std::is_mov...assignable,(3)

💖 NAME 💖

Okay bestie, welcome to the std::is_mov...assignable,(3) page from the C++ Standard Library manual — the one all about move-assignability type traits. It covers std::is_move_assignable, std::is_trivially_move_assignable, and std::is_nothrow_move_assignable. No cap, these are the vibe-checkers for “can I move-assign this type?” 💅

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✨ Synopsis ✨

Defined in header <type_traits>:

template< class T >                  (1) (since C++11)
struct is_move_assignable;
template< class T >                  (2) (since C++11)
struct is_trivially_move_assignable;
template< class T >                  (3) (since C++11)
struct is_nothrow_move_assignable;

Here’s the value table, bestie: the T is a referenceable type column gives the trait expression below, and the T is not a referenceable type column is false (the extracted text shows false on the (2) row). Table translation era 💖

• (1) — std::is_assignable<T, T&&>::value
• (2) — std::is_trivially_assignable<T, T&&>::value
• (3) — std::is_nothrow_assignable<T, T&&>::value

Behavior undefined alert 🚨 If T is not a complete type, (possibly cv-qualified) void, or an array of unknown bound, the behavior is undefined.

Another UB warning 💀 If an instantiation of a template above depends, directly or indirectly, on an incomplete type, and that instantiation could yield a different result if that type were hypothetically completed, the behavior is undefined.

Specialization warning 🙈 If the program adds specializations for any of the templates described on this page, the behavior is undefined.

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🫶 Member constants 🫶

• value — true if T is move-assignable, false otherwise. It’s [static] — public static member constant. Iconic little boolean.

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🌸 Member functions 🌸

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

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🎀 Member types 🎀

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

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🧚 Possible implementation 🧚

template<class T>
struct is_move_assignable
    : std::is_assignable<typename std::add_lvalue_reference<T>::type,
                         typename std::add_rvalue_reference<T>::type>
{};

template<class T>
struct is_trivially_move_assignable
    : std::is_trivially_assignable<typename std::add_lvalue_reference<T>::type,
                                   typename std::add_rvalue_reference<T>::type> {};

template<class T>
struct is_nothrow_move_assignable
    : std::is_nothrow_assignable<typename std::add_lvalue_reference<T>::type,
                                 typename std::add_rvalue_reference<T>::type> {};

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🔥 Notes 🔥

The trait std::is_move_assignable is less strict than MoveAssignable because it does not check the type of the result of the assignment (which, for a MoveAssignable type, must be T&), nor the semantic requirement that the target’s value after the assignment is equivalent to the source’s value before the assignment.

The type does not have to implement a move assignment operator in order to satisfy this trait; see MoveAssignable for details.

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💜 Example 💜

// Run this code

#include <iostream>
#include <string>
#include <type_traits>

struct Foo { int n; };

struct NoMove
{
    // prevents implicit declaration of default move assignment operator
    // however, the class is still move-assignable because its
    // copy assignment operator can bind to an rvalue argument
    NoMove& operator=(const NoMove&) { return *this; }
};

int main()
{
    std::cout << std::boolalpha
              << "std::string is nothrow move-assignable? "
              << std::is_nothrow_move_assignable<std::string>::value << '\n'
              << "int[2] is move-assignable? "
              << std::is_move_assignable<int[2]>::value << '\n'
              << "Foo is trivially move-assignable? "
              << std::is_trivially_move_assignable<Foo>::value << '\n'
              << "NoMove is move-assignable? "
              << std::is_move_assignable<NoMove>::value << '\n'
              << "NoMove is nothrow move-assignable? "
              << std::is_nothrow_move_assignable<NoMove>::value << '\n';
}

🍓 Output: 🍓

std::string is nothrow move-assignable? true
int[2] is move-assignable? false
Foo is trivially move-assignable? true
NoMove is move-assignable? true
NoMove is nothrow move-assignable? false

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🌈 See also 🌈

• is_assignable (C++11), is_trivially_assignable (C++11), is_nothrow_assignable (C++11) — checks if a type has an assignment operator for a specific argument (class template).
• is_copy_assignable (C++11), is_trivially_copy_assignable (C++11), is_nothrow_copy_assignable (C++11) — checks if a type has a copy assignment operator (class template).

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Source/version/date energy: http://cppreference.com, 2024.06.10, page std::is_mov...assignable,(3). That’s the whole manual page, bestie — go move-assign something responsibly! 💖✨🎀