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

 

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std::vector

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vector::vector vector::~vector vector::operator= vector::assign vector::assign_range (C++23)   vector::get_allocator Element access vector::operator[] vector::at vector::data vector::front vector::back Iterators vector::beginvector::cbegin (C++11) vector::endvector::cend (C++11) vector::rbeginvector::crbegin (C++11) vector::rendvector::crend (C++11)	Capacity vector::empty vector::size vector::max_size vector::reserve vector::capacity vector::shrink_to_fit (DR*) Modifiers vector::clear vector::erase vector::insert vector::insert_range (C++23) vector::append_range (C++23) vector::emplace (C++11) vector::emplace_back (C++11) vector::push_back vector::pop_back vector::resize vector::swap
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operator==operator<=> (C++20) swap(std::vector) erase(std::vector)erase_if(std::vector) (C++20)(C++20)	operator!=operator<operator>operator<=operator>= (until C++20)(until C++20)(until C++20)(until C++20)(until C++20)
Deduction guides(C++17)

 

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.

If T is not MoveInsertable into std::vector<T, Allocator>, the behavior is undefined.

(since C++11)

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++98 C++11	1. the complexity requirement was missing (C++98) 2. T was not required to be MoveInsertable (C++11)	1. added 2. required
LWG 2223	C++98 C++11	1. references, pointers, and iterators were not invalidated (C++98) 2. there was no exception safety guarantee (C++11)	1. they may be invalidated 2. added

See also

size	returns the number of elements (public member function)
capacity	returns the number of elements that can be held in currently allocated storage (public member function)