NAME

std::vector::resize - std::vector::resize

Synopsis

void resize( size_type count ); (since C++11)
(until C++20)
constexpr void resize( size_type count ); (since C++20)
void resize( size_type count, T value = T() ); (1) (until C++11)
void resize( size_type count, const value_type& (since C++11)
value ); (2) (until C++20)
constexpr void resize( size_type count, const (since C++20)
value_type& value );

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,

1) additional default-inserted elements 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

-
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 (since C++11)
constructor. If it throws, the guarantee is waived and the effects are
unspecified.

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(const auto& el: c) std::cout << el << ' ';
std::cout << '\n';
c.resize(5);
std::cout << "After resize up to 5: ";
for(const auto& el: c) std::cout << el << ' ';
std::cout << '\n';
c.resize(2);
std::cout << "After resize down to 2: ";
for(const auto& el: c) std::cout << el << ' ';
std::cout << '\n';
c.resize(6, 4);
std::cout << "After resize up to 6 (initializer = 4): ";
for(const auto& el: c) std::cout << el << ' ';
std::cout << '\n';
}

Output:

The vector 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