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

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

Inserts elements at the specified location in the container.

1-2) inserts value before pos

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)

The behavior is undefined if first and last are iterators into *this.

5) inserts elements from initializer list ilist before pos.

All iterators, including the past-the-end iterator, are invalidated. References are invalidated too, unless pos == begin() or pos == end(), in which case they are not invalidated.

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, can't be iterators into container for which insert is called
ilist	-	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 overload (4,5).
-`T` must meet the requirements of Swappable, MoveAssignable, MoveConstructible and MoveInsertable in order to use overload (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 lesser of the distances between pos and either of the ends of the container.

3) Linear in count plus linear in the lesser of the distances between pos and either of the ends of the container.

4) Linear in std::distance(first, last) plus linear in the lesser of the distances between pos and either of the ends of the container.

5) Linear in ilist.size() plus linear in the lesser of the distances between pos and either of the ends of the container.

Exceptions

If an exception is thrown when inserting a single element at either end, this function has no effect (strong exception guarantee).

Example

#include <iostream>
#include <iterator>
#include <deque>
 
void print(int id, const std::deque<int>& container)
{
    std::cout << id << ". ";
    for (const int x: container) {
         std::cout << x << ' ';
    }
    std::cout << '\n';
}
 
int main ()
{
    std::deque<int> c1(3, 100);
    print(1, c1);
 
    auto it = c1.begin();
    it = c1.insert(it, 200);
    print(2, c1);
 
    c1.insert(it, 2, 300);
    print(3, c1);
 
    // reset `it` to the begin:
    it = c1.begin();
 
    std::deque<int> c2(2, 400);
    c1.insert(std::next(it, 2), c2.begin(), c2.end());
    print(4, c1);
 
    int arr[] = { 501,502,503 };
    c1.insert(c1.begin(), arr, arr + std::size(arr));
    print(5, c1);
 
    c1.insert(c1.end(), { 601,602,603 } );
    print(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