std::function -> function pointer

Question

Here is a code:

#include <functional>
using namespace std::tr1;

typedef void(*fp)(void);

void foo(void)
{

}

void f(fp)
{

}

int main()
{
  function<void(void)> fun = foo;
  f(fun); // error
  f(foo); // ok
}

Originally i need to make a function pointer from non-static class method because i need to save data between function callings. I tried std::tr1::bind and boost::bind, but they return functional object, not pointer, which, as i can see, can't be "casted" to pure functional pointer. While the function signature (SetupIterateCabinet) demands a pure func pointer exactly.

I need an advise how to solve the problem. Thank you.

Answer 1

You've greatly oversimplified your real problem and turned your question into an XY problem. Let's get back to your real question: how to call SetupIterateCabinet with a non-static member function as a callback.

Given some class:

class MyClass
{
public:
    UINT MyCallback(UINT Notification, UINT_PTR Param1, UINT_PTR Param2)
    {
        /* impl */
    }
};

In order to use MyClass::MyCallback as the third argument to SetupIterateCabinet, you need to pass a MyClass* for the Context argument and use a plain shim function to take that Context argument and do the right thing with it:

UINT MyClassCallback(PVOID Context, UINT Notification, UINT_PTR Param1, UINT_PTR Param2)
{
    return static_cast<MyClass*>(Context)->MyCallback(Notification, Param1, Param2);
}

int main()
{
    MyClass mc;
    SetupIterateCabinet(_T("some path"), 0, MyClassCallback, &mc);
}

Answer 2

You can't convert a std::function to a function pointer(you can do the opposite). You should use either function pointers, or std::functions. If you can use std::function instead of pointers, then you should.

This makes your code work:

typedef function<void(void)> fp;