std::map::insert(3)

💅 std::map::insert — C++ Standard Library manual page, but make it bestie-core 💅

okay so we're deep in the std::map::insert(3) manual page from the C++ Standard Library, and this is part 1 of 2. buckle up, because inserting into a map is literally the main character move of associative containers ✨

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🌸 NAME 🌸

std::map::insert - std::map::insert

yes bestie, it's the same thing twice. iconic behavior, no notes 💅

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📜 SYNOPSIS 📜

std::pair<iterator, bool> insert( const value_type& value );                 (1)
template< class P >
std::pair<iterator, bool> insert( P&& value );                               (2) (since C++11)
std::pair<iterator, bool> insert( value_type&& value );                      (3) (since C++17)
(4) iterator insert( iterator pos, const value_type& value );                (until C++11)
    iterator insert( const_iterator pos, const value_type& value );          (since C++11)
template< class P >
iterator insert( const_iterator pos, P&& value );                            (5) (since C++11)
iterator insert( const_iterator pos, value_type&& value );                   (6) (since C++17)
template< class InputIt >
void insert( InputIt first, InputIt last );                                  (7)
void insert( std::initializer_list<value_type> ilist );                      (8) (since C++11)
insert_return_type insert( node_type&& nh );                                 (9) (since C++17)
iterator insert( const_iterator pos, node_type&& nh );                       (10) (since C++17)

🧠 the tea: what insert actually does 🧠

Inserts element(s) into the container, if the container doesn't already contain an element with an equivalent key. no duplicates allowed, this map is exclusive 💅

1-3) Inserts value.

Overload (2) is equivalent to emplace(std::forward<P>(value)) and only participates in overload resolution if std::is_constructible<value_type, P&&>::value == true.

4-6) Inserts value in the position as close as possible to the position just prior to pos.

Overload (5) is equivalent to emplace_hint(hint, std::forward<P>(value)) and only participates in overload resolution if std::is_constructible<value_type, P&&>::value == true.

7. Inserts elements from range [first, last). If multiple elements in the range have keys that compare equivalent, it is unspecified which element is inserted (pending LWG2844).
8. Inserts elements from initializer list ilist. If multiple elements in the range have keys that compare equivalent, it is unspecified which element is inserted (pending LWG2844).
9. If nh is an empty node handle, does nothing. Otherwise, inserts the element owned by nh into the container, if the container doesn't already contain an element with a key equivalent to nh.key(). The behavior is undefined if nh is not empty and get_allocator() != nh.get_allocator().
10. If nh is an empty node handle, does nothing and returns the end iterator. Otherwise, inserts the element owned by nh into the container, if the container doesn't already contain an element with a key equivalent to nh.key(), and returns the iterator pointing to the element with key equivalent to nh.key() (regardless of whether the insert succeeded or failed). If the insertion succeeds, nh is moved from, otherwise it retains ownership of the element. The element is inserted as close as possible to the position just prior to pos. The behavior is undefined if nh is not empty and get_allocator() != nh.get_allocator().

No iterators or references are invalidated.

If the insertion is successful, pointers and references to the element obtained while it is held in the node handle are invalidated, and pointers and references obtained to that element before it was extracted become valid. (since C++17)

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🫶 PARAMETERS 🫶

• pos — iterator to the position before which the new element will be inserted
• value — element value to insert
• first, last — range of elements to insert
• ilist — initializer list to insert the values from
• nh — a compatible node handle

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🧚 TYPE REQUIREMENTS 🧚

• InputIt must meet the requirements of LegacyInputIterator.

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🎁 RETURN VALUE 🎁

• 1-3) A pair consisting of an iterator to the inserted element (or to the element that prevented the insertion) and a bool value set to true if and only if the insertion took place.
• 4-6) An iterator to the inserted element, or to the element that prevented the insertion.
• 7,8) (none)
• 9) An object of insert_return_type with the members initialized as follows:
  • If nh is empty, inserted is false, position is end(), and node is empty.
  • Otherwise if the insertion took place, inserted is true, position points to the inserted element, and node is empty.
  • If the insertion failed, inserted is false, node has the previous value of nh, and position points to an element with a key equivalent to nh.key().
• 10) End iterator if nh was empty, iterator pointing to the inserted element if insertion took place, and iterator pointing to an element with a key equivalent to nh.key() if it failed.

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😭 EXCEPTIONS 😭

• 1-6) If an exception is thrown by any operation, the insertion has no effect.

This section is incomplete Reason: cases 7-10

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⚡ COMPLEXITY ⚡

• 1-3) Logarithmic in the size of the container, O(log(size())).
• 4-6) Amortized constant if the insertion happens in the position just after (until C++11) before (since C++11) pos, logarithmic in the size of the container otherwise.
• 7,8) O(N·log(size() + N)), where N is the number of elements to insert.
• 9) Logarithmic in the size of the container, O(log(size())).
• 10) Amortized constant if the insertion happens in the position just before pos, logarithmic in the size of the container otherwise.

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🗒️ NOTES 🗒️

The hinted insert (4-6) does not return a boolean in order to be signature-compatible with positional insert on sequential containers, such as std::vector::insert. This makes it possible to create generic inserters such as std::inserter. One way to check success of a hinted insert is to compare size() before and after.

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🌈 EXAMPLE 🌈

// Run this code

#include <iomanip>
#include <iostream>
#include <map>
#include <string>
using namespace std::literals;

template<typename It>
void print_insertion_status(It it, bool success)
{
    std::cout << "Insertion of " << it->first
              << (success ? " succeeded\n" : " failed\n");
}

int main()
{
    std::map<std::string, float> heights;

    // Overload 3: insert from rvalue reference
    const auto [it_hinata, success] = heights.insert({"Hinata"s, 162.8});
    print_insertion_status(it_hinata, success);

    {
        // Overload 1: insert from lvalue reference
        const auto [it, success2] = heights.insert(*it_hinata);
        print_insertion_status(it, success2);
    }
    {
        // Overload 2: insert via forwarding to emplace
        const auto [it, success] = heights.insert(std::pair{"Kageyama", 180.6});
        print_insertion_status(it, success);
    }
    {
        // Overload 6: insert from rvalue reference with positional hint
        const std::size_t n = std::size(heights);
        const auto it = heights.insert(it_hinata, {"Azumane"s, 184.7});
        print_insertion_status(it, std::size(heights) != n);
    }
    {
        // Overload 4: insert from lvalue reference with positional hint
        const std::size_t n = std::size(heights);
        const auto it = heights.insert(it_hinata, *it_hinata);
        print_insertion_status(it, std::size(heights) != n);
    }
    {
        // Overload 5: insert via forwarding to emplace with positional hint
        const std::size_t n = std::size(heights);
        const auto it = heights.insert(it_hinata, std::pair{"Tsukishima", 188.3});
        print_insertion_status(it, std::size(heights) != n);
    }

    auto node_hinata = heights.extract(it_hinata);
    std::map<std::string, float> heights2;

    // Overload 7: insert from iterator range
    heights2.insert(std::begin(heights), std::end(heights));

    // Overload 8: insert from initializer_list
    heights2.insert({{"Kozume"s, 169.2}, {"Kuroo", 187.7}});

    // Overload 9: insert node
    const auto status = heights2.insert(std::move(node_hinata));
    print_insertion_status(status.position, status.inserted);

    node_hinata = heights2.extract(status.position);
    {
        // Overload 10: insert node with positional hint
        const std::size_t n = std::size(heights2);
        const auto it = heights2.insert(std::begin(heights2), std::move(node_hinata));
        print_insertion_status(it, std::size(heights2) != n);
    }

    // Print resulting map
    std::cout << std::left << '\n';
    for (const auto& [name, height] : heights2)
        std::cout << std::setw(10) << name << " | " << height << "cm\n";
}

🍓 OUTPUT 🍓

Insertion of Hinata succeeded
Insertion of Hinata failed
Insertion of Kageyama succeeded
Insertion of Azumane succeeded
Insertion of Hinata failed
Insertion of Tsukishima succeeded
Insertion of Hinata succeeded
Insertion of Hinata succeeded

Azumane    | 184.7cm
Hinata     | 162.8cm
Kageyama   | 180.6cm
Kozume     | 169.2cm
Kuroo      | 187.7cm
Tsukishima | 188.3cm

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🐛 DEFECT REPORTS 🐛

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

• LWG 233 — Applied to: C++98. Behavior as published: pos was just a hint, it could be totally ignored. Correct behavior: the insertion is required to be as close as possible to the position just prior to pos.
• LWG 264 — Applied to: C++98. Behavior as published: the complexity of overload (7) was required to be linear if the range [first, last) is sorted according to Compare. Correct behavior: removed the linear requirement in this special case.
• LWG 316 — Applied to: C++98. Behavior as published: in the return value of overload (1), it was not specified which bool value indicates a successful insertion. Correct behavior: success is indicated by true.
• LWG 2005 — Applied to: C++11. Behavior as published: overloads (2,5) were poorly described. Correct behavior: improved the description.

✨ SEE ALSO ✨

okay bestie these are the lil sibling functions you'll wanna meet too, they're all public member functions except the last one which is just a vibe:

• 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_or_assign — inserts an element or assigns to the current element if the key already exists (C++17) (public member function)
• inserter — creates a std::insert_iterator of type inferred from the argument (function template)

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🫶 Category

• Todo with reason

no cap, that's literally just the category tag the page is filed under — an empty todo note with a reason attached, bestie. don't read into it 💜

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and that's the whole page, you ate that 💅✨ go forth and insert responsibly, mwah 🎀

source/version: http://cppreference.com · dated 2024.06.10 · page: std::map::insert(3) — periodt, she's iconic, bookmark her 🌈 🦋