# std::set<Key,Compare,Allocator>::set

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< cpp‎ | container‎ | set

set(); explicit set( const Compare& comp, const Allocator& alloc = Allocator() ); |  |
explicit set( const Allocator& alloc ); | (1) | (since C++11)
| (2) |
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& alloc) : set(first, last, Compare(), alloc) {} |  | (since C++14)
set( const set& other ); | (3) |
set( const set& other, const Allocator& alloc ); | (3) | (since C++11)
set( set&& other ); | (4) | (since C++11)
set( set&& other, const Allocator& alloc ); | (4) | (since C++11)
| (5) |
set( std::initializer_list<value_type> init, const Compare& comp = Compare(), const Allocator& alloc = Allocator() ); |  | (since C++11)
set( std::initializer_list<value_type> init, const Allocator& alloc ) : set(init, Compare(), alloc) {} |  | (since C++14)

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 std::allocator_traits<allocator_type>::select_on_container_copy_construction(other.get_allocator()).
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`.
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 §23.2.1[container.requirements.general]/12, and a more direct guarantee is under consideration via LWG 2321.

### 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)

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