Check at compile-time is a template type a vector

Question

I can imagine the following code:

template  class X
{
  public:
   T container;

   void foo()
   {
      if(is_vector(T))
         contain         


        

Answer 1

Here's the typical method using void_t:

template 
using void_t = void;  // C++17 std::void_t

template   // I'm using C for "container" instead of T, but whatever.
struct has_push_back_impl : std::false_type {};

template 
struct has_push_back_impl().push_back(typename C::value_type{}))>>
    : std::true_type {};  // Note that void_t is technically not needed in this case, since the 'push_back' member function actually returns void anyway, but it the general method to pass the type into void_t's template argument to obtain void.  For example, the 'insert' function from std::set and std::map do NOT return void, so 'has_insert' will need to use void_t.

template 
using has_push_back = has_push_back_impl;  // void passed to the second template argument by default, thus allowing the second specialization to be used instead of the primary template whenever C has a push_back member function.

This method will work for has_insert for associative containers, even though std::set, std::map's insert function return std::pair while std::multimap::insert returns std::multimap::iterator (this is one case where Ze Blob's method will not work).