Chained invocation of C++11 std::bind doesn't work - c++

Question

I have a problem when invoking nested std::bind expressions. The following code demonstrates the problem. It fails to compile with libc++, but works with boost:
#define BOOST 0
#if BOOST
#include <boost/function.hpp>
#include <boost/bind.hpp>
using boost::function;
using boost::bind;
#else
#include <functional>
using std::function;
using std::bind;
using std::placeholders::_1;
#endif
int sum(int a, int b) { return a+b; }
// works
template <typename F>
int yeah(F f, int c)
{
return f(c);
}
// breaks with libc++
template <typename F>
int nope(F f, int c)
{
return bind(f, c)();
}
// fixes the problem
template <typename F>
int fix1(F f, int c)
{
function<int(int)> g = f;
return bind(g, c)();
}
template <typename F>
class protect_t
{
public:
typedef typename F::result_type result_type;
explicit protect_t(F f): f_(f) {}
template <typename... Args>
result_type operator()(Args&&... args)
{
return f_(std::forward<Args>(args)...);
}
private:
F f_;
};
template <typename F>
protect_t<F> protect(F f)
{
return protect_t<F>(f);
}
// compilation fails with libc++
template <typename F>
int fix2(F f, int c)
{
return bind(protect(f), c)();
// F copy(f); // fails due to this!
}
#include <iostream>
int main()
{
std::cout << yeah(bind(sum, _1, 4), 5) << std::endl; // works
std::cout << nope(bind(sum, _1, 4), 5) << std::endl; // breaks
std::cout << fix1(bind(sum, _1, 4), 5) << std::endl; // fixes
std::cout << fix2(bind(sum, _1, 4), 5) << std::endl; // doesn't compile
}
Wrapping the bind expression in a std::function (see fix1) remedies the problem, albeit by sacrificing speed due to run-time polymorphism disabling inlining (haven't measured it though).
Wrapping the bind expression in protect_t (see fix2) is inspired by boost::protect, however, compilation with libc++ fails due to bind expressions not being copyable. Which makes me wonder why wrapping them in std::function works anyway.
Any idea how to solve this problem? What's wring with std::bind anyway? First I thought the problem is related to eager evaluation of bind expressions dictated by the C++11 standard (see here), but that would not be a problem here, would it?

Answer 1

The standard says that any callable can be wrapped using std::bind, including those produced by a preceding call to std::bind. Your problem is caused by a deficiency in the implementation of the standard library you're using, the solution is either to upgrade or, if this bug isn't still fixed, to switch to a different implementation.