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

From cppreference.com
< cpp‎ | container‎ | vector
 
 
 
 
void shrink_to_fit();
(until C++20)
constexpr void shrink_to_fit();
(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.

Contents

[edit] Parameters

(none)

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

[edit] Return value

(none)

[edit] 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)

[edit] Notes

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

[edit] Example

#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

[edit] 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

[edit] See also

returns the number of elements
(public member function) [edit]
returns the number of elements that can be held in currently allocated storage
(public member function) [edit]