Question

I am trying to make a class function take another class unknown function. I hope that made any sense. What I am trying to do is… if button1 (or any declared button) is clicked, it will execute whatever function (any “void function”) that was inputted into the first parameter of function Event__MouseClicked.

bool MouseClicked( void ){ /**/ };

namespace XE
{
    class Window
    {
    public:
        const char* Title;
    };

    class Button
    {
    public:
        void Event__MouseClicked( void( *Function )( void ) ) {
            if( MouseClicked( ) )
                ( *Function )( );
        }
    };
};

class XorrWindow1 : public XE::Window
{
public:
    XorrWindow1( void ) {
        Initialize( );
    }
protected:
    ~XorrWindow1( void );
private:
    XE::Button Button1;
    XE::Button Button2;
private:
    // EVENT PROTOTYPES
    void DoSomething( void );
    void RandomFunction( void );

    void Initialize( void )
    {
        // INITIALIZE CONTROLS
        // AND OTHER STUFF BELOW
        this->Title = "XorrWindow1";

        // How can I resolve this problem?
        this->Button1.Event__MouseClicked( &XorrWindow1::DoSomething );
        this->Button2.Event__MouseClicked( &XorrWindow1::RandomFunction );
    };
};

void XorrWindow1::DoSomething( void ) {
    ::MessageBoxA( NULL, this->Title, "Button1 clicked!", MB_OK );
};

void XorrWindow1::RandomFunction( void ) {
    ::MessageBoxA( NULL, this->Title, "Button2 clicked!", MB_OK );
};

The error is this:

'XE::Button::Event__MouseClicked' : cannot convert parameter 1 from 'void (__thiscall XorrWindow1::* )(void)' to 'void (__cdecl *)(void)'

I absolutely understand what caused the error. But I don’t know how to fix it because it must be able to take any unknown function of class Window1.

Answer 1

Like Karel Petranek said, you need templates to accommodate all cases. Here is the most generic method (utilized by STL everywhere):

template<typename f>
void Event__MouseClicked(f&& func)
{
    if( MouseClicked( ) )
        func();
}

Basically Event__MouseClicked accepts a callable object. In your case you would call it like Marcelo Cantos described:

Button1.Event__MouseClicked([&]{ DoSomething(); });

But the benefit of this method is that passing any callable object that takes no arguments into Event__MouseClicked will compile and work. You can pass a function pointer, the result of std::bind, a std::function, a lambda, etc etc.

This is more optimal than forcibly accepting a std::function in some cases, for example when accepting a straight function pointer.