Pointer to a class method in c++ [duplicate] - c++

Question

This question already has answers here:
C++ Call Pointer To Member Function
(4 answers)
Closed 5 years ago.
A have a lot of B classes, a class A has one object b. This object b has a function (calc) that needs a pointer to a method in an object of A. This method (fun) acess private variables in class (in my example just return 3).
class A;
class B {
public:
virtual int calc ( int (A::*fun)()) { return 2*fun(); };
};
class A {
B* b;
public:
A (B* b_) : b (b_) {};
int fun() { return 3; };
int run(){ return b->calc(&A::fun); };
};
int main() {
B* b = new B();
A a(b);
a.run();
return 0;
}
How can I use a pointer to a method correctly in definition of calc method in class B?
I am getting this error message:
teste.cpp:10:58: error: must use ‘.*’ or ‘->*’ to call pointer-to-member function in ‘fun (...)’, e.g. ‘(... ->* fun) (...)’
virtual int calc ( int (A::*fun)()) { return 2*fun(); };
^

Answer 1

I recommend the std::function approach if it's feasible for you. However, for the sake of completeness, here's how you would correctly use pointer-to-member functions.
The pointer-to-member itself doesn't store the "current" instance of A, so you need to pass that explicitly. Then you use the special ->* (or .*) syntax to call it.
virtual int calc (A* value, int (A::*fun)()) {
return 2 * (value->*fun)();
};
Then you would call it as b->calc(this, &A::fun);.

Answer 2

You can do it your way, but the member function must be called on a particular instance:
class A;
class B {
public:
virtual int calc(A* a, int (A::*fun)()) { return 2 * (a->*fun)(); };
};
class A {
B* b;
public:
A(B* b_) : b(b_) {};
int fun() { return 3; };
int run() { return b->calc(this, &A::fun); }; // now also passing this pointer
};
int main() {
B* b = new B();
A a(b);
a.run();
return 0;
}
If you can live without calc() being virtual then a lambda is also an option:
class A;
class B {
public:
template<typename T>
int calc(T fun) { return 2 * fun(); };
};
class A {
B* b;
public:
A(B* b_) : b(b_) {};
int fun() { return 3; };
int run() {
return b->calc([this]() {return fun(); } );
};
};
int main() {
B* b = new B();
A a(b);
a.run();
return 0;
}

Answer 3

A pointer-to-class method is defined and initialized as (assuming SomeFn matches the signature):
RetType (ClassName::*pfn)(Args) = &ClassName::SomeFn;
And is called as:
ClassName * ptr = GetClassPtr();
(ptr->*pfn)(arg, arg);

Answer 4

If you are able to use C++11, then you should use std::function and std::bind: Otherwise you need to use a pointer to member function + a pointer to the instance.
With C++11
#include <functional>
class B {
public:
virtual int calc (std::function<int()>&& fun) { return 2 * fun(); }; };
};
class A {
B* b;
public:
A (B* b_) : b (b_) {};
int fun() { return 3; };
int run() { return b->calc(std::bind(&A::fun, this)); };
};
Without C++11
class B {
public:
virtual int calc (int(A::*fnptr)(), A* a) { return 2 * (a->*fnptr)(); };
};
class A {
B* b;
public:
A (B* b_) : b (b_) {};
int fun() { return 3; };
int run() { return b->calc(&A::fun, this); };
};
See example here.