Question

Why is the nested std::bind in the below code not implicitly converted to an std::function<void()> by any of the major compilers (VS2010/2012, gcc, clang)? Is this standard behavior, or a bug?

#include <functional>

void bar(int, std::function<void()>) { }
void foo() { }

int main()
{
    std::function<void(int, std::function<void()>)> func;
    func = std::bind(bar, 5, std::bind(foo));

    std::cin.get();
    return 0;
}

Answer 1

This is explained in the boost documentation:

The inner bind expressions are evaluated, in unspecified order, before
the outer bind when the function object is called; the results of the
evaluation are then substituted in their place when the outer bind is
evaluated. In the example above, when the function object is called
with the argument list (x), bind(g, _1)(x) is evaluated first,
yielding g(x), and then bind(f, g(x))(x) is evaluated, yielding the
final result f(g(x)).

Boost even provides protect to prevent this evaluation:

#include <boost/bind/protect.hpp>
...
func = std::bind(bar, 5, boost::protect(std::bind(foo)));

However, to call func you have to provide both arguments like this (thanks to David Rodríguez – dribeas for pointing that out), so this example is definitely not good:

func(1, std::function<void()>());