std::unorde...::operator=(3)

💅 std::unordered_multiset::operator= (the C++ Standard Library one, bestie)

okay so picture this: you've got an unordered_multiset and you want it to hold different stuff now. that's literally it. this operator is the glow-up button. it replaces the contents of the container. three flavors, all iconic, all since C++11 except where noted. 🦋

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✨ SYNOPSIS ✨

unordered_multiset& operator=( const unordered_multiset& other );   (1) (since C++11)

unordered_multiset& operator=( unordered_multiset&& other );        (2) (since C++11) (until C++17)

unordered_multiset& operator=( unordered_multiset&& other )         (2) (since C++17)
    noexcept(/* see below */);

unordered_multiset& operator=( std::initializer_list<value_type> ilist );  (3) (since C++11)

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🌸 DESCRIPTION 🌸

Replaces the contents of the container. yes queen, wholesale replacement. here's the tea on each overload:

1) Copy assignment operator. Replaces the contents with a copy of the contents of other.

If std::allocator_traits<allocator_type>::propagate_on_container_copy_assignment::value is true, the allocator of *this is replaced by a copy of other. If the allocator of *this after assignment would compare unequal to its old value, the old allocator is used to deallocate the memory, then the new allocator is used to allocate it before copying the elements. Otherwise, the memory owned by *this may be reused when possible. In any case, the elements originally belonging to *this may be either destroyed or replaced by element-wise copy-assignment. (lowkey a whole saga, but ok 💀)

2) Move assignment operator. Replaces the contents with those of other using move semantics (i.e. the data in other is moved from other into this container). other is in a valid but unspecified state afterwards. 🫠

If std::allocator_traits<allocator_type>::propagate_on_container_move_assignment::value is true, the allocator of *this is replaced by a copy of that of other. If it is false and the allocators of *this and other do not compare equal, *this cannot take ownership of the memory owned by other and must move-assign each element individually, allocating additional memory using its own allocator as needed. In any case, all elements originally belonging to *this are either destroyed or replaced by element-wise move-assignment.

3) Replaces the contents with those identified by initializer list ilist. cute, direct, no drama ✨

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🎀 PARAMETERS 🎀

• other — another container to use as data source
• ilist — initializer list to use as data source

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👑 RETURN VALUE 👑

*this

(the main character returns herself, as she should 💅)

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⭐ COMPLEXITY ⭐

• 1) Linear in the size of *this and other.
• 2) Linear in the size of *this unless the allocators do not compare equal and do not propagate, in which case linear in the size of *this and other.
• 3) Linear in the size of *this and ilist.

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🌈 EXCEPTIONS 🌈

until C++17:

• 1–3) May throw implementation-defined exceptions.

since C++17:

• 1,3) May throw implementation-defined exceptions.
• 2) noexcept specification:

noexcept(std::allocator_traits<Allocator>::is_always_equal::value
         && std::is_nothrow_move_assignable<Hash>::value
         && std::is_nothrow_move_assignable<Pred>::value)

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🦋 NOTES 🦋

After container move assignment (overload (2)), unless element-wise move assignment is forced by incompatible allocators, references, pointers, and iterators (other than the end iterator) to other remain valid, but refer to elements that are now in *this. The current standard makes this guarantee via the blanket statement in [container.reqmts]/67, and a more direct guarantee is under consideration via LWG issue 2321. 📎 (yes we cite our sources, we're not messy)

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💖 EXAMPLE 💖

The following code uses operator= to assign one std::unordered_multiset to another:

// Run this code

#include <initializer_list>
#include <iostream>
#include <iterator>
#include <unordered_set>

void print(auto const comment, auto const& container)
{
    auto size = std::size(container);
    std::cout << comment << "{ ";
    for (auto const& element : container)
        std::cout << element << (--size ? ", " : " ");
    std::cout << "}\n";
}

int main()
{
    std::unordered_multiset<int> x{1, 2, 3}, y, z;
    const auto w = {4, 5, 6, 7};

    std::cout << "Initially:\n";
    print("x = ", x);
    print("y = ", y);
    print("z = ", z);

    std::cout << "Copy assignment copies data from x to y:\n";
    y = x;
    print("x = ", x);
    print("y = ", y);

    std::cout << "Move assignment moves data from x to z, modifying both x and z:\n";
    z = std::move(x);
    print("x = ", x);
    print("z = ", z);

    std::cout << "Assignment of initializer_list w to z:\n";
    z = w;
    print("w = ", w);
    print("z = ", z);
}

Possible output:

Initially:
x = { 3, 2, 1 }
y = { }
z = { }
Copy assignment copies data from x to y:
x = { 3, 2, 1 }
y = { 3, 2, 1 }
Move assignment moves data from x to z, modifying both x and z:
x = { }
z = { 3, 2, 1 }
Assignment of initializer_list w to z:
w = { 4, 5, 6, 7 }
z = { 7, 6, 5, 4 }

like… notice how x gets emptied by the move? that's the "valid but unspecified state" vibe check in action. don't come for her, she's fine, just empty now 😭

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🧚 SEE ALSO 🧚

• constructor — constructs the unordered_multiset (public member function)

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Category: conditionally noexcept (we love a conditional queen 🔥)

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okay that's the whole page bestie, no cap. this was served fresh from cppreference.com and dated 2024.06.10 — the std::unordered_multiset::operator= page, manual section 3, mwah 💋✨ go assign something slay 🎀