boost::factory and std::function

Question

I'm trying to convert some code to C++11 on gcc 4.7.2. Part of the code included the assignment of a boost::factory to a boost::function. However, the compilation fails with lots of warnings if I change it to std::function. Here's sample code the reproduces the error:

#include <boost/functional/factory.hpp>
#include <boost/function.hpp>
#include <functional>

struct S {
  S(int) {}
};

int main() {

  boost::function<S*(int)> b = boost::factory<S*>(); // ok

  std::function<S*(int)> s = boost::factory<S*>();   // error
}

This gives error: no match for call to ‘(boost::factory<S*>) (int)’

Interestingly, if S takes no arguments, std::function<S*()> assigns to the boost::factory without complaint. Why doesn't this work? Is there a work around to allow assignment of a boost::factory to an std::function? I'm using Boost.1.53

Answer 1

It seems that boost::factory<S*> insists on its argument to be bound to a reference:

boost::factory<S*> factory;

factory(17); // error

int argument(17);
factory(argument); // OK

Based on this, it seems that boost::function<S*(int)> passes the argument as int without perfectly forwarding it while std::function<S*(int)> seems to use perfect forwarding on the argument. As a result, the argument cannot be bound to boost::factory<S*> argument.

When using gcc's std::function<...> passing the argument by const& works:

std::function<S*(int const&)> factory = boost::factory<S*>();

Unfortunately, the same work-around does not work with the std::function<...> from libc++. Since you said that you tried to compile the code with gcc that may not be much of an issue, though.