std::vector shrink_to_fit() method

since C++20

// Non const version only
constexpr void shrink_to_fit();

since C++11

// Non const version only
void shrink_to_fit();

Requests the removal of unused capacity. It is a non-binding request to reduce capacity() to size(). It depends on the implementation whether the request is fulfilled.

Invalidation

If reallocation occurs, all iterators, including the past-the-end iterator, and all references to the elements are invalidated.

If no reallocation takes place, no iterators or references are invalidated.

Parameters

(none)

Type requirements

• T (the container's element type) must meet the requirements of MoveInsertable.

Return value

(none)

Complexity

At most linear in the size of the container - O(size()).

Exceptions

If an exception is thrown other than by T's move constructor, there are no effects (strong exception guarantee).

Example

Main.cpp

#include <iostream>
#include <vector>
 
int main()
{
    std::vector<int> v;
    std::cout << "Default-constructed capacity is " << v.capacity() << '\n';
    v.resize(100);
    std::cout << "Capacity of a 100-element vector is " << v.capacity() << '\n';
    v.resize(50);
    std::cout << "Capacity after resize(50) is " << v.capacity() << '\n';
    v.shrink_to_fit();
    std::cout << "Capacity after shrink_to_fit() is " << v.capacity() << '\n';
    v.clear();
    std::cout << "Capacity after clear() is " << v.capacity() << '\n';
    v.shrink_to_fit();
    std::cout << "Capacity after shrink_to_fit() is " << v.capacity() << '\n';
    for (int i = 1000; i < 1300; ++i)
        v.push_back(i);
    std::cout << "Capacity after adding 300 elements is " << v.capacity() << '\n';
    v.shrink_to_fit();
    std::cout << "Capacity after shrink_to_fit() is " << v.capacity() << '\n';
}

Possible output

Default-constructed capacity is 0
Capacity of a 100-element vector is 100
Capacity after resize(50) is 100
Capacity after shrink_to_fit() is 50
Capacity after clear() is 50
Capacity after shrink_to_fit() is 0
Capacity after adding 300 elements is 512
Capacity after shrink_to_fit() is 300