std::map::try_emplace(3)

💖 NAME 💖

okay so this is the manual page for std::map::try_emplace(3) from the C++ Standard Library, and it is literally the std::map::try_emplace member function. NAME says std::map::try_emplace - std::map::try_emplace, and the whole vibe is: insert only if the key is not already there. periodt. 💅

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✨ Synopsis ✨

Here’s the signature lineup, bestie — keep these exactly as written, no cap:

1. (1) — since C++17

template< class... Args >
std::pair<iterator, bool> try_emplace( const Key& k, Args&&... args );

2. (2) — since C++17

template< class... Args >
std::pair<iterator, bool> try_emplace( Key&& k, Args&&... args );

3. (3) — since C++26

template< class K, class... Args >
std::pair<iterator, bool> try_emplace( K&& k, Args&&... args );

4. (4) — since C++17

template< class... Args >
iterator try_emplace( const_iterator hint, const Key& k, Args&&... args );

5. (5) — since C++17

template< class... Args >
iterator try_emplace( const_iterator hint, Key&& k, Args&&... args );

6. (6) — since C++26

template< class K, class... Args >
iterator try_emplace( const_iterator hint, K&& k, Args&&... args );

So like, if a key equivalent to k already exists in the container, it does nothing. Otherwise, it inserts a new element into the container with key k and value constructed with args. In such case:

1. Behaves like emplace except that the element is constructed as

value_type(std::piecewise_construct,
           std::forward_as_tuple(k),
           std::forward_as_tuple(std::forward<Args>(args)...))

2. Behaves like emplace except that the element is constructed as

value_type(std::piecewise_construct,
           std::forward_as_tuple(std::move(k)),
           std::forward_as_tuple(std::forward<Args>(args)...))

3. Behaves like emplace except that the element is constructed as

value_type(std::piecewise_construct,
           std::forward_as_tuple(std::forward<K>(k)),
           std::forward_as_tuple(std::forward<Args>(args)...))

4. Behaves like emplace_hint except that the element is constructed as

value_type(std::piecewise_construct,
           std::forward_as_tuple(k),
           std::forward_as_tuple(std::forward<Args>(args)...))

5. Behaves like emplace_hint except that the element is constructed as

value_type(std::piecewise_construct,
           std::forward_as_tuple(std::move(k)),
           std::forward_as_tuple(std::forward<Args>(args)...))

6. Behaves like emplace_hint except that the element is constructed as

value_type(std::piecewise_construct,
           std::forward_as_tuple(std::forward<K>(k)),
           std::forward_as_tuple(std::forward<Args>(args)...))

⚠️ 1-6) If value_type is not EmplaceConstructible into map from the corresponding expression, the behavior is undefined. bestie please respect this 💀

Overload (3) is picky: it participates in overload resolution only if all following conditions are satisfied:

• std::is_convertible_v<K&&, const_iterator> and std::is_convertible_v<K&&, iterator> are both false.
• The qualified-id Compare::is_transparent is valid and denotes a type.

If equal_range(u.first) == equal_range(k) is false, the behavior is undefined, where u is the new element to be inserted.

Overload (6) is also picky: it participates in overload resolution only if the qualified-id Compare::is_transparent is valid and denotes a type.

If equal_range(u.first) == equal_range(k) is false, the behavior is undefined, where u is the new element to be inserted.

No iterators or references are invalidated. iconic, honestly. ✨

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💅 Parameters 💅

• k — the key used both to look up and to insert if not found
• hint — iterator to the position before which the new element will be inserted
• args — arguments to forward to the constructor of the element

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🎀 Return value 🎀

• 1-3) Same as for emplace.
• 4-6) Same as for emplace_hint.

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✨ Complexity ✨

• 1-3) Same as for emplace.
• 4-6) Same as for emplace_hint.

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💜 Notes 💜

Unlike insert or emplace, these functions do not move from rvalue arguments if the insertion does not happen, which makes it easy to manipulate maps whose values are move-only types, such as std::map<std::string, std::unique_ptr<foo>>. In addition, try_emplace treats the key and the arguments to the mapped_type separately, unlike emplace, which requires the arguments to construct a value_type (that is, a std::pair).

Overloads (3,6) can be called without constructing an object of type Key.

🦋 Feature-test macro tea 🦋

• __cpp_lib_map_try_emplace — value 201411L, std (C++17), feature: std::map::try_emplace, std::map::insert_or_assign
• __cpp_lib_associative_heterogeneous_insertion — value 202311L, std (C++26), feature: Heterogeneous overloads for the remaining member functions in ordered and unordered associative containers. Overloads (3) and (6).

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🍓 Example 🍓

// Run this code

#include <iostream>
#include <string>
#include <map>
#include <utility>

void print_node(const auto& node)
{
    std::cout << '[' << node.first << "] = " << node.second << '\n';
}

void print_result(auto const& pair)
{
    std::cout << (pair.second ? "inserted: " : "ignored:  ");
    print_node(*pair.first);
}

int main()
{
    using namespace std::literals;
    std::map<std::string, std::string> m;

    print_result(m.try_emplace("a", "a"s));
    print_result(m.try_emplace("b", "abcd"));
    print_result(m.try_emplace("c", 10, 'c'));
    print_result(m.try_emplace("c", "Won't be inserted"));

    for (const auto& p : m)
        print_node(p);
}

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🌈 Output 🌈

inserted: [a] = a
inserted: [b] = abcd
inserted: [c] = cccccccccc
ignored:  [c] = cccccccccc
[a] = a
[b] = abcd
[c] = cccccccccc

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👑 See also 👑

• emplace — constructs element in-place; (C++11); public member function
• emplace_hint — constructs elements in-place using a hint; (C++11); public member function
• insert — inserts elements or nodes; (since C++17); public member function

mwah 💋 source/version/date sign-off: http://cppreference.com · 2024.06.10 · std::map::try_emplace(3)