std::map::map(3)

💅 std::map::map(3) — C++ Standard Library 💅

okay bestie, this is the std::map constructor page and it is lowkey iconic. the manual name is literally std::map::map — yes, same name, no notes 💀. this is part 1, so buckle up 🎀.

🌸 NAME 🌸

std::map::map — std::map::map (that's the whole name, and honestly it ate)

✨ SYNOPSIS ✨

here's the whole overload lineup, all 13 of them, with their standard-version stickers. the labels are doing their own thing in places, but every single one is here 🫶:

map();
(until C++11)
map()
(since C++11)
    : map(Compare()) {}
explicit map( const Compare& comp,                                (2)
              const Allocator& alloc = Allocator() );
explicit map( const Allocator& alloc );                           (3)
(since C++11)
template< class InputIt >

map( InputIt first, InputIt last,                                 (4)
     const Compare& comp = Compare(),

     const Allocator& alloc = Allocator() );
template< class InputIt >

map( InputIt first, InputIt last,                                 (5)
(since C++14)
     const Allocator& alloc )

    : map(first, last, Compare(), alloc) {}
map( const map& other );                                          (6)
map( const map& other, const Allocator& alloc );                  (7)
(since C++11)
map( map&& other );                                               (8)
(since C++11)
map( map&& other, const Allocator& alloc );                       (9)
(since C++11)
map( std::initializer_list<value_type> init,                  (1)

     const Compare& comp = Compare(),
(10) (since C++11)

     const Allocator& alloc = Allocator() );
map( std::initializer_list<value_type> init,

     const Allocator& alloc )
(11) (since C++14)

    : map(init, Compare(), alloc) {}
template< container-compatible-range<value_type> R >

map( std::from_range_t, R&& rg,
(12) (since C++23)
     const Compare& comp = Compare(),

     const Allocator& alloc = Allocator() );
template< container-compatible-range<value_type> R >

map( std::from_range_t, R&& rg,
     const Allocator& alloc )
(13) (since C++23)

    : map(std::from_range, std::forward<R>(rg), Compare(),
alloc) {}

okay so what does this actually do? Constructs new container from a variety of data sources and optionally using user supplied allocator alloc or comparison function object comp. that's the whole vibe, periodt.

1. 1–3) — Constructs an empty container.
2. 4,5) — Constructs the container with the contents of the range [first, last). If multiple elements in the range have keys that compare equivalent, it is unspecified which element is inserted (pending LWG2844). If [first, last) is not a valid range, the behavior is undefined. 🚨
3. 6,7) — Copy constructor. Constructs the container with the copy of the contents of other.

If alloc is not provided, allocator is obtained by calling std::allocator_traits<allocator_type>::select_on_container_copy_construction(other.get_allocator()). (since C++11)

During class template argument deduction, only the first argument contributes to the deduction of the container's Allocator template parameter. (since C++23)

4. 8,9) — Move constructor. Constructs the container with the contents of other using move semantics. If alloc is not provided, allocator is obtained by move-construction from the allocator belonging to other.

During class template argument deduction, only the first argument contributes to the deduction of the container's Allocator template parameter. (since C++23)

5. 10,11) — Initializer-list constructor. Constructs the container with the contents of the initializer list init. If multiple elements in the range have keys that compare equivalent, it is unspecified which element is inserted (pending LWG2844).
6. 12,13) — Constructs the container with the contents of rg. If multiple elements in the range have keys that compare equivalent, it is unspecified which element is inserted (pending LWG2844).

🌈 PARAMETERS 🌈

• alloc — allocator to use for all memory allocations of this container.
• comp — comparison function object to use for all comparisons of keys.
• first, last — the range to copy the elements from.
• other — another container to be used as source to initialize the elements of the container with.
• init — initializer list to initialize the elements of the container with.
• rg — a container compatible range, that is, an input_range whose elements are convertible to value_type.

🦋 TYPE REQUIREMENTS 🦋

• InputIt must meet the requirements of LegacyInputIterator.
• Compare must meet the requirements of Compare.
• Allocator must meet the requirements of Allocator.

🔥 COMPLEXITY 🔥

• 1–3) — Constant.
• 4,5) — N·log(N) where N is std::distance(first, last) in general, linear in N if [first, last) is already sorted by value_comp().
• 6,7) — Linear in size of other.
• 8,9) — Constant. If alloc is given and alloc != other.get_allocator(), then linear.
• 10,11) — N·log(N) where N is init.size() in general, linear in N if init is already sorted by value_comp().
• 12,13) — N·log(N) where N is ranges::distance(rg) in general, linear in N if rg is already sorted by value_comp().

💀 EXCEPTIONS 💀

Calls to Allocator::allocate may throw.

🧚 NOTES 🧚

After container move construction (overload (8,9)), 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.

Although not formally required until C++23, some implementations has already put the template parameter Allocator into non-deduced contexts in earlier modes.

Feature-test macro situation:

• __cpp_lib_containers_ranges — value 202202L, standard (C++23), feature: Ranges-aware construction and insertion; overloads (12,13).

🍓 EXAMPLE 🍓

// Run this code

        #include <iomanip>
        #include <iostream>
        #include <map>
        #include <string>

        template<typename Key, typename Value, typename Cmp>
        std::ostream& operator<<(std::ostream& os, std::map<Key, Value, Cmp>
       const& map)
        {
            os << "{ ";
            for (auto comma{map.size()}; auto const& p : map)
                os << '\'' << p.first << "' is " << p.second << (--comma ? ", "
       : " ");
            return os << "}\n";
        }

        struct Point
        {
            double x, y;

            friend std::ostream& operator<<(std::ostream& os, Point pt)
            {
                return os << '(' << pt.x << ", " << pt.y << ')';
            }
        };

        struct PointCmp
        {
            bool operator()(const Point& lhs, const Point& rhs) const
            {
                return lhs.x < rhs.x; // NB: y is intentionally ignored
            }
        };

        int main()
        {
            // (1) Default constructor
            std::map<std::string, int> map1;
            map1["something"] = 69;
            map1["anything"] = 199;
            map1["that thing"] = 50;
            std::cout << "map1 = " << map1;

            // (4) Range constructor
            std::map<std::string, int> iter(map1.find("anything"), map1.end());
            std::cout << "\niter = " << iter;
            std::cout << "map1 = " << map1;

            // (6) Copy constructor
            std::map<std::string, int> copied(map1);
            std::cout << "\ncopied = " << copied;
            std::cout << "map1 = " << map1;

            // (8) Move constructor
            std::map<std::string, int> moved{std::move(map1)};
            std::cout << "\nmoved = " << moved;
            std::cout << "map1 = " << map1;

            // (10) Initializer list constructor
            const std::map<std::string, int> init
            {
                {"this", 100},
                {"can", 100},
                {"be", 100},
                {"const", 100}
            };
            std::cout << "\ninit = " << init;

            std::cout << "\nCustom Key class option 1:\n";
            // Use a comparison struct
            std::map<Point, double, PointCmp> mag =
            {
                {{5, -12}, 13},
                {{3, 4}, 5},
                {{-8, -15}, 17}
            };
            std::cout << "mag = " << mag << '\n';

            std::cout << "Custom Key class option 2:\n";
            // Use a comparison lambda
            // This lambda sorts points according to their magnitudes, where
            // these magnitudes are taken from the local variable mag.
            auto cmpLambda = [&mag](const Point& lhs, const Point& rhs)
            {
                return mag[lhs] < mag[rhs];
            };

            // You could also use a lambda that is not dependent on local
       variables, like this:
            // auto cmpLambda = [](const Point& lhs, const Point& rhs){ return
       lhs.y < rhs.y; };
            std::map<Point, double, decltype(cmpLambda)> magy(cmpLambda);

            // Various ways of inserting elements:
            magy.insert(std::pair<Point, double>({5, -12}, 13));
            magy.insert({{3, 4}, 5});
            magy.insert({Point{-8.0, -15.0}, 17});
            std::cout << "magy = " << magy << '\n';

            std::cout << "Construction from a range:\n";
            using PS = std::pair<const std::string, int>;
            const auto rg = {PS{"one", 1}, {"one", 101}, {"two", 2}, {"three",
       3}};
        #if __cpp_lib_containers_ranges
            std::map<std::string, int> nums(std::from_range, rg); // overload
       (12)
        #else
            std::map<std::string, int> nums(rg.begin(), rg.end()); // fallback
       to (4)
        #endif
            std::cout << "nums = " << nums << '\n';
        }

and the output is literally this, bestie — exact vibes, no edits:

Output:
        map1 = { 'anything' is 199, 'something' is 69, 'that thing' is 50 }

        iter = { 'anything' is 199, 'something' is 69, 'that thing' is 50 }
        map1 = { 'anything' is 199, 'something' is 69, 'that thing' is 50 }

        copied = { 'anything' is 199, 'something' is 69, 'that thing' is 50 }
        map1 = { 'anything' is 199, 'something' is 69, 'that thing' is 50 }

        moved = { 'anything' is 199, 'something' is 69, 'that thing' is 50 }
        map1 = { }

        init = { 'be' is 100, 'can' is 100, 'const' is 100, 'this' is 100 }

        Custom Key class option 1:
        mag = { '(-8, -15)' is 17, '(3, 4)' is 5, '(5, -12)' is 13 }

        Custom Key class option 2:
        magy = { '(3, 4)' is 5, '(5, -12)' is 13, '(-8, -15)' is 17 }

        Construction from a range:
        nums = { 'one' is 1, 'three' is 3, 'two' is 2 }

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💀 Defect reports 💀

okay so here's the lore: the following behavior-changing defect reports were applied retroactively to previously published C++ standards. yes, retroactively. the committee really said “timeline? never met her” 💅

• LWG 2076 — applied to C++11. Behavior as published: overload (4) conditionally required Keyand T to be CopyInsertable into *this. Correct behavior: not required.
• LWG 2193 — applied to C++11. Behavior as published: the default constructor was explicit. Correct behavior: made non-explicit.

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👀 See also 👀

• operator= — assigns values to the container (public member function).

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catch u later bestie 🫶 this one’s std::map::map(3), source/version is cppreference.com, and the date is 2024.06.10 — iconic, periodt. 💖