std::vector resize() method

since C++20

// prism-push-types:size_type,value_type
// (1) Non const version only
constexpr void resize( size_type count );
// (2) Non const version only
constexpr void resize( size_type count, value_type const& value );

since C++11

// prism-push-types:size_type,value_type
// (1) Non const version only
void resize( size_type count );
// (2) Non const version only
void resize( size_type count, value_type const& value );

until C++11

// prism-push-types:size_type
// (2) Non const version only
void resize( size_type count, T value = T() );

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 is the container's element type in this list.

• (1) - T must meet the requirements of MoveInsertable and DefaultInsertable.
• (2) - T must meet the requirements of CopyInsertable.

Return value

(none)

Complexity

Linear in the difference between the size() and count - O(size() - count).

important

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).

since C++11

In overload (1), if Ts 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.

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

Main.cpp

#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