Applying SFINAE to Non-Type Template Parameters

Yes, SFINAE can be applied to non-type template parameters as well, such as integers, enums, or pointers.

For example, let's say we want to write a ToString function that converts an integral value to a string, but we only want to enable it for unsigned integral types. We can achieve this using SFINAE on a non-type template parameter:

1#include <type_traits>
2#include <string>
3
4using std::enable_if_t, std::is_unsigned_v;
5template<typename T,
6  enable_if_t<is_unsigned_v<T>, int> = 0
7>
8std::string ToString(T value) {
9  return std::to_string(value);
10}

Here, the second template parameter is an unused non-type parameter of type int. The std::enable_if condition checks if T is an unsigned type using the std::is_unsigned type trait.

If T is unsigned, the second parameter will have a type of int and the template will be viable. If T is not unsigned, a substitution failure occurs, removing this template from the overload set.

We can test it like this:

1#include <type_traits>
2#include <string>
3
4using std::enable_if_t, std::is_unsigned_v;
5template<typename T,
6  enable_if_t<is_unsigned_v<T>, int> = 0
7>
8std::string ToString(T value) {
9  return std::to_string(value);
10}
11
12int main() {
13  unsigned int x = 42;
14  int y = -10;
15
16  // OK
17  std::string sx = ToString(x);
18
19  // Substitution failure, int is not unsigned
20  std::string sy = ToString(y);
21}

So SFINAE works consistently with both type and non-type template parameters. The key is to introduce an unused parameter whose validity depends on the properties you want to constrain.