# std::vector<T,Allocator>::insert

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< cpp‎ | container‎ | vector

iterator insert( const_iterator pos, const T& value ); | (1) | (constexpr since C++20)
iterator insert( const_iterator pos, T&& value ); | (2) | (since C++11) (constexpr since C++20)
iterator insert( const_iterator pos, size_type count, const T& value ); | (3) | (constexpr since C++20)
template< class InputIt > iterator insert( const_iterator pos, InputIt first, InputIt last ); | (4) | (constexpr since C++20)
iterator insert( const_iterator pos, std::initializer_list <T> ilist ); | (5) | (since C++11) (constexpr since C++20)

Inserts elements at the specified location in the container.

1) Inserts a copy of value before pos.
2) Inserts value before pos, possibly using move semantics.
3) Inserts count copies of the value before pos.
4) Inserts elements from range `[`first`,` last`)` before pos.

This overload has the same effect as overload (3) if `InputIt` is an integral type. | (until C++11)
This overload participates in overload resolution only if `InputIt` qualifies as LegacyInputIterator, to avoid ambiguity with the overload (3). | (since C++11)

If first and last are iterators into *this, the behavior is undefined.
5) Inserts elements from initializer list ilist before pos.

If after the operation the new `size()` is greater than old `capacity()` a reallocation takes place, in which case all iterators (including the `end()` iterator) and all references to the elements are invalidated. Otherwise, only the iterators and references before the insertion point remain valid.

### Parameters

pos | - | iterator before which the content will be inserted (pos may be the `end()` iterator)
value | - | element value to insert
count | - | number of elements to insert
first, last | - | the range of elements to insert, cannot be iterators into container for which insert is called
ilist | - | std::initializer_list to insert the values from
Type requirements
- `T` must meet the requirements of CopyAssignable and CopyInsertable in order to use overload (1).
- `T` must meet the requirements of MoveAssignable and MoveInsertable in order to use overload (2).
- `T` must meet the requirements of CopyAssignable and CopyInsertable in order to use overload (3).
- `T` must meet the requirements of EmplaceConstructible in order to use overloads (4,5).
- `T` must meet the requirements of MoveAssignable and MoveInsertable in order to use overload (4). required only if `InputIt` satisfies LegacyInputIterator but not LegacyForwardIterator. (until C++17)
- `T` must meet the requirements of Swappable, MoveAssignable, MoveConstructible and MoveInsertable in order to use overloads (4,5). (since C++17)

### Return value

1,2) Iterator pointing to the inserted value.
3) Iterator pointing to the first element inserted, or pos if count == 0.
4) Iterator pointing to the first element inserted, or pos if first == last.
5) Iterator pointing to the first element inserted, or pos if ilist is empty.

### Complexity

1,2) Constant plus linear in the distance between pos and end of the container.
3) Linear in count plus linear in the distance between pos and end of the container.
4) Linear in std::distance (first, last) plus linear in the distance between pos and end of the container.
5) Linear in ilist.size() plus linear in the distance between pos and end of the container.

### Exceptions

If an exception is thrown other than by

- the copy constructor of `T`,

- the move constructor of `T`,

(since C++11)

- the copy assignment operator of `T`,

- the move assignment operator of `T`,

(since C++11)

- any `InputIt` operation,

these functions have no effect (strong exception safety guarantee).

If an exception is thrown when inserting a single element at the end, and `T` is CopyInsertable into *this or std::is_nothrow_move_constructible <T>::value is true, this function has no effect (strong exception guarantee). Otherwise, if an exception is thrown by the move constructor of a non-CopyInsertable `T`, the effects are unspecified. | (since C++11)

### Example

Run this code

    #include <iostream>
    #include <iterator>
    #include <string_view>
    #include <vector>

    namespace stq {
    void println(std::string_view rem, const std::vector<int>& container)
    {
        std::cout << rem.substr(0, rem.size() - 2) << '[';
        bool first{true};
        for (const int x : container)
            std::cout << (first ? first = false, "" : ", ") << x;
        std::cout << "]\n";
    }
    }

    int main()
    {
        std::vector<int> c1(3, 100);
        stq::println("1. {}", c1);

        auto pos = c1.begin();
        pos = c1.insert(pos, 200); // overload (1)
        stq::println("2. {}", c1);

        c1.insert(pos, 2, 300); // overload (3)
        stq::println("3. {}", c1);

        // pos no longer valid, get a new one:
        pos = c1.begin();

        std::vector<int> c2(2, 400);
        c1.insert(std::next(pos, 2), c2.begin(), c2.end()); // overload (4)
        stq::println("4. {}", c1);

        int arr[] = {501, 502, 503};
        c1.insert(c1.begin(), arr, arr + std::size(arr)); // overload (4)
        stq::println("5. {}", c1);

        c1.insert(c1.end(), {601, 602, 603}); // overload (5)
        stq::println("6. {}", c1);
    }

Output:

    1. [100, 100, 100]
    2. [200, 100, 100, 100]
    3. [300, 300, 200, 100, 100, 100]
    4. [300, 300, 400, 400, 200, 100, 100, 100]
    5. [501, 502, 503, 300, 300, 400, 400, 200, 100, 100, 100]
    6. [501, 502, 503, 300, 300, 400, 400, 200, 100, 100, 100, 601, 602, 603]

### 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 149 | C++98 | overloads (3) and (4) returned nothing | returns an iterator
LWG 247 | C++98 | the complexity was only specified for overload (3) | specified for all overloads
LWG 406 | C++98 | the strong exception guarantee also applied if the exception is thrown by an `InputIt` operation | no guarantee in this case

### See also

emplace (C++11) | constructs element in-place (public member function)
push_back | adds an element to the end (public member function)
inserter | creates a std::insert_iterator of type inferred from the argument (function template)

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