std::default_initializable(3)

🌸 NAME 🌸

okay bestie, welcome to the std::default_initializable(3) page from the C++ Standard Libary manual 🫶 and the name entry is literally std::default_initializable — std::default_initializable (yes, twice, and honestly? iconic).

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✨ Synopsis ✨

It’s defined in header <concepts>, and it’s been a thing since C++20. The synopsis is giving exactly this:

Defined in header <concepts>
template< class T >

concept default_initializable = std::constructible_from<T> &&
(since C++20)
requires { T{}; } &&

                                        /* T t; is well-formed, see below */;

So what does the default_initializable concept actually check? It checks whether variables of type T can be:

• value-initialized (T() is well-formed);
• direct-list-initialized from an empty initializer list (T{} is well-formed); and
• default-initialized (T t; is well-formed).

And this part is important, no cap: access checking is performed as if in a context unrelated to T. Only the validity of the immediate context of the variable initialization is considered. 💅

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🔥 Possible implementation 🔥

Here’s one possible implementation, and yes it’s lowkey minimal:

template<class T>
concept default_initializable =
    std::constructible_from<T> &&
    requires { T{}; } &&
    requires { ::new T; };

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👀 See also 👀

• constructible_from (C++20) — specifies that a variable of the type can be constructed from or bound to a set of argument types (concept).
• is_default_constructible (C++11) — checks if a type has a default constructor (class template).
• is_trivially_default_constructible (C++11) — checks if a type has a default constructor (class template).
• is_nothrow_default_constructible (C++11) — checks if a type has a default constructor (class template).

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mwah 💋 that’s the page! source/version/date vibes: http://cppreference.com, 2024.06.10, page std::default_initializable(3). bye bestie 🫶