std::vector::resize

C++

Language

Headers

Freestanding and hosted implementations

Named requirements

Language support library

Concepts library (C++20)

Diagnostics library

Utilities library

Strings library

Containers library

Algorithms library

Iterators library

Numerics library

Input/output library

Localizations library

Regular expressions library (C++11)

Atomic operations library (C++11)

Thread support library (C++11)

Filesystem library (C++17)

Technical Specifications

Containers library

array (C++11)

vector

deque

forward_list (C++11)

list

set

multiset

map

multimap

unordered_set (C++11)

unordered_multiset (C++11)

unordered_map (C++11)

unordered_multimap (C++11)

stack

queue

priority_queue

span (C++20)

std::vector

Member functions

vector::vector

vector::~vector

vector::operator=

vector::assign

vector::get_allocator

Element access

vector::at

vector::operator[]

vector::front

vector::back

vector::data (C++11)

Iterators

vector::beginvector::cbegin (C++11)

vector::endvector::cend (C++11)

vector::rbeginvector::crbegin (C++11)

vector::rendvector::crend (C++11)

Capacity

vector::empty

vector::size

vector::max_size

vector::reserve

vector::capacity

vector::shrink_to_fit (C++11)

Modifiers

vector::clear

vector::insert

vector::emplace (C++11)

vector::erase

vector::push_back

vector::emplace_back (C++11)

vector::pop_back

vector::resize

vector::swap

Deduction guides(C++17)

void resize( size_type count, T value = T() );		(until C++11)
void resize( size_type count );	(1)	(since C++11)
void resize( size_type count, const value_type& value );	(2)	(since C++11)

Resizes the container to contain count elements.

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, additional elements are appended and initialized with copies of value.	(until C++11)
If the current size is less than count, 1) additional default-inserted elements are appended 2) additional copies of value are appended	(since C++11)

Parameters

count	-	new size of the container
value	-	the value to initialize the new elements with
Type requirements
- T must meet the requirements of MoveInsertable and DefaultInsertable in order to use overload (1).
- T must meet the requirements of CopyInsertable in order to use overload (2).

Return value

(none)

Complexity

Linear in the difference between the current size and count. Additional complexity possible due to reallocation if capacity is less than count

Exceptions

If an exception is thrown, this function has no effect (strong exception guarantee).

In overload (1), if T's move constructor is not noexcept and T is not CopyInsertable into *this, vector will use the throwing move constructor. If it throws, the guarantee is waived and the effects are unspecified.

(since C++11)

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.

Vector capacity is never reduced when resizing to smaller size because that would invalidate all iterators, rather than only the ones that would be invalidated by the equivalent sequence of pop_back() calls.

Example

Run this code

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

Output:

The vector holds: 1 2 3
After resize up 5: 1 2 3 0 0
After resize down to 2: 1 2

See also

size	returns the number of elements (public member function)
insert	inserts elements (public member function)
erase	erases elements (public member function)