# std::deque<T,Allocator>::deque

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< cpp‎ | container‎ | deque

deque(); | (1) |
explicit deque( const Allocator& alloc ); | (2) |
| (3) |
explicit deque( size_type count, const T& value = T(), const Allocator& alloc = Allocator() ); |  | (until C++11)
deque( size_type count, const T& value, const Allocator& alloc = Allocator() ); |  | (since C++11)
| (4) |
explicit deque( size_type count ); |  | (since C++11) (until C++14)
explicit deque( size_type count, const Allocator& alloc = Allocator() ); |  | (since C++14)
template< class InputIt > deque( InputIt first, InputIt last, const Allocator& alloc = Allocator() ); | (5) |
deque( const deque& other ); | (6) |
deque( const deque& other, const Allocator& alloc ); | (7) | (since C++11)
deque( deque&& other ); | (8) | (since C++11)
deque( deque&& other, const Allocator& alloc ); | (9) | (since C++11)
deque( std::initializer_list<T> init, const Allocator& alloc = Allocator() ); | (10) | (since C++11)
template< container-compatible-range<T> R > deque( std::from_range_t, R&& rg, const Allocator& alloc = Allocator() ); | (11) | (since C++23)

Constructs a new container from a variety of data sources, optionally using a user supplied allocator alloc.

1) Default constructor. Constructs an empty container with a default-constructed allocator.
2) Constructs an empty container with the given allocator alloc.
3) Constructs the container with count copies of elements with value value.
4) Constructs the container with count default-inserted instances of `T`. No copies are made.
5) Constructs the container with the contents of the range `[`first`,` last`)`.

This constructor has the same effect as deque(static_cast<size_type>(first), static_cast<value_type>(last), a) if `InputIt` is an integral type. | (until C++11)
This overload participates in overload resolution only if `InputIt` satisfies LegacyInputIterator, to avoid ambiguity with the overload (3). | (since C++11)

6) Copy constructor. Constructs the container with the copy of the contents of other.

The allocator is obtained as if by calling std::allocator_traits<allocator_type>::select_on_container_copy_construction( other.get_allocator()). | (since C++11)

7) Constructs the container with the copy of the contents of other, using alloc as the allocator.

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

8) Move constructor. Constructs the container with the contents of other using move semantics. Allocator is obtained by move-construction from the allocator belonging to other.
9) Allocator-extended move constructor. Using alloc as the allocator for the new container, moving the contents from other; if alloc != other.get_allocator(), this results in an element-wise move.

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

10) Constructs the container with the contents of the initializer list init.
11) Constructs the container with the contents of the range rg.

### Parameters

alloc | - | allocator to use for all memory allocations of this container
count | - | the size of the container
value | - | the value to initialize elements of the container with
first, last | - | the range `[`first`,` last`)` 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 `T`

### Complexity

1,2) Constant.
3,4) Linear in count.
5) Linear in distance between first and last.
6,7) Linear in size of other.
8) Constant.
9) Linear if alloc != other.get_allocator(), otherwise constant.
10) Linear in size of init.
11) Linear in ranges::distance(rg).

### Exceptions

Calls to Allocator::allocate may throw.

### Notes

After container move construction (overload (8)), 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.rev.reqmts]/17, and a more direct guarantee is under consideration via LWG issue 2321.

Feature-test macro | Value | Std | Comment
--- | --- | --- | ---
`__cpp_lib_containers_ranges` | 202202L | (C++23) | Ranges-aware construction and insertion; overload (11)

### Example

Run this code

    #include <deque>
    #include <iostream>
    #include <string>

    template<typename T>
    std::ostream& operator<<(std::ostream& s, const std::deque<T>& v)
    {
        s.put('{');
        char comma[3]{'\0', ' ', '\0'};
        for (const auto& e : v)
        {
            s << comma << e;
            comma[0] = ',';
        }
        return s << "}\n";
    }

    int main()
    {
        // C++11 initializer list syntax:
        std::deque<std::string> words1{"the", "frogurt", "is", "also", "cursed"};
        std::cout << "1: " << words1;

        // words2 == words1
        std::deque<std::string> words2(words1.begin(), words1.end());
        std::cout << "2: " << words2;

        // words3 == words1
        std::deque<std::string> words3(words1);
        std::cout << "3: " << words3;

        // words4 is {"Mo", "Mo", "Mo", "Mo", "Mo"}
        std::deque<std::string> words4(5, "Mo");
        std::cout << "4: " << words4;
    }

Output:

    1: {the, frogurt, is, also, cursed}
    2: {the, frogurt, is, also, cursed}
    3: {the, frogurt, is, also, cursed}
    4: {Mo, Mo, Mo, Mo, Mo}

### 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 144 | C++98 | the complexity requirement of overload (5) was the same as that of the corresponding overload of std::vector | changed to linear complexity
LWG 237 | C++98 | the complexity requirement of overload (5) was linear in first - last | changed to linear in std::distance(first, last)
LWG 868 | C++98 | for overload (4), the elements in the container were default constructed | they are value-initialized
LWG 2193 | C++11 | the default constructor is explicit | made non-explicit

### See also

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

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