std::deque::emplace_back(3)

💅 std::deque::emplace_back(3)

okay so bestie, welcome to the C++ Standard Library page for std::deque::emplace_back — yes the manual name is literally just C++ Standard Libary (they spelled it like that, we're not fixing it, we respect the typo 👑). this is the function that glues a brand-new element onto the back of your deque, and it does it in place like the main character it is. let's get into it ✨

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🌸 NAME 🌸

std::deque::emplace_back — std::deque::emplace_back

(no notes. that's the name. it's giving minimalist. periodt.)

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✨ Synopsis ✨

template< class... Args >                  (since C++11)
void emplace_back( Args&&... args );       (until C++17)

template< class... Args >                  (since C++17)
reference emplace_back( Args&&... args );

so what actually happens? it appends a new element to the end of the container 🫶. the element gets constructed through std::allocator_traits::construct, which typically uses placement-new to construct the element in-place at the location provided by the container. and the arguments args... get forwarded to the constructor as std::forward<Args>(args).... iykyk, that's the perfect-forwarding slay 💅

⚠️ heads up bestie: all iterators (including the end() iterator) are invalidated. no references are invalidated though, so your refs stay safe. don't come for me when your iterator goes stale 😭

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🎀 Parameters 🎀

• args — the arguments you forward to the constructor of the element. literally just hand them over and the element builds itself. chef's kiss.

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🦋 Type requirements 🦋

• T (the container's element type) must meet the requirements of EmplaceConstructible. no cap, if T isn't EmplaceConstructible this whole vibe check fails 🙈

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💖 Return value 💖

• (none) — (until C++17)
• A reference to the inserted element. — (since C++17) 👑

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⭐ Complexity ⭐

Constant. that's it. that's the whole section. iconic. 🔥

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🌈 Exceptions 🌈

if an exception is thrown, this function has no effect — the strong exception guarantee. so like, it either works or it doesn't, no messy half-finished situations 🧚

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🍓 Example 🍓

the following code uses emplace_back to append an object of type President to a std::deque. it demonstrates how emplace_back forwards parameters to the President constructor and shows how using emplace_back avoids the extra copy or move operation required when using push_back 💜

// Run this code

#include <deque>
#include <cassert>
#include <iostream>
#include <string>

struct President
{
    std::string name;
    std::string country;
    int year;

    President(std::string p_name, std::string p_country, int p_year)
        : name(std::move(p_name)), country(std::move(p_country)), year(p_year)
    {
        std::cout << "I am being constructed.\n";
    }

    President(President&& other)
        : name(std::move(other.name)), country(std::move(other.country)), year(other.year)
    {
        std::cout << "I am being moved.\n";
    }

    President& operator=(const President& other) = default;
};

int main()
{
    std::deque<President> elections;
    std::cout << "emplace_back:\n";
    auto& ref = elections.emplace_back("Nelson Mandela", "South Africa", 1994);
    assert(ref.year == 1994 && "uses a reference to the created object (C++17)");

    std::deque<President> reElections;
    std::cout << "\npush_back:\n";
    reElections.push_back(President("Franklin Delano Roosevelt", "the USA", 1936));

    std::cout << "\nContents:\n";
    for (President const& president: elections)
        std::cout << president.name << " was elected president of "
                  << president.country << " in " << president.year << ".\n";

    for (President const& president: reElections)
        std::cout << president.name << " was re-elected president of "
                  << president.country << " in " << president.year << ".\n";
}

🌸 Output 🌸

emplace_back:
I am being constructed.

push_back:
I am being constructed.
I am being moved.

Contents:
Nelson Mandela was elected president of South Africa in 1994.
Franklin Delano Roosevelt was re-elected president of the USA in 1936.

see how emplace_back only says "I am being constructed." once? no extra move. that's the whole flex 💅✨

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👑 See also 👑

• push_back — adds an element to the end (public member function)
• emplace — constructs element in-place (C++11) (public member function)

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okay that's the page bestie, you're officially a deque girlie now 🫶💖

source: cppreference.com — date: 2024.06.10 — mwah, go build something iconic 😘🎀