This question already has answers here:
Function pointer to member function
(8 answers)
Class member function pointer
(3 answers)
Closed 6 years ago.
I have a class that contains two similar non-static functions, I want to assign one of them on the fly to a function pointer:
A.h:
class A{
private:
void (*funcPtr)(int);
void do_sth(int i);
void do_sth_else(int i);
}
A.cpp:
A::A(int i){
if(i == 1)
A::funcPtr = A::do_sth; // or &A::do_sth;
else
A::funcPtr = A::do_sth_else; // or &A::do_sth_else;
}
But I get such error:
error: cannot convert 'A::do_sth' from type 'void (A::)(int)' to type 'void (*)(int)'
I read multiple similar issues, But cannot apply their solutions on my problem.
These are member functions. Either make funcPtr a member pointer by qualifying it with class name, or make do_sth(), do_sth_else() static member functions. I'd suggest using & in front of function name when taking the address of it.
Related
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How can I pass a member function where a free function is expected?
(9 answers)
how to pass a non static-member function as a callback?
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Pass Member Function as Parameter to other Member Function (C++ 11 <function>) [duplicate]
(2 answers)
Closed 4 months ago.
I need to change the prototype of a function pointer of a class. So, I was hoping inheriting it and doing the following would work, but it doesn't ("invalid use of non-static member function 'void B::myIntCallback(unsigned int)"):
class A {
public:
typedef void (*intCallback_t)(unsigned int myInt);
A(intCallback_t intCallback) {}
};
class B : A {
public:
typedef void (*charCallback_t)(unsigned char myChar);
B(charCallback_t charCallback) : A(this->myIntCallback) {
charCallback_ = charCallback;
}
private:
charCallback_t charCallback_;
void myIntCallback(unsigned int myInt) {
charCallback_((unsigned char)myInt);
}
};
Does anybody know how I can solve this? I can't change class A.
You are trying to pass a member function (B::myIntCallback) to a function pointer argument. Since the member function needs an object to be called on, his would require some kind of capturing, e.g. a lambda capturing this or an std::bind expression. Unfortunately, neither is possible with a plain function pointer, see also Passing capturing lambda as function pointer.
If possible, you may want to consider changing the class A to accept either a std::function or a template argument as the type of the callback. See also Should I use std::function or a function pointer in C++?.
This question already has answers here:
C++ Call Pointer To Member Function
(4 answers)
Calling C++ member functions via a function pointer
(10 answers)
Closed 9 months ago.
Consider this class:
class Downloader {
private:
bool video_or_audio;
// other variables [...]
// [...]
void downloadVideo(std::string videoURL);
void downloadAudio(std::string audioURL);
public:
void download();
}
Now, download() is defined this way:
void Downloader::download(){
std::ifstream url_list;
void (*download_func)(std::string) = video_or_audio == 0 ? downloadVideo : downloadAudio; // Compiler says here: "Reference to non static member function must be called".
if(video_or_audio == 0){
url_list.open("video_list.txt");
}
else{
url_list.open("audio_list.txt");
}
std::string url;
while(std::getline(url_list, url)){
download_func(url); // Calling the function pointed by the pointer defined in line 2 of the function download().
}
}
My compiler (clang) says: "Reference to non static member function must be called" in the second line of function download() definition. Why is this happening and how can I solve this problem?
A solution appears to be defining downloadVideo() and downloadAudio() functions to be static in the class declaration. However, if I do so, I cannot access private variables members of class Downloader, that's not desirable, as I need these variables.
Thank you!
Once calling a member function by pointer, you need to provide two things:
Member function itself
Address of particular instance of given class at which the member function to be called (because the function can access class members thus you have to advise which of the instances is the right one)
void (Downloader::*download_func)(std::string) = video_or_audio == 0 ? &Downloader::downloadVideo : &Downloader::downloadAudio; // Correct the signatures.
...
while(std::getline(url_list, url)){
(this->*download_func)(url);
}
So, here we changed download_func from "just function pointer" to a member function pointer. Then, later in the loop body, we call this member function on this instance (however you can pass an instance as a param if necessary).
This question already has answers here:
Function pointer to member function
(8 answers)
Closed 4 years ago.
I'm trying to create a function pointer. My code:
Header file:
#pragma once
#include <stdio.h>
class my_class
{
private:
int function(int x);
int *(*foo)(int);
public:
my_class();
~my_class();
};
css file:
#include "my_class.h"
int my_class::function(int x) {
return 1;
}
my_class::my_class() {
foo = &function;
}
my_class::~my_class()
{
}
But the line inside my_class::my_class() gives this error:
error C2276: '&': illegal operation on bound member function expression
Putting the mouse cursor over the = in said line makes the following tooltip appear:
a value of type "int (my_class::*)(int x)" cannot be assigned to an entity of type "int *(*)int"
How can I get it to work?
What you think the compiler will do for you is an implicit type cast from a pointer to a member function to a pointer to a normal function. But this is not what happens.
A pointer to a member function is usually not used, because the instance of an object is what is assigned a certain amount of memory and thereby a pointer to a member function might not exactly point to the function and might lead to erroneous results.
You can fix it in two ways:
Define your member function as a static function. Static member functions are not attached to any particular object and can be used without the scope resolution operator.
Another probable fix you can use is to use the this pointer to create a pointer to a member function.
This question already has answers here:
Using generic std::function objects with member functions in one class
(6 answers)
Closed 4 years ago.
I have a similar situation:
class A {
void callingFunction() {
calledFunction(passedFunction);
}
void calledFunction(std::function<void(int)> foo) {
}
void passedFunction(int arguments) {
}
};
The compiler error is
error: invalid use of non-static member function
How do I achieve this without making the passedFunction static?
Doing this:
calledFunction(std::bind(&A::passedFunction, this);
Creates this error:
error:static assertion failed: Wrong number of arguments foor pointer-to-member
Does it mean I have to provide all the arguments in the callingFunctionwhen passing the passedFunction? This is not possible as the arguments for the passedFunction are specified in the calledFunction
You can write a lambda capturing this, on which passedFunction could be called.
calledFunction([this](int v) { this->passedFunction(v); });
This question already has answers here:
How can I pass a member function where a free function is expected?
(9 answers)
Closed 6 years ago.
I was trying to call a function with a function pointer where the function pointer is a class member.
I simplifed the code to only showcase the problem but the foo() function in the oiginal code is already writen so I cannot change it.
(In my actual code, foo is from GLFW (glfwSetKeyCallback) and A is an input handler class.)
The code is:
#include <iostream>
// this four line can not be changed
typedef void(*fptr)(int);
void foo(fptr f) {
f(0);
}
void testOutA(int i) {
}
class A {
public:
void testInA(int i) {
}
};
int main()
{
foo(testOutA); // works fine as it should be
A * a = new A();
foo(a->testInA); // ERROR
return 0;
}
The compilation error message is:
ERROR: error C3867: 'A::testInA': non-standard syntax; use '&' to create a pointer to member
The type of the expression a->testInA is a pointer to a non-static member function.
It does not have the same type as fptr, so the compiler emits an error, albeit a cryptic one.
Compilation would pass if testInA were static.
See C++ function pointer (class member) to non-static member function for more details.