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

void resize( size_type count );

constexpr void resize( size_type count );

void resize( size_type count, const value_type& value );

constexpr void resize( size_type count, const value_type& value );

Resizes the container to contain count elements, does nothing if count == size().

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,

Parameters

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). Although not explicitly specified, std::length_error is thrown if the capacity required by the new vector would exceed max_size().

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

#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';
}

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

Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

See also