NAME¶

std::set::set - std::set::set

Synopsis¶

set();

explicit set( const Compare& comp,

const Allocator& alloc = Allocator() );
explicit set( const Allocator& alloc ); (1) (since
C++11)
template< class InputIt >

set( InputIt first, InputIt last,
const Compare& comp = Compare(),

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

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

: set(first, last, Compare(), alloc) {}
set( const set& other ); (3)
set( const set& other, const Allocator& alloc ); (2) (3) (since
C++11)
set( set&& other ); (4) (since
C++11)
set( set&& other, const Allocator& alloc ); (4) (since
C++11)
set( std::initializer_list<value_type> init,
(since
const Compare& comp = Compare(), C++11)
(5)
const Allocator& alloc = Allocator() );
set( std::initializer_list<value_type> init, const (since
Allocator& alloc ) C++14)
: set(init, Compare(), alloc) {}

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

1) Default constructor. Constructs empty container.
2) Range constructor. 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).
3) 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)

4) 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.

5) 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).

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) Constant
2) N log(N) where N = std::distance(first, last) in general, linear in N if the
range is already sorted by value_comp().
3) Linear in size of other
4) Constant. If alloc is given and alloc != other.get_allocator(), then linear.
5) 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 (4)), 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 <iostream>
#include <string>
#include <set>
#include <cmath>

struct Point { double x, y; };
struct PointCmp {
bool operator()(const Point& lhs, const Point& rhs) const {
return std::hypot(lhs.x, lhs.y) < std::hypot(rhs.x, rhs.y);
}
};

int main()
{
// (1) Default constructor
std::set<std::string> a;
a.insert("cat");
a.insert("dog");
a.insert("horse");
for(auto& str: a) std::cout << str << ' ';
std::cout << '\n';

// (2) Iterator constructor
std::set<std::string> b(a.find("dog"), a.end());
for(auto& str: b) std::cout << str << ' ';
std::cout << '\n';

// (3) Copy constructor
std::set<std::string> c(a);
c.insert("another horse");
for(auto& str: c) std::cout << str << ' ';
std::cout << '\n';

// (4) Move constructor
std::set<std::string> d(std::move(a));
for(auto& str: d) std::cout << str << ' ';
std::cout << '\n';
std::cout << "moved-from set is ";
for(auto& str: a) std::cout << str << ' ';
std::cout << '\n';

// (5) Initializer list constructor
std::set<std::string> e {"one", "two", "three", "five", "eight"};
for(auto& str: e) std::cout << str << ' ';
std::cout << '\n';

// custom comparison
std::set<Point, PointCmp> z = {{2, 5}, {3, 4}, {1, 1}};
z.insert({1, -1}); // this fails because the magnitude of 1,-1 equals 1,1
for(auto& p: z) std::cout << '(' << p.x << ',' << p.y << ") ";
std::cout << '\n';
}

Output:¶

cat dog horse
dog horse
another horse cat dog horse
cat dog horse
moved-from set is
eight five one three two
(1,1) (3,4) (2,5)

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)