std::deque::operator=(3)

📦 version 2024.07.05, in nixpkgs unstable, opensuse-leap 16.0

👑 std::deque::operator=(3) 👑

okay bestie, so this one's straight out of the C++ Standard Libary manual (page 3, mwah) — it's the assignment operator for std::deque, and it's literally the glow-up button for your container. It replaces the contents of the container. That's it, that's the whole vibe. 💅

✨ SYNOPSIS ✨

deque& operator=( const deque& other );                          (1)

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

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

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

Replaces the contents of the container. no cap, that's the entire job description 🫶

🌸 DESCRIPTION 🌸

1) the copy assignment operator bestie — it replaces the contents with a copy of the contents of other. 📋

so here's the tea: if std::allocator_traits<allocator_type>::propagate_on_container_copy_assignment::value is true, then the allocator of *this gets replaced by a copy of other. and if the allocator of *this after assignment would compare unequal to its old value, the old allocator is used to deallocate the memory, and then the new allocator is used to allocate it before copying the elements (that part's (since C++11)). otherwise, the memory owned by *this may be reused when possible. and in any case, the elements originally belonging to *this may be either destroyed or replaced by element-wise copy-assignment. either way they're not sticking around as-is, periodt 💀

2) the move assignment operator 🔥 — it 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 left in a valid but unspecified state afterwards. very "we don't know her anymore" of it.

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's 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. and 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. little braced-list moment, we love her 🎀

💖 PARAMETERS 💖

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

⭐ RETURN VALUE ⭐

*this — and honestly? iconic. returns herself. main character energy 👑

🦋 COMPLEXITY 🦋

Linear in the size of *this and other.
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.
Linear in the size of *this and ilist.

🌈 EXCEPTIONS 🌈

1-3) May throw implementation-defined exceptions. (until C++17)

1,3) May throw implementation-defined exceptions.

(since C++17) — noexcept specification:

noexcept(std::allocator_traits<Allocator>::is_always_equal::value)

bestie please actually read that noexcept clause, it depends entirely on is_always_equal — don't just assume it's a no-throw situation 🙈

🧚 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. iykyk 💜

💀 EXAMPLE 💀

the following code uses operator= to assign one std::deque to another:

// Run this code

#include <initializer_list>
#include <iostream>
#include <iterator>
#include <deque>

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::deque<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);
}

Output

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

see how x literally gets emptied into z? ate and left no crumbs 😭

🎀 SEE ALSO 🎀

constructor — constructs the deque (public member function)
assign — assigns values to the container (public member function)

std::deque::operator=(3)