std::chrono::is_clock(3)

💅 std::chrono::is_clock(3) — the C++ Standard Library, but make it cute

Okay so bestie, this is the std::chrono::is_clock page from the C++ Standard Library, and it's genuinely one of those tiny traits that does exactly one thing and does it iconically. Let's get into it 🫶

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✨ NAME ✨

std::chrono::is_clock — literally just std::chrono::is_clock. That's the name. She's self-titled, main character energy, no notes 💅

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🌸 SYNOPSIS 🌸

Defined in header <chrono>:

template< class T >         (since C++20)
struct is_clock;

So here's the vibe: if T satisfies the Clock requirements, this trait hands you a member constant value equal to true. For any other type? value is false. Periodt. ✨

Now for the fine print, and listen up because this part actually matters: for the purpose of this trait, how far an implementation goes to determine that a type can't meet the Clock requirements is unspecified — with one exception: a minimum T shall not qualify as a Clock unless it meets all of the following conditions:

• Each of these qualified identifiers has to be valid and has to denote a type:
  • T::rep
  • T::period
  • T::duration
  • T::time_point
• And each of these expressions has to be well-formed when it's treated as an unevaluated operand:
  • T::is_steady
  • T::now()

🚨 Okay so real talk: if your program adds specializations for std::is_clock or std::is_clock_v, the behavior is undefined. Like, don't. Just don't do it. That's the whole warning. 😭

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🦋 Template parameters 🦋

• T — a type to check 👀

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🎀 Helper variable template 🎀

template< class T >                                     (since C++20)
inline constexpr bool is_clock_v = is_clock<T>::value;

The little _v shortcut bestie. We love a convenience alias. 💖

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🌈 Inherited from std::integral_constant 🌈

She inherits, she doesn't reinvent. Iconic behavior honestly.

⭐ Member constants

• value — true if T satisfies the Clock requirements, false otherwise. [static] (public static member constant)

💜 Member functions

• operator bool — converts the object to bool, returns value (public member function)
• operator() — returns value (C++14) (public member function)

🍓 Member types

• value_type — defined as bool
• type — defined as std::integral_constant<bool, value>

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🔥 Possible implementation 🔥

Okay this is the fun part, here's roughly how someone could build it:

template<class>
struct is_clock : std::false_type {};

template<class T>
    requires
        requires
        {
            typename T::rep;
            typename T::period;
            typename T::duration;
            typename T::time_point;
            T::is_steady; // not required to be bool
            T::now();     // not required to be T::time_point
        }
struct is_clock<T> : std::true_type {};

See those comments? T::is_steady is not required to be bool, and T::now() is not required to be T::time_point. The trait is just checking the expressions are well-formed, not that they're the "right" types. Sneaky but valid 🧚

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💀 Example 💀

// Run this code

#include <chrono>
#include <ratio>

int main()
{
    static_assert
    (
        std::chrono::is_clock_v<std::chrono::utc_clock>
        and
        not std::chrono::is_clock_v<std::chrono::duration<int, std::exa>>
    );
}

utc_clock? A clock, slay ✅. duration<int, std::exa>? Not a clock, sorry babe ❌. Both asserted at compile time, no runtime drama. 👑

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🧚 Sign-off 🧚

That's the whole page, bestie — sourced from http://cppreference.com, dated 2024.06.10, and filed under std::chrono::is_clock(3). Go forth and check your clocks at compile time, fr fr 💖✨🎀