Why does C++11 not support 'std::function<void(int, …)>'?

Question

#include <functional>

void f1(int)
{}

void f2(int, ...)
{}

int main()
{
    std::function<void(int)>      g1 = f1; // OK.
    std::function<void(int, ...)> g2 = f2; // Error! Not standard C++!
}

Why does C++11 not provide a specialized template class as follows:

template<class ResultType, class... ArgTypes>
class function<ResultType(ArgTypes......)>
{
    // ... ... ...
};

Answer 1

I do not mean to provide the ultimate reason why that specialization is not provided (I do not know that), but maybe I could hint at some of the technical obstacles that could be encountered when trying to implement it. That will hopefully give you a feeling of why the specialization is not there.

Let's consider first how the std::function<> class template itself could be implemented. The type erasure technique that underlies its design could be sketched as follows (this is just an illustrative simplification , the real implementation is way more complex):

#include <memory>

template<typename T>
struct function { };

template<typename R, typename... Args>
struct function<R(Args...)>
{

public:

    template<typename F>
    function(F&& f) : _holder(
        new holder<typename std::decay<F>::type>(std::forward<F>(f))
        )
    { }

    R operator () (Args&&... args)
    { _holder->call(std::forward<Args>(args)...); }

private:

    struct holder_base
    { virtual R call(Args&&... args) = 0; };

    template<typename F>
    struct holder : holder_base
    {
        holder(F&& f) : _f(std::forward<F>(f)) { }
        R call(Args&&... args) { return _f(std::forward<Args>(args)...); }
        F _f;
    };

    std::unique_ptr<holder_base> _holder;
};

Now let's see how the specialization for ellipses would look like. First of all, the number and type of the arguments provided to a variadic function is not fixed in that function's signature. Therefore, the call operator of our specialized template must be a function template accepting any number and type of arguments:

template<typename R, typename... Args>
struct function<R(Args.......)>
{
    ...

    template<typename... Ts>
    R operator () (Args&&... args, Ts&&... ts)
    { _holder->call(std::forward<Args>(args)..., std::forward<Ts>(ts)...); }

    ...

This forces us, in turn, to make holder<> 's call operator a variadic function template. However, in order to realize type erasure, that same call operator must be virtual , and function templates cannot be virtual in C++.

Things would certainly be easier if variadic arguments (I'm talking about ellipses here) could be easily forwarded without having to recur to variadic template parameters and perfect forwarding. I am not aware of a simple way to achieve that though, and especially not if no other argument is to be passed to the function than those matching the variadic list.