“Overload” function template based on function object operator() signature in C++98

Question

I want to make a template function that takes a function and a vector and uses the function to map that vector to another vector that will be returned by the function templa

Answer 1

Here's my approach:

template <std::size_t, typename T = void> struct ignore_value {typedef T type;};

template <typename T>
T& declval();

template<typename F, typename T>
typename ignore_value<sizeof(declval<F>()(declval<T const>())),
       std::vector<T> >::type map_vec(F fnc, const std::vector<T>& source);

template<typename F, typename T>
typename ignore_value<sizeof(declval<F>()
                         (declval<T const>(), declval<const std::vector<T> >())),
       std::vector<T> >::type map_vec(F fnc, const std::vector<T>& source);

It works with the same Demo that 0x499602D2 used, with both GCC and Clang in C++98 mode.