How to write std::chrono::high_resolution clock class template specialization only if it is not alias

Question

I want to write class template specializations for std::chrono::system_clock, std::chrono::steady_clock, and std::chrono::high_resolution_clock.

I wrote a straight forward code as follows:

#include <chrono>

template <typename T>
struct foo;

template <>
struct foo<std::chrono::system_clock> 
{};

template <>
struct foo<std::chrono::steady_clock> 
{};

// pseudo code    
//
// if constexpr (
//     !std::is_same_v(std::chrono::high_resolution_clock, std::chrono::system_clock> &&
//     !std::is_same_v(std::chrono::high_resolution_clock, std::chrono::steady_clock>
// ) {
// 

template <>
struct foo<std::chrono::high_resolution_clock> 
{};

// }

int main() {
}

Compile result: https://wandbox.org/permlink/8SqPZsMYdT8WKai3

If std::chrono::high_resolution_clock is an alias of std::chrono::system_clock or std::chrono::steady_clock, then I got the error redefinition of the same class template specialization.

I'm looking for a way to enable std::chrono::high_resolution_clock class template specialization only if it is not alias. I wrote comments (pseudo code) in the code that demonstrates what I want to do.

Is there any good way ?

score 4 · Accepted Answer · answered Jun 12 '20 at 12:26

You could provide an additional template parameter that depends on the condition you want to check. The primary template would be:

template <typename T, bool = true>
struct foo;

and then the specialization would be:

template <>
struct foo<std::chrono::high_resolution_clock,
           !std::is_same_v<std::chrono::high_resolution_clock, 
                           std::chrono::system_clock>
           &&
           !std::is_same_v<std::chrono::high_resolution_clock, 
                           std::chrono::steady_clock>
          >
{};

Here's a demo.

Thank you! Your answer is exactly what I want to do. – Takatoshi Kondo Jun 12 '20 at 13:06 — Takatoshi Kondo, Jun 12 '20 at 13:06

How to write std::chrono::high_resolution clock class template specialization only if it is not alias

1 Answers1