C++ function pointer to member function inside class

Question

I'd like to store a function pointer to a private member function of my object inside my class. Basically I want to do this:

MyWindow::MyWindow()
{
  std::function<void(int&)> func = this->memberFunction; // ERROR
}

void MyWindow::memberFunction(int& i)
{
  // do something
}

When I try to build it the compiler throws an error:

Error C3867: 'MyWindow::memberFunction': non-standard syntax; use '&' to create a pointer to member

Answer 1

The error message for C3867 tells you the first problem: you need to use & to form a function pointer.

Your next problem is that you do not have a function pointer, but a pointer-to- member -function, and that is not compatible with your std::function , which is missing anywhere to store or pass the implicit first argument that holds the "this" pointer.

You can, however, "bind" a value for that argument, in effect hardcoding it into the functor.

You're also going to need to spell it MyWindow::memberFunction rather than this->memberFunction , because that's just the way it is (to quote GCC: "ISO C++ forbids taking the address of an unqualified or parenthesized non-static member function to form a pointer to member function").

So:

using std::placeholders;
std::function<void(int&)> func = std::bind(&MyWindow::memberFunction, this, _1);

Or, using modern techniques:

std::function<void(int&)> func = [this](int& x) { memberFunction(x); };

Or just:

auto func = [this](int& x) { memberFunction(x); };