Creating a function alias

Question

EDIT: This question was originally titled "Using std::bind to create inline function," but that's not really what I'm after: I just want a simple way to alias functions.

I would like to expose std::chrono::high_resolution_clock::now as a standalone function. That is, I would like to do the following:

auto current_time = std::bind(std::chrono::high_resolution_clock::now);

Unfortunately, since this is in a header file, it results in multiple definitions of current_time at link-time. Is there a way to return an inline function from std::bind?

Answer 1

Here's what I do if I want to create a simple function alias

constexpr auto &&now = std::chrono::high_resolution_clock::now;

and if I want to create a full wrapper alias that will be inlined

template<typename ... Args>
inline constexpr auto now(Args &&... args) -> decltype(std::chrono::high_resolution_clock::now(std::forward<Args>(args)...)){
    return std::chrono::high_resolution_clock::now(std::forward<Args>(args)...);
}

The reason why I use a universal reference auto&& in the alias definition is because of the possibility of addressof(now) == addressof(std::chrono::high_resolution_clock::now).

On my system with G++ 4.9.2 running this:

constexpr auto &&now_ref = std::chrono::high_resolution_clock::now;
constexpr auto now_var = std::chrono::high_resolution_clock::now;

template<typename ... Args>
inline constexpr auto now_wrapper(Args &&... args)
    -> decltype(std::chrono::high_resolution_clock::now(std::forward<Args>(args)...)){
    return std::chrono::high_resolution_clock::now(std::forward<Args>(args)...);
}

int main(int argc, char *argv[]){
    std::cout << std::hex << std::showbase;
    std::cout << (uintptr_t)std::addressof(std::chrono::high_resolution_clock::now) << '\n';
    std::cout << (uintptr_t)std::addressof(now_wrapper<>) << '\n';
    std::cout << (uintptr_t)std::addressof(now_var) << '\n';
    std::cout << (uintptr_t)std::addressof(now_ref) << '\n';
}

I get the following results:

0x4007c0
0x400a50
0x400ae8
0x4007c0

Showing that only the auto&& is actually a direct alias of the function, whereas all other methods have some level of indirection. (although, after compilation they may be replaced by inlined function calls. maybe.)