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Showing content from https://en.cppreference.com/w/cpp/container/vector/shrink_to_fit.html below:

std::vector<T,Allocator>::shrink_to_fit - cppreference.com

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.

If reallocation occurs, all iterators (including the end() iterator) and all references to the elements are invalidated. If no reallocation occurs, no iterators or references are invalidated.

At most linear in the size of the container.

If an exception is thrown other than by the move constructor of a non-CopyInsertable T, there are no effects.

In libstdc++, shrink_to_fit() is not available in C++98 mode.

#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

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

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