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

void assign( size_type count, const T& value );
 (1) 
template< class InputIt >
void assign( InputIt first, InputIt last );
 (2) 
void assign( std::initializer_list<T> ilist );
 (3) (since C++11)

Replaces the contents of the container.

1) Replaces the contents with count copies of value value.
2) Replaces the contents with copies of those in the range [first, last). The behavior is undefined if either argument is an iterator into *this.

This overload has the same effect as overload (1) if InputIt is an integral type.

 (until C++11)

This overload participates in overload resolution only if InputIt satisfies LegacyInputIterator.

 (since C++11)
3) Replaces the contents with the elements from the initializer list ilist.

All iterators, pointers and references to the elements of the container are invalidated. The past-the-end iterator is also invalidated.

Parameters

count - the new size of the container
value - the value to initialize elements of the container with
first, last - the range to copy the elements from
ilist - initializer list to copy the values from

Complexity

1) Linear in count.
2) Linear in distance between first and last.
3) Linear in ilist.size().

Example

The following code uses assign to add several characters to a std::deque<char>:

#include <deque>
#include <iostream>
#include <string>
 
int main()
{
    std::deque<char> characters;
 
    auto print_deque = [&]()
    {
        for (char c : characters)
            std::cout << c << ' ';
        std::cout << '\n';
    };
 
    characters.assign(5, 'a');
    print_deque();
 
    const std::string extra(6, 'b');
    characters.assign(extra.begin(), extra.end());
    print_deque();
 
    characters.assign({'C', '+', '+', '1', '1'});
    print_deque();
}

Output:

a a a a a
b b b b b b
C + + 1 1

See also

constructs the deque
(public member function)

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https://en.cppreference.com/w/cpp/container/deque/assign