C++: Assigning a function to a tr1::function object

Question

One of our classes provides tr1::function callback object. When I try assigning a member function to it, though, I get a compiler error.

The examples below are untested and just to illustrate:

Foo.h:

class Foo()
{
public:
  Foo();
  std::tr1::function<void (int x)> F;
}

Bar.h:

class Bar()
{
public:
   Bar();
   Foo* foo;
   void HookUpToFoo();
   void Respond(int x);
}

Bar.cpp:

Bar()
{
    this->foo = new Foo();
    this->HookUpToFoo();
}

void Bar::HookUpToFoo()
{
  this->foo->F = &Bar::Respond; // error
}

void Bar::Respond(int x)
{
       // do stuff
}

The compiler error we get refers to a line in xrefwrap and is Error 1 error C2296: '.*' : illegal, left operand has type 'int' C:\Program Files\Microsoft Visual Studio 9.0\VC\include\xrefwrap 64

..What am I doing wrong in assigning a delegate? I want to go the more modern route and use tr1::function rather than function pointers.

Answer 1

A member function accepts an additional hidden parameter: this. This makes it incompatible with your function object which only accepts one parameter. You can bind a member function to a particular instance using bind, which is also available in tr1:

using namespace std::tr1::placeholders;

this->foo->F = std::tr1::bind(&Bar::Respond, this, _1);

This ensures that Bar::Respond will be called with the correct value bound to this. The _1 is a placeholder for the additional parameter (x).