Using SFINAE with Multiple Template Parameters

When applying SFINAE to a template with multiple parameters, you can introduce additional unused template parameters for each one that needs to be constrained.

For example, let's say we have a ProcessPair template that takes two type parameters T and U, and we want to ensure that T and U both have a process() member function:

1#include <type_traits>
2#include <iostream>
3
4struct SomeType {
5  int Value;
6  void process() {
7    std::cout << "Processing " << Value << '\n';
8  }
9};
10
11using std::enable_if_t, std::is_same_v,
12  std::declval;
13
14template<typename T, typename U,
15  enable_if_t<is_same_v<decltype(
16    declval<T>().process()), void>, int> = 0,
17  enable_if_t<is_same_v<decltype(
18    declval<U>().process()), void>, int> = 0
19>
20void ProcessPair(T a, U b) {
21  a.process();
22  b.process();
23  std::cout << "Processed pair of objects";
24}
25
26int main() {
27  ProcessPair(SomeType{1}, SomeType{2});
28}

1Processing 1
2Processing 2
3Processed pair of objects

Here we've added two unused template parameters, one for each of T and U. Each one uses std::enable_if with a condition that checks if the type has a process() member function returning void.

If either T or U doesn't satisfy the condition, a substitution failure will occur, removing this template from the overload set.

By chaining multiple unused parameters like this, we can constrain any number of the template's type parameters using SFINAE.