // forward declaration
class A {
B& b_; // reference
public:
explicit A(B& b) : b_(b) {}
};
class B {
A ... |
70,775,205 | 70,776,161 | Implementing template specialized functions | I'm trying to allow a class to implement an interface, where the interface is templated to either allow an update internally or not. The code looks like this and works if the implementation is within the class declaration. If it is moved out (as shown here), I get a compiler error on Visual Studio 2019, and I can't und... | 1. Microsoft-specific syntax
The syntax you're using to override GetValue() is not standard c++.
class X : public IXQ<EntryType::Read> {
public:
float IXQ<EntryType::Read>::GetValue() { /* ... */ }
}
This is a Microsoft-Specific Extension for C++ called Explicit Overrides (C++) and is intended to be used with __in... |
70,775,446 | 70,778,214 | Can OpenMP's SIMD directive vectorize indexing operations? | Say I have an MxN matrix (SIG) and a list of Nx1 fractional indices (idxt). Each fractional index in idxt uniquely corresponds to the same position column in SIG. I would like to index to the appropriate value in SIG using the indices stored in idxt, take that value and save it in another Nx1 vector. Since the indices ... | Probably no, but I would expect manually vectorized version to be faster.
Below is an example of that inner loop, untested. It doesn’t use AVX only SSE up to 4.1, and should be compatible with these Eigen matrices you have there.
The pIndex input pointer should point to the j-th element of your idxt vector, and pSignsC... |
70,776,453 | 70,776,798 | What is the difference between __declspec( dllimport ) extern "C" AND extern "C" __declspec( dllimport ) | Would there be any difference between both notations when compiling in a .c file or within a .cpp file ?
| extern "C" and __declspec(dllimport) are totally orthogonal.
extern "C" means that C linkage should be used for that symbol. I.e. no C++ name mangling will be applied. It also limits functions' interfaces to C-compatible things: built-in types, trivial structs, and pointers. Essentially, it tells the compiler and l... |
70,776,943 | 70,777,056 | gcc.exe vs cpp.exe vs g++.exe | I am using Visual Studio Code for the coding and for C/C++ I uses MinGW. When I click on configure default build task then I can see various options like C/C++:cpp.exe build active file(compiler: C:/MinGW/bin/cpp.exe),C/C++:g++.exe build active file (compiler: C:/MinGW/bin/g++.exe),C/C++:gcc.exe build active file(compi... |
cpp - The C and C++ preprocessor that replaces macros etc. Since you don't have a "Task generated by debugger" for this, it'll most likely be used in the same way for both debug and release builds.
gcc - The C compiler that is used to compile C programs.
g++ - The C++ compiler that is used to compile C++ programs
|
70,777,047 | 70,777,397 | Difference in a C++ pointer behavior when incremented in the for loop definition vs inside for loop | I'm onto a leetcode problem (Floyd’s Cycle Detection Algorithm for Linked List) wherein I noticed a strange behavior on the pointer states.
When I change the pointer state inside the for loop, the program executes correctly (pointer states move to the right states):
ListNode *detectCycle(ListNode *head) {
... | A
for (init-statement; condition; iteration-expression)
{
dostuff();
}
maps to
{
init-statement
while ( condition )
{
dostuff();
iteration-expression ;
}
}
so we get
{
slow = head, fast = head;
while (fast && fast->next)
{
slow = slow->next, fast = fast->next->... |
70,777,258 | 70,805,849 | Constructing native object from NodeJS Addon | Expectation
I want to to implement using a native NodeJS module that works like the javascript module below
class C1{
make(){return new C2()}
}
class C2{}
module.exports({C1, C2})
Approach
My attempt was, (among some other failed attempts)
// so.cpp
#include <napi.h>
class C1: public Napi::ObjectWrap<C1> {
public:
... | You are not allowed to use constructor outside the function that created it - it is an automatic v8::Local reference that is allocated on the stack - you need a persistent reference to do this:
#include <napi.h>
class C1: public Napi::ObjectWrap<C1> {
public:
C1(const Napi::CallbackInfo &info);
static void Initial... |
70,777,344 | 70,777,836 | How to write an infinite sequence compatible with std::ranges? | I would like to write a struct which will generate an infinite sequence of fibonacci numbers in a way compatible with std::ranges and range adaptors.
So if I wanted the first 5 even fibonacci numbers, I would write something like this:
#include <iostream>
#include <ranges>
using namespace std;
int main() {
auto is_... | Edit: As it was pointed out by 康桓瑋 in the comments, there has been a better, cleaner solution presented at Cppcon, link, by Tristan Brindle.
I think this could serve as a quick reference to make custom iterator-based generators.
By your requirements, something must be a std::ranges::view, meaning it must be a moveable ... |
70,778,047 | 70,782,227 | Access pixel value of mask using opencv | I got a problem where I need to access pixels of a opencv Mat image container.
I use opencv inRange function to create a mask. In that mask I need to check the value of different pixels, but I won't receive the values I expect to receive.
// convert image to hsv for better color-detection
cv::Mat img_hsv, maskR, maskY,... | I finally found the answer to my own question.
It works when I use uchar:
maskR.at<uchar>(position.x, position.y) == 255
I thought this wouldn't work because printing this with std::cout wouldn't give me an output, but the reason for that is that I forgot to cast uchar so it could be printed in the console
|
70,778,633 | 70,778,698 | Time/space complexity of string splitting/object creation in c++ | I'am struggling to figure out the time complexity of a piece of code that I have that takes a user input from 1 line e.g open 1 0, and splits the input via spaces, then allows the user to create a new 'account' object on the heap. I am thinking it is an O(n^2) operation as it contains 2 while loops, plus an additional ... | The complexity is linear in the total user input length on both space and time.
All the individual operations iterate over user input only a constant amount of times, including the inner loop. Therefore with n the total length of user input time the time complexity is Theta(n).
Similarly memory is only used to store a ... |
70,778,770 | 70,779,029 | Launching executable in C++ | I wish to launch an executable by API call, but I am not sure if one is supposed to check for executable bit using posix APIs before hand. What is best practice in this regard?
| All Posix APIs you'll use to launch a process (e.g. execve() and friends) will do any and all required filesystem permissions checks in a context that is atomic and secure (and doing so in user space is neither). Best practice is to simply use the syscalls that are available.
As a side note, you can browse the source c... |
70,779,104 | 70,779,135 | How to create a wrapper for `std::make_unique<T>`? | I'd like to create a wrapper for std::unique_ptr<T> and std::make_unique<T> because I think they look ugly and take too long to type. (Yes, I'm that kind of person).
I have completed my UniquePtr type alias with no problem but cannot get my MakeUnique to work. It seems to be a bit of a rabbit hole, and was wondering if... | You need to forward properly the values, and you need to expand the pack.
First, make it compile:
template<typename T, typename... Args>
UniquePtr<T> MakeUnique(Args... args) // not recursive
{ // ^---- no need for <T> when defining function template
return std::make_unique<T>(args...); // the ..... |
70,779,376 | 70,779,499 | How to initialize std::span<const T*>? Problems with const-ness | I am trying to initialize a span<const T*> — that is, a list of pointers to const data.
However, the rules for const conversion amongst pointers and span<>'s available constructors are thwarting me.
I'm sure there's a way to do it, but I cannot find the right casting/invocation.
Aside: I'm actually not using C++20, but... | What's wrong with:
#include<span>
#include<vector>
#include<iostream>
using std::cout;
using std::endl;
int main() {
int a = 0;
int b = 1;
int c = 2;
std::vector<int*> pointers = { &a, &b, &c };
std::span<const int*> cspan(const_cast<const int**>(pointers.data()), pointers.size());
std::sor... |
70,779,554 | 70,779,922 | Error when trying to linking libcurl in static | I'm new in C++ and I got a problem about linking libcurl in static, I downloaded sources and build it myself:
./configure && make install
Everything is okay, I write my cmake file, build my program and run, it's working, so I try to link in static and I got many errors about ssl and thread, so I go back in the doc and... | If you link your app with the static libcurl.a, you have to link your app with all other libs that would be linked with the dynamic libcurl.so. You need to build and link your app with following flags:
target_compile_options($(CMAKE_PROJECT_NAME) -pthread)
target_link_libraries($(CMAKE_PROJECT_NAME) curl pthread z)
Pe... |
70,780,287 | 70,780,904 | Negative indexing of 2d array in C++ | In the following code section:
// Convolution Operation + Sigmoid Activation
for (int filter_dim=0; filter_dim<5; filter_dim++) {
for (int i=0; i<28; i++) {
for (int j=0; j<28; j++) {
max_pooling[filter_dim][i][j] =0;
conv_layer[filter_dim][i][j] = 0;
... |
Wouldn't that be index out of bound?
Yes it is out of bounds of the array which leads to undefinded behavior.
Undefined behavior means anything1 can happen including but not limited to the program giving your expected output. But never rely(or make conclusions based) on the output of a program that has undefined beh... |
70,780,820 | 70,782,300 | How to use bool member function in is_integral_v? | From this cppreference link, is_integral has bool as a member function. How to use it? I could understand other members, and am able to code a simple example as follows. But how to use the bool member function?
#include <iostream>
#include <type_traits>
using namespace std;
int main(){
cout << boolalpha;
// me... | When you call is_integral<float>(), you are actualy calling the operator bool. The following calls the operator() instead:
is_integral<float>()()
It can be seen more clear in the following code:
std::is_integral<int> x;
if (x) { // operator bool
std::cout << "true" << std::endl;
}
std::cout << ... |
70,781,635 | 70,781,930 | Making a memory card flipping game in C++ | I need some help with this program:
Write a program that presents the backs of a number of cards to the player. You should have at
least 10 cards. The player selects 2 cards (one at a time), if they match, the player gets a point and
the cards remain face up. If they do not match, the cards must turn back over. The gam... | I don't know what these boards' functions are, but if you want to judge if the selected pair is match, you can try this:
if (displayLetter[firstChoice / 5][firstChoice % 5] == displayLetter[secondChoice / 5][secondChoice % 5]) {
cout << "Match!" << endl;
Sleep(2000);
}
Then you can chan... |
70,781,838 | 70,782,561 | c++ virtual inheritance doesn't work, how do I use the multiple parents' members? | Example of using virtual inheritance
class AA
{
public:
AA() { cout << "AA()" << endl; };
AA(const string& name, int _a):n(name),a(_a) {};
AA(const AA& o) :n(o.n), a(o.a) {};
virtual ~AA() { cout << "~AA()" << endl; };
protected:
int a = 0;
std::string n;
};
class A1 : public virtual AA
{
publi... | The result is correct, while the real reason is when you are using virtual inheritance, D ctor first uses AA's default ctor, n is not assigned.
If you want to use AA(name, a), you need to explicitly use that in D.
|
70,781,863 | 70,784,310 | Is it possible to increment setw? | I need to display something that looks like:
/
-
/
/
-
/
/
/
-
I currently have the following which does not print the slashes properly.
for (int i = 1; i <= numberOfDolls; i++) {
for(int j = 1; j <= i; j++)
cout << setw(numberOfDolls) << '/' << endl;
cout << "-" ... | I am not sure about your doubt but you will get desired output using following:
for (int i = 1; i <= numberOfDolls; i++) {
for(int j = 1, n=numberOfDolls; j <= i; j++)
cout << setw(n--) << '/' << endl;
cout <<setw(numberOfDolls-1)<< "-" << endl;
}
|
70,782,051 | 70,782,100 | unique_ptr inside a variant. Is it safe to use? | Id like to ask, is safe having a variant like this?
struct A
{
unique_ptr<T> anything;
};
struct B
{
int x = 0;
int y = 0;
};
variant<A, B> myVar;
myVar = ... A object;
myVar = ... B object;
myVar = ... another A object;
Would the std::unique_ptr destructor be invoked for all compilers? The idea would be ... | The paragraph you quoted is a guarantee by the standard;
As with unions, if a variant holds a value of some object type T, the
object representation of T is allocated directly within the object
representation of the variant itself. Variant is not allowed to
allocate additional (dynamic) memory.
It means std::variant ... |
70,782,158 | 70,782,833 | How to read paragraphs from a file while avoiding the blank space between them into an array? | I have been trying to figure this out. I'm very new to C++. I have tried using this: (what modifications would be needed?)
while(!openFile.fail())
{
getline(openFile,myArray[size])
}
My array size is the same as the number of paragraphs. However, I can't figure out how to consider the paragraphs as one element of th... | I'm not entirely sure what step you're having trouble with, but if I understand correctly. You have a file of some kind that you want to read from and a std::string array initialized to have size equaling the number of lines in the file. You can easily open a file with <ifstream> using open() and then read the file lin... |
70,782,217 | 70,782,235 | Qt5 slot lambda why captured local variable is random for the first time lambda is called | In the following code, I think p2 should print 10. But in fact it prints random value. The I starts to understand why it is not 10, with the following (1)-(3) thougths.
{
int m = 10; // local var m
qDebug() << "m0: " << &m;
connect(btn2, &QPushButton::clicked, this, [&]() { qDebug() << "m2: " << &m; qDebug(... | Because you capture it by reference.
Once the function ends, so does the life-time of all its local variables. The local variables cease to exist. Any reference or pointer to them will become invalid. Using such references or dereferencing such pointers will lead to undefined behavior.
The life-time of all variables ca... |
70,782,546 | 70,791,991 | Merge and/or Override flags of different configure presets in CMakePresets.json | I have two configure presets in my CMakePresets.json were I would like to merge the flags of the inherit configurePresets (gcc) with the other preset (gcc-arm-embedded)
Here is a simplified version:
"configurePresets": [
{
"name": "gcc",
"hidden": true,
"cacheVariables": {
"CMAKE_CXX_FLA... | First, simplify the inherits entry to:
"inherits": ["gcc-arm-embedded"]
The resolution order works left to right, never overwriting, but since gcc-arm-embedded inherits from gcc already, there's no need to specify both. So that gets you to this:
CMAKE_CXX_FLAGS: "-ffunction-sections -fdata-sections"
CMAKE_EXE_LINKER_F... |
70,782,552 | 70,782,820 | What is wrong with this code? The answer should be 24 for this question right? | What is wrong with this code? The answer should be 24 for this question right?
int t;
vector<int>arr={24,434};
for(int i=arr.size()-1;i>=(arr.size()-2);i--)
{
t=i;
}
cout<<arr[t];
| This loop
for(int i=arr.size()-1;i>=(arr.size()-2);i--)
is an infinite loop.
When i is equal to 0 it is decremented due to the third expression of the loop and becomes equal to -1.
And then in this condition
i>=(arr.size()-2)
as the operand arr.size()-2 has unsigned integer type with the rank not less than the rank o... |
70,782,570 | 70,785,675 | Dart/Flutter FFI: Convert List to Array | I have a struct that receives an array of type Float from a C++ library.
class MyStruct extends Struct{
@Array.multi([12])
external Array<Float> states;
}
I am able to receive data and parse it in Dart.
Now I want to do the reverse. I have a List<double> which I want to assign to this struct and pass to C++.... | There's no way to get around copying elements into FFI arrays.
for (var i = 0; i < listObject.length; i++) {
my_struct.states[i] = listObject[i];
}
This may seem inefficient, but consider that depending on the specialization of listObject, the underlying memory layout of the data may differ significantly from the co... |
70,782,643 | 70,783,249 | warning: '*' in boolean context, suggest '&&' instead [-Wint-in-bool-context]? | i've this code (which is pretty all "float"):
#define sincf(x) (x == 0.0f) ? (1.0f) : (sinf(M_PI * x) / (M_PI * x))
// ...
for (int i = 0; i < num_taps; i++)
proto[i] = 2.0f * f * sincf(2.0f * f * (i - m / 2.0f));
// ...
why gcc says warning: '' in boolean context, suggest '&&' instead [-Wint-in-bool-context]* for... | After the macro has been substituted, this part
2.0f * f * sincf(2.0f * f * (i - m / 2.0f));
becomes
2.0f * f * (2.0f * f * (i - m / 2.0f) == 0.0f) ? ...
and according to operator precedence, the multiplication 2.0f * f * condition will be done before checking if the condition is true (with ?). Like so:
(2.0f * f * ... |
70,782,888 | 70,783,876 | Strange errors of compiling pcl1.8 with cuda 11.3 | System: cuda 11.3, gcc 7.5, boost 1.65.1, pcl 1.8.0
When I compile code that uses PCL library, it shows the following error
/usr/include/pcl-1.8/pcl/io/file_io.h(264): error: namespace "boost" has no member "numeric_cast"
/usr/include/pcl-1.8/pcl/io/file_io.h(264): error: type name is not allowed
/usr/include/pcl-1.8... | Right, I solved it by using normal gcc compiler... DONT USE NVCC!
|
70,782,966 | 70,783,451 | Why does requires-expression behave differently in template and not-template for checking private member access? | In the following code class A has a private member function f. I want to write a static assertion that will check whether this function is accessible from the current context (as was suggested in the comment to this question).
There are 3 similar cases all based on requires-expression:
class A{ void f(); };
// #1: acc... | Whilst https://eel.is/c++draft/expr.prim.req.general#1 describes that
A requires-expression provides a concise way to express requirements on template arguments
the grammar of a requires-expression
requires-expression:
requires requirement-parameter-list(opt) requirement-body
requirement-body:
{ requirement-seq... |
70,783,028 | 70,783,471 | Cast uint8_t-array to readable format | I have this code, which receives a row of byte-values, inserted in an array. How can I show the content "readable" and not like this: R$⸮⸮>⸮⸮⸮
Here's the relevant code:
// Read data from UART.
uint8_t myData[8];
String myDataString = "";
int length = 0;
ESP_ERROR_CHECK(uart_get_buffered_data_len(uart_num, (si... | You are correct with your assumption that myData[i] gets interpreted as ASCII-code, but you can easily convert it into a HEX-string as follows:
myDataString += String(myData[i], HEX);
|
70,783,449 | 70,783,627 | Question about Bitwise Shift in Microsoft C++ | I am doing the following bitwise shift in Microsoft C++:
uint8_t arr[3] = {255, 255, 255};
uint8_t value = (arr[1] << 4) >> 4;
The result of these operations confused me quite a bit:
value = 255
However, if I do the bitwise shift separately:
value = (arr[i] << 4);
value = value >> 4;
the answer is different and make... | Expression (arr[1] << 4) will implicitly promote the value of arr[1] to type unsigned int before applying the shift operation, such that the "intermediate" result will not "loose" any bits (cf, for example, the explanation in implicit conversions).
However, when you write value = (arr[i] << 4);, then this "intermediate... |
70,783,542 | 70,788,205 | Preprocessor definitions based on visual studio configuration type | I am working on a C++ project in Visual Studio 2019, which should have the ability to be compiled as an executable and as a DLL. It is legacy code and contains the preprocessor flags AS_EXE and AS_DLL, which is presumably the flags I need to set. I just don't know how to do this based on different values in Project -> ... | You can use Condition attribute of ItemDefinitionGroup node in your .vcxproj.
<ItemDefinitionGroup Condition="'$(Configuration)'=='MyConfigForDLL'">
<ClCompile>
<PreprocessorDefinitions>AS_DLL;%(PreprocessorDefinitions)</PreprocessorDefinitions>
...
</ClCompile>
</ItemDefinitionGroup>
<ItemDefinitionGroup C... |
70,783,880 | 70,783,952 | Unit tests for C++ criterion | I'm trying to make unit tests with criterion for my C++ code but I can't figure out how to test a function that only print and do not return anything. Here's what I tried:
//the function to test
#include <iostream>
#include <fstream>
void my_cat(int ac, char **av)
{
if (ac <= 1)
std::cout << "my_cat: Usag... | With gtest, I think this can help you
testing::internal::CaptureStdout();
std::cout << "My test";
std::string output = testing::internal::GetCapturedStdout();
refer from : How to capture stdout/stderr with googletest?
|
70,784,182 | 70,784,444 | Where exactly are variables stored while vectors store in RAM? | I used this and compiled it:
#include <iostream>
#include <string>
#include <vector>
int main() {
std::vector<std::string> articles (119999999,"ads");
std::cout << articles[1];
getchar();
return 0;
}
Then a memory error occurred that the memory is full (because no loss of information occurred). I o... | sizeof(std::string) is typically 32 bytes. Even with short string optimisation, the memory request is a contiguous block of 119999999 * 32 bytes. That's of the order of 4Gb and beyond the capability of your computer.
If you require the storage of duplicate strings then consider std::reference_wrapper as the vector elem... |
70,784,323 | 70,785,115 | What data type should I use in cv::Mat.at<data_type> when using CV_8SC3, CV_16FC3, CV_16FC1? | When looing through the image and to index the (i, j) , OpenCV provides the method at in cv::Mat. In order to get the correct value of that pixel we need to specify data type carefully ,or we might get some unexpected value.
For example, if you use CV_16SC3 to create cv::Mat like below :
cv::Mat img(h, w, CV_16SC3, cv... | You can define your own specialization on your demand. cv::Vecxx are just a bunch of shorter aliases for for the most popular specializations of Vec<T,n>
So:
for CV_8SC3, you might use cv::Vec<char, 3>,
for CV_16FC3, you might use cv::Vec<cv::float16_t, 3>.
for CV_16FC1, you might use cv::Vec<cv::float16_t, 1>.
Besid... |
70,784,593 | 70,823,620 | Confused with some Vulkan concept | i have some few questions about Vulkan concepts.
1. vkCmdBindDescriptorSets arguments
// Provided by VK_VERSION_1_0
there is prototype:
void vkCmdBindDescriptorSets(
VkCommandBuffer commandBuffer,
VkPipelineBindPoint pipelineBindPoint,
VkPipelineLayout ... |
Understanding Vulkan uniform layout's 'set' index
vkCmdBindDescriptorSets update the current descriptorSet array.
the index of set define your shader use the one of array[index].
|
70,784,873 | 70,784,960 | C++ Using Boost logging with Cmake fails to compile | I am trying to compile C++ using boost logging library with CMake.
Heres a simple example
my CPP file - logtests.cpp
#include <iostream>
#include <boost/log/trivial.hpp>
#include <boost/log/utility/setup/file.hpp>
#include <boost/log/expressions.hpp>
namespace logging = boost::log;
void init_logging()
{
logging:... | If you want to use dynamic multi-threaded Boost libraries, you need to set the Boost_USE_STATIC_LIBS and Boost_USE_MULTITHREAD flags before looking for Boost:
set(Boost_USE_STATIC_LIBS OFF)
set(Boost_USE_MULTITHREAD ON)
FIND_PACKAGE(Boost log log_setup)
These flags are documented.
I also recommend you use the Boost::... |
70,784,886 | 70,787,564 | How can I compare system_clock::now() to a local time in c++20? | I am testing a library like follows:
std::chrono::system_clock::time_point FromDateTime(const DateTime& dateTime)
{
std::tm tm{};
tm.tm_year = dateTime.Year - 1900;
tm.tm_mon = dateTime.Month - 1;
tm.tm_mday = dateTime.Day;
tm.tm_hour = dateTime.Hour;
tm.tm_min = dateTime.Minute;
tm.tm_sec =... | Here's the equivalent C++20 code to your first version of FromDateTime:
std::chrono::system_clock::time_point
FromDateTime(const DateTime& dateTime)
{
using namespace std::chrono;
auto local_tp = local_days{year{dateTime.Year}/dateTime.Month/dateTime.Day} +
hours{dateTime.Hour} + minutes{dateTime.Minute... |
70,785,189 | 70,785,575 | How to push pair from priority queue into a 2D array? | I am doing a leetcode problem, and I am required to return a 2d array of results. But I am using a priority queue for that and am unable to move elements to a 2d array. I am not able to come up with a syntax for this.
The push_back(), is not working when I try to push the pair in a 2d array.
here is the link to the pro... | // if closet should be your answer then its size will be k
/* Here we tell how many rows
the 2D vector is going to have. And Each row will contain a vector of size 2 */
int row = k;
vector<vector<int>>closest(row, vector<int>(2));
int index = 0;
while(heap.size() > 0) {
pair<int,int>ptr = heap.top().second;
... |
70,785,205 | 70,785,376 | Access Derived class (forward declaration) using base class pointer | How to access derived class member variable in my unit test framework(i.e. Gtest_main.cpp) from below scenario.
imp.cpp
class base{
public:
virtual void foo()=0;
};
class derived : public base{
public:
int abc; // how to access this from Gtest_main.cpp
... |
How to access abc in Gtest_main.cpp
You can't. Without seeing the full definition of class derived, the compiler has no clue what derived::abc means.
|
70,785,297 | 70,998,018 | How to make C++ program a Terminal program (UNIX) | I wrote this simple C++ program to compare two strings for a match. Although basic, it's very useful to me as I often need to verify details multiple times a day.
I want to initiate the program with a command name e.g. check4match (the program name) so I can run the program in the terminal.
#include <iostream>
#include... | Ok, as always relatively simple in the end. So chmod a+x didn't work to make the cpp program executable. Simple make filename (without the .cpp extension).
Then I moved the newly made .exe file to /usr/local/bin. I had to drag & drop the file, as moving via terminal command wasn't allowed, even with sudo.
|
70,785,390 | 74,406,914 | Code Runner in vsCode ignoring my compiler arguments in my tasks.json file | I am using vsCode with the C/C++ extension and code runner. I want to compile with a -DLOCAL flag so that I can do
#ifdef LOCAL
...
#endif
The problem is that when I compile my code with the code runner extension, it seems to ignore that flag and the #ifdef doesn't work. However, when I use run and debug, the ifdef wo... | It looks like the code runner is not honoring the tasks.json at all. For me I need to modify the the Code Runner's executorMap by going to:
Extensions -> Code Runner -> extension settings -> edit in JSON -> add the -DLOCAL flag in the "cpp" command entry.
|
70,785,430 | 70,785,741 | How can you catch memory corruption in C++? | I have an std::string which appears to be getting corrupted somehow. Sometimes the string destructor will trigger an access violation, and sometimes printing it via std::cout will produce a crash.
If I pad the string in a struct as follows, the back_padding becomes slightly corrupted at a relatively consistant point in... | In general you have to solve problem of code sanitation, which is quite a broad topic. It sounds like you may have either out-of-bound write, or use of a dangling pointer or even a race condition in using a pointer, but in latter case bug's visibility is affected by obsevation, like the proverbial cat in quantum superp... |
70,785,853 | 70,786,413 | Iterating over pointer array C++ | I am new to C++ but I have an experience in C. Now I am trying to iterate over an array of objects but I am getting segmentation fault because it runs out of the range of the array.
I have a class ZombieHorede which keeps a pointer to an array of zombies.
#pragma once
#include "Zombie.hpp"
class ZombieHorde {
private... |
question is how to iterate over the zombies array in order to not get a segmentation fault.
One way to resolve this would be to have a data member called zombieSize that stores the value of N as shown below in the modified snippets:
class ZombieHorde {
private:
static std::string namepool[7];
sta... |
70,785,866 | 70,786,578 | C++ coroutine's return_void's and return_value's return type | return_void
consider the coroutine's ReturnObject below and note the comments before the method ReturnObject::promise_type::return_void :
struct ReturnObject {
struct promise_type {
int val_{23};
ReturnObject get_return_object() {
return {std::coroutine_handle<promise_typ... | [stmt.return.coroutine]/2 says that the expressions
promise_object.return_value(...)
and
promise_object.return_void()
(whichever is used by the co_return) shall be of type void.
So the return type of either function must be void (if it is used by a co_return).
MSVC does correctly reject the program:
<source>(31): err... |
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