How to pass pointer to a member within class to outer class? - c++

Question

I am trying to pass a pointer to memeber of class(Dialog::handler) from its method(in scope of Dialog::render) to some outer method(Button::OnClick).
Here is a small example:
class Button
{
public:
void OnClick(void (*handler)())
{
handler();
}
};
class Dialog
{
public:
void handler()
{
//do stuff
}
void render()
{
auto button = new Button;
//Source of problem
button->OnClick(this->*handler);
}
};
But compiler shows error:
non-standard syntax; use '&' to create a pointer to member
Also I triend other combinations, like:
this->handler.
&this.handler.
this.*handler.
etc
But obviously they failed.

Answer 1

You could use std::function and pass it a lambda in which you've caught this of the object you'd like to call back:
#include <functional>
#include <iostream>
class Button {
public:
void OnClick(std::function<void()> handler) {
handler();
}
};
class Dialog {
public:
void handler() {
std::cout << "Dialog::handler\n";
}
void render() {
auto button = new Button;
// a lambda catching "this" Dialog.
button->OnClick([this] { this->handler(); });
delete button; // you didn't delete your button
}
};
int main() {
Dialog d;
d.render();
}
But it looks like you should probably inherit from a common base class that has a virtual void handler() so you can pass object pointers/references around instead. A rough idea:
#include <iostream>
class VisualBase {
public:
virtual void handler() = 0;
virtual ~VisualBase() = 0;
};
VisualBase::~VisualBase() {}
class Button : public VisualBase {
public:
void handler() override {}
void OnClick(VisualBase* caller) {
caller->handler();
}
};
class Dialog : public VisualBase {
public:
void handler() override {
std::cout << "Dialog::handler\n";
}
void render() {
Button button;
button.OnClick(this);
}
};
int main() {
Dialog d;
d.render();
}