Pointer to a member-function - c++

Question

I would like to do the following:
I have two classes, A and B, and want to bind a function from A to a function from B so that whenever something calls the function in B, the function from A is called.
So basically, this is the scenario:
(important A and B should be independent classes)
This would be class A:
class A {
private:
// some needed variables for "doStuff"
public:
void doStuff(int param1, float *param2);
}
This is class B
class B {
private:
void callTheFunction();
public:
void setTheFunction();
}
And this is how I would like to work with these classes:
B *b = new B();
A *a = new A();
b->setTheFunction(a->doStuff); // obviously not working :(
I've read that this could be achieved with std::function, how would this work? Also, does this have an impact in the performance whenever callTheFunction() is called? In my example, its a audio-callback function which should call the sample-generating function of another class.

Answer 1

Solution based on usage C++11 std::function and std::bind.
#include <functional>
#include <stdlib.h>
#include <iostream>
using functionType = std::function <void (int, float *)>;
class A
{
public:
void doStuff (int param1, float * param2)
{
std::cout << param1 << " " << (param2 ? * param2 : 0.0f) << std::endl;
};
};
class B
{
public:
void callTheFunction ()
{
function (i, f);
};
void setTheFunction (const functionType specificFunction)
{
function = specificFunction;
};
functionType function {};
int i {0};
float * f {nullptr};
};
int main (int argc, char * argv [])
{
using std::placeholders::_1;
using std::placeholders::_2;
A a;
B b;
b.setTheFunction (std::bind (& A::doStuff, & a, _1, _2) );
b.callTheFunction ();
b.i = 42;
b.f = new float {7.0f};
b.callTheFunction ();
delete b.f;
return EXIT_SUCCESS;
}
Compile:
$ g++ func.cpp -std=c++11 -o func
Output:
$ ./func
0 0
42 7

Answer 2

Here's a basic skeleton:
struct B
{
A * a_instance;
void (A::*a_method)(int, float *);
B() : a_instance(nullptr), a_method(nullptr) {}
void callTheFunction(int a, float * b)
{
if (a_instance && a_method)
{
(a_instance->*a_method)(a, b);
}
}
};
Usage:
A a;
B b;
b.a_instance = &a;
b.a_method = &A::doStuff;
b.callTheFunction(10, nullptr);

Answer 3

This i basic a solution
class A {
private:
// some needed variables for "doStuff"
public:
void doStuff(int param1, float *param2)
{
}
};
typedef void (A::*TMethodPtr)(int param1, float *param2);
class B {
private:
TMethodPtr m_pMethod;
A* m_Obj;
void callTheFunction()
{
float f;
(m_Obj->*m_pMethod)(10, &f);
}
public:
void setTheFunction(A* Obj, TMethodPtr pMethod)
{
m_pMethod = pMethod;
m_Obj = Obj;
}
};
void main()
{
B *b = new B();
A *a = new A();
b->setTheFunction(a, A::doStuff); // now work :)
}