Creating a Custom Type Trait to Check for a Member Function

To create a custom type trait that checks for the presence of a specific member function in a type, you can use SFINAE (Substitution Failure Is Not An Error) techniques. Here's an example of how to create a type trait has_render that checks if a type has a Render member function:

1#include <iostream>
2#include <type_traits>
3
4// Helper struct for SFINAE
5template <typename T>
6struct has_render {
7  template <typename U>
8  static auto test(U* p)
9    -> decltype(p->Render(), std::true_type{});
10
11  template <typename>
12  static std::false_type test(...);
13
14  static constexpr bool value =
15    decltype(test<T>(nullptr))::value;
16};
17
18// _v version for convenience
19template <typename T>
20inline constexpr bool has_render_v =
21  has_render<T>::value;
22
23// Example classes
24class WithRender {
25 public:
26  void Render() {}
27};
28
29class WithoutRender {};
30
31int main() {
32  std::cout << std::boolalpha;
33  std::cout << "WithRender has Render: "
34    << has_render_v<WithRender> << "\n";
35  std::cout << "WithoutRender has Render: "
36    << has_render_v<WithoutRender> << "\n";
37}

1WithRender has Render: true
2WithoutRender has Render: false

In this example, the has_render struct uses SFINAE to detect the presence of a Render member function. The test function is overloaded based on whether Render can be called on a pointer to the type T. If Render is present, the first overload is selected, and std::true_type is returned. Otherwise, the second overload is selected, and std::false_type is returned.

The has_render_v variable template is provided for convenience, similar to the _v suffix used in standard library type traits.

This technique can be adapted to check for the presence of other member functions or member variables in a type.