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

 

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std::deque

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deque::deque deque::~deque deque::operator= deque::assign deque::assign_range (C++23)   deque::get_allocator Element access deque::at deque::operator[] deque::front deque::back Iterators deque::begindeque::cbegin (C++11) deque::enddeque::cend (C++11) deque::rbegindeque::crbegin (C++11) deque::renddeque::crend (C++11)	Capacity deque::empty deque::size deque::max_size deque::shrink_to_fit (DR*) Modifiers deque::clear deque::insert deque::insert_range (C++23) deque::emplace deque::erase deque::push_front deque::emplace_front (C++11) deque::prepend_range (C++23) deque::pop_front deque::push_back deque::emplace_back (C++11) deque::append_range (C++23) deque::pop_back deque::resize deque::swap
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operator==operator<=> (C++20) swap(std::deque) erase(std::deque)erase_if(std::deque) (C++20)(C++20)	operator!=operator<operator>operator<=operator>= (until C++20)(until C++20)(until C++20)(until C++20)(until C++20)
Deduction guides(C++17)

 

void resize( size_type count );	(1)	(constexpr since C++26)
void resize( size_type count, const value_type& value );	(2)	(constexpr since C++26)

Resizes the container to contain count elements:

• If count is equal to the current size, does nothing.
• If the current size is greater than count, the container is reduced to its first count elements.
• If the current size is less than count, then:

1) Additional copies of T()(until C++11)default-inserted elements(since C++11) are appended.

2) Additional copies of value are appended.

Parameters

count	-	new size of the container
value	-	the value to initialize the new elements with
Type requirements
- If the following condition is satisfied, the behavior is undefined: 1) T is not DefaultInsertable or MoveInsertable into deque. 2) T is not CopyInsertable into deque. (since C++11)

Complexity

Linear in the difference between the current size and count.

Notes

If value-initialization in overload (1) is undesirable, for example, if the elements are of non-class type and zeroing out is not needed, it can be avoided by providing a custom Allocator::construct.

Example

Run this code

#include <deque>
#include <iostream>
 
void print(auto rem, const std::deque<int>& c)
{
    for (std::cout << rem; const int el : c)
        std::cout << el << ' ';
    std::cout << '\n';
}
 
int main()
{
    std::deque<int> c = {1, 2, 3};
    print("The deque holds: ", c);
 
    c.resize(5);
    print("After resize up to 5: ", c);
 
    c.resize(2);
    print("After resize down to 2: ", c);
 
    c.resize(6, 4);
    print("After resize up to 6 (initializer = 4): ", c);
}

Output:

The deque holds: 1 2 3
After resize up to 5: 1 2 3 0 0
After resize down to 2: 1 2
After resize up to 6 (initializer = 4): 1 2 4 4 4 4

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 679	C++98	resize() passed value by value	passes by const reference
LWG 1418	C++98	the behavior of resize(size()) was not specified	specified
LWG 2033	C++11	1. elements were removed by using erase() 2. T was not required to be MoveInsertable	1. uses pop_back() 2. required
LWG 2066	C++11	overload (1) did not have the exception safety guarantee of overload (2)	added
LWG 2160	C++11	elements were removed by using pop_back() due to the resolution of LWG 2033	does not specify the method of removing elements

See also

max_size	returns the maximum possible number of elements (public member function)
size	returns the number of elements (public member function)
empty	checks whether the container is empty (public member function)