Overloading function where input has certain member function

user_6844405 0
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I'm attempting to overload a function depending on whether the passed in sequence container has push_back as a member function.

#include <vector>
#include <forward_list>
#include <list>
#include <array>
#include <iostream>

template<class T, typename std::enable_if_t<std::is_member_function_pointer_v<decltype(&T::push_back)>, int> = 0>
void has_push_back(T p)
{
  std::cout << "Has push_back" << std::endl;
}

template<class T, typename std::enable_if_t<!std::is_member_function_pointer_v<decltype(&T::push_back)>, int> = 0>
void has_push_back(T p)
{
  std::cout << "No push_back" << std::endl;
}

int main() {
  std::vector<int> vec = { 0 };
  std::array<int, 1> arr = { 0 };
  std::forward_list<int> f_list = { 0 };

  has_push_back(vec);
  has_push_back(arr);
  has_push_back(f_list);
  
  return 0;
}

This results in the following compiler error for each of the calls to has_push_back():

error C2672: 'has_push_back': no matching overloaded function found
error C2783: 'void has_push_back(T)': could not deduce template argument for '__formal'

Expected result:

Has push_back
No push_back
No push_back
enable-if c++17 templates c++
1 Answers

You could use expression SFINAE for this:

#include <iostream>
#include <vector>
#include <array>
#include <forward_list>

void has_push_back(...)
{
  std::cout << "No push_back" << std::endl;
}

template<class T>
auto has_push_back(T&& t) 
  -> decltype(t.push_back(t.front()), void()) 
  // ^^ expression SFINAE, only considered 
  // if the whole expression within decltype is valid
{
  std::cout << "Has push_back" << std::endl;
}

int main() {
  std::vector<int> vec = { 0 };
  std::array<int, 1> arr = { 0 };
  std::forward_list<int> f_list = { 0 };

  has_push_back(vec);
  has_push_back(arr);
  has_push_back(f_list);
  
  return 0;
}

Result:

Has push_back
No push_back
No push_back

https://godbolt.org/z/dbzafs1nY

&T::push_back may not be well formed when push_back is a template or overloaded. Instead, I check if a call with t.front() to push_back is valid or not. Of course this also requires the type to have a suitable front member.

user_7781170
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