[Boost-users] Bind/Function: use reference arguments in template variadic functions

Subject: [Boost-users] Bind/Function: use reference arguments in template variadic functions
From: Yannick POTIN (yannp63_at_[hidden])
Date: 2013-04-11 16:57:09

Hello,

I'm working on a little project which use the template variadic technology.
I use them with boost::bind and boost::function and found a very annoying bug.

Because I have a lot of files very long and (sorry :-( ) uncommented.
I made a simple but explicit example like this one :

----------------------------------------------
#include <boost/function.hpp>
#include <boost/bind.hpp>

#include <iostream>

// A classic class...
class A {
public:
  // ... with a constructor
  A(int a) : a(a) { }

  // ... an operator
  void operator()() {
    std::cout << (void*)this << "::" << __func__ << "(): " << a << std::endl;
  }

  // ... and a member
  int a;
};

// A stupid function...
void call_a(A &a) {
  // ... which do something usefull
  // please not that this function NEEDs a reference
  a();
}

// A template variadic function...
template<class ...classes>
boost::function0<void> call_something(classes... c) {
  // ... here we are:
  return boost::bind(call_a,boost::ref(c...));
}

// Ok, this one, I think you know it more than I do
int main(void) {

  // Let's create some objets
  A a1(1);

  A a2(2);

  // Call them to print the memory
  // position (see A::operator()())
  a1();

  a2();

  // Create a boost::function0<void> and call it
  call_something<A>(boost::ref(a1))();

  // Again
  call_something<A>(boost::ref(a2))();

  return 0;
}

----------------------------------------------
After compilation, I get this output (watch the memory addresses):
0xbfc8f994::operator()(): 1
0xbfc8f990::operator()(): 2
0xbfc8f984::operator()(): 1
0xbfc8f984::operator()(): 2
I really don't know what happened (even if I can imagine).
But references are just lost and this is not good.

So, here is my question: How can I make this working ?

May I precise that this example only use one argument but my project
actually use at most five so 
the boost::ref() call like in the call_something function is impossible 
(and does not work anyway).

Before someone ask me if I can use pointer instead, I answer: 
Yes I can, but it will take a lot of time 
and if I can avoid this, I will.

Thank you for reading, for your patience and for your answers ;-)

Regards,

Yannick POTIN