Example of field in a class `Button`, where we may contain reference to any other class member function.

dowolna_fcja_w_polu.cpp

#include <iostream>
#include <functional>

// In "some classes" you wanna have some functions.
struct SomeClass
{
    void f1( int a )
    {
        std::cout<< a << std::endl;
    };
    
    void f2( int a, float b)
    {
        std::cout<< a << " "<< b << std::endl;
    };
    
    void f3()
    {
        std::cout<<"void" << std::endl;
    };
};

template< typename T, typename... Args >
struct FunctionHandler
{
    std::function<void( T*, Args ... args )> function;
};
/*
template< typename... Args >
struct FunctionHandler
{
    std::function<void( Args ... args )> function;
};
*/
// here's your button
class Button
{
public:
    void* handler; // NOPE ;/
    
    template< class T, typename... Args >
    void bind_function( void(T::*&&f )( Args ... args ) )
    {
        auto* p = new FunctionHandler< T, Args ...  >(); // here we create content of this pointer
        handler = p;
        p->function = ( std::function<void( T*, Args ... args )> )f;  // bind a function. We may use std::bind() as well. NOTE: we want get address of function in class, not object,
    }
    
    template< class T, typename... Args >
    void call_function( T& t, Args ... args )
    {
        auto* p = ( FunctionHandler< T, Args ... >* ) ( handler ); // Here we cast a void pointer.
        (*p).function( &t, args ... ); // here we call that function
    }
    
    template< class T, typename... Args >
    void unbind_function()
    {
        auto* p = ( FunctionHandler< T, Args ... >* ) (handler); // Here we cast a void pointer.
        delete p; // delete helper
        handler = nullptr; // set ptr to null;
    }
};

int main()
{
    Button b; // create some buttons
    SomeClass o; // create some object. It should work with static objects, or even static class members (if so, you should change template a bit).
    // Set first function
	b.bind_function< SomeClass, int >( &SomeClass::f1 );
	b.call_function< SomeClass, int >( o, 1 );
	b.unbind_function< SomeClass, int >();
	
	b.bind_function< SomeClass, int, float >( &SomeClass::f2 );
	b.call_function< SomeClass, int, float >( o, 1, 2.0f );
	b.unbind_function< SomeClass, int, float >();
	
	b.bind_function< SomeClass >( &SomeClass::f3 );
	b.call_function< SomeClass >( o );
	b.unbind_function< SomeClass >();
    return 0;
};