NAME¶

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

Synopsis¶

map(); (1)
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( 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,

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

const Allocator& alloc = Allocator() );
map( std::initializer_list<value_type> init, (11) (since C++14)
const Allocator& );

Constructs new container from a variety of data sources and optionally using user
supplied allocator alloc or comparison function object comp.

1-3) Constructs an empty container.
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).
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 (since
std::allocator_traits<allocator_type>::select_on_container_copy_construction( C++11)
other.get_allocator()).
The template parameter Allocator is only deduced from the first argument (since
while used in class template argument deduction. C++23)

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.

The template parameter Allocator is only deduced from the first (since C++23)
argument while used in class template argument deduction.

10-11) 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).

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

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 = std::distance(first, last) in general, linear in N if the
range 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 = init.size() in general, linear in N if init 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.requirements.general]/12, and a more direct guarantee is
under consideration via LWG 2321.

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

Example¶

// Run this code

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

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

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

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 }
};

for(auto p : mag)
std::cout << "The magnitude of (" << p.first.x
<< ", " << p.first.y << ") is "
<< p.second << '\n';

std::cout << "\nCustom Key class option 2:\n";
// Use a comparison lambda
// This lambda sorts points according to their magnitudes, where note that
// 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});

for(auto p : magy)
std::cout << "The magnitude of (" << p.first.x
<< ", " << p.first.y << ") is "
<< p.second << '\n';
}

Output:¶

map1 = { (anything:199) (something:69) (that thing:50) }

iter = { (anything:199) (something:69) (that thing:50) }
map1 = { (anything:199) (something:69) (that thing:50) }

copied = { (anything:199) (something:69) (that thing:50) }
map1 = { (anything:199) (something:69) (that thing:50) }

moved = { (anything:199) (something:69) (that thing:50) }
map1 = { }

init = { (be:100) (can:100) (const:100) (this:100) }

Custom Key class option 1:
The magnitude of (-8, -15) is 17
The magnitude of (3, 4) is 5
The magnitude of (5, -12) is 13

Custom Key class option 2:
The magnitude of (3, 4) is 5
The magnitude of (5, -12) is 13
The magnitude of (-8, -15) is 17

Defect reports

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

DR Applied to Behavior as published Correct behavior
LWG 2193 C++11 the default constructor is explicit made non-explicit

See also¶

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