std::vector::operator=(3)

💅 std::vector::operator=(3) 💅

okay bestie, so this is the C++ Standard Library page for std::vector::operator= — yep, the assignment operator, the one that goes brrr and replaces what's inside your container. She's from std::vector::operator= (yes the NAME is literally just the same thing twice, we love a self-referential icon).

---

✨ Synopsis ✨

vector& operator=( const vector& other );      (1) (constexpr since C++20)

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

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

vector& operator=( std::initializer_list<value_type> ilist );   (3)
                                                          (since C++11)
                                                          (constexpr since C++20)

So what does she actually do? Replaces the contents of the container. That's the whole vibe. 💖

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

Now, plot twist 🫢 — 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. And 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 (that whole allocator-swap clause is since C++11, fyi). 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. either way bestie, they're not surviving as-is.

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 — so like, she's fine, she's just not the same girl anymore, yk? 🙈

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. But 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. 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, curated, list-shaped. 🎀

---

🌸 Parameters 🌸

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

---

🦋 Return value 🦋

*this — she returns herself. main character energy, literally. 👑

---

🔥 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.

---

😭 Exceptions 😭

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

(since C++17) — 1,3) May throw implementation-defined exceptions. 2) noexcept specification:

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

---

💀 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 — the standard's out here doing blanket statements and LWG's still workshopping the direct one. 📝

---

🧚 Example 🧚

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

// Run this code

#include <initializer_list>
#include <iostream>
#include <iterator>
#include <vector>

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::vector<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 gets absolutely emptied out by the move? she gave everything she had to z, that's real friendship fr fr. 🫶

---

🌈 See also 🌈

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

---

💜 Category 💜

• conditionally noexcept

---

so yeah, that's the whole page 💅 slay, you now understand std::vector::operator=. this drop is brought to you by cppreference.com, dated 2024.06.10, and it's tagged std::vector::operator=(3) — go forth and assign responsibly bestie, ily, bye! 🍓✨