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

void shrink_to_fit();		(constexpr since C++20)

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.

Parameters

(none)

Type requirements
- `T` must meet the requirements of MoveInsertable into *this. (since C++11)

Return value

(none)

Complexity

At most linear in the size of the container.

Exceptions

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

(since C++11)

Notes

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

Example

Run this code

#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

Defect reports

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

DR	Applied to	Behavior as published	Correct behavior
LWG 755	C++98	`std::vector` lacked explicit shrink-to-fit operations	provided
LWG 2033	C++11	1. `T` was not required to be MoveInsertable 2. the complexity requirement was missing	1. required 2. added

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