std::list::emplace_back(3)

std::list::emplace_back(3) 💅

okay so bestie, welcome to the C++ Standard Library page for std::list::emplace_back — yes it's a mouthful, yes it's iconic, and yes we're gonna get through it together 🫶

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✨ NAME ✨

std::list::emplace_back — literally the function you call when you want a brand-new element to appear at the end of your list, built right there, no extra copies, no drama 💅

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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 here's the tea: it appends a new element to the end of the container. The element is constructed through std::allocator_traits::construct, which typically uses placement-new to construct the element in-place at the location provided by the container. The arguments args... are forwarded to the constructor as std::forward<Args>(args)....

No iterators or references are invalidated. Not one. She said "I'm not touching your pointers, bestie" and honestly? Respect 😌

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🎀 Parameters 🎀

• args — the arguments you're forwarding to the constructor of the element. they get yeeted straight into the constructor, iykyk

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💜 Type requirements 💜

• T (the container's element type) must meet the requirements of EmplaceConstructible. that's the vibe check it has to pass, no cap

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🦋 Return value 🦋

• (none) — until C++17 girlie gave you nothing back 😭
• A reference to the inserted element. — since C++17 and now she's serving main character energy, handing you a reference like it's a gift 🎁

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⭐ Complexity ⭐

Constant. fr fr, that's it. constant. slay ✨

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🔥 Exceptions 🔥

If an exception is thrown, this function has no effect — that's the strong exception guarantee. so like, if things go sideways, she rolls back the whole thing and pretends it never happened 🙈 iconic behaviour honestly

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💖 Example 💖

okay so this code uses emplace_back to append an object of type President to a std::list. 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. like, watch the output — you'll see it 👀

// Run this code

#include <list>
#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::list<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::list<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 that? emplace_back gave you one line, push_back gave you two. the move constructor said "not today" 💀 that's the whole point bestie ✨

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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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lowkey the moral of the story: emplace_back builds it in place at the end of the container, forwards your args, invalidates nothing, and keeps its promises even when exceptions show up. periodt 💅

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and that's the whole page, bestie 🎀✨ this one came from http://cppreference.com, dated 2024.06.10, and yes the page itself is std::list::emplace_back(3) — mwah, go forth and emplace 🫶💖