NAME¶

std::deque::shrink_to_fit - std::deque::shrink_to_fit

Synopsis¶

void shrink_to_fit();

Requests the removal of unused capacity.

It is a non-binding request to reduce the memory usage without changing the size of
the sequence. It depends on the implementation whether the request is fulfilled.

All iterators (including the end() iterator) and 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¶

(since C++11)
If an exception is thrown other than by the move constructor of a
non-CopyInsertable T, there are no effects.

Notes¶

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

Example¶

// Run this code

#include <cstddef>
#include <deque>
#include <iostream>
#include <new>

// Minimal C++11 allocator with debug output.
template<class Tp>
struct NAlloc
{
typedef Tp value_type;

NAlloc() = default;

template<class T> NAlloc(const NAlloc<T>&) {}

Tp* allocate(std::size_t n)
{
n *= sizeof(Tp);
std::cout << "allocating " << n << " bytes\n";
return static_cast<Tp*>(::operator new(n));
}

void deallocate(Tp* p, std::size_t n)
{
std::cout << "deallocating " << n*sizeof*p << " bytes\n";
::operator delete(p);
}
};
template<class T, class U>
bool operator==(const NAlloc<T>&, const NAlloc<U>&) { return true; }
template<class T, class U>
bool operator!=(const NAlloc<T>&, const NAlloc<U>&) { return false; }

int main()
{
// std::queue has no capacity() function (like std::vector).
// Because of this, we use a custom allocator to show the
// working of shrink_to_fit.

std::cout << "Default-construct deque:\n";
std::deque<int, NAlloc<int>> deq;

std::cout << "\nAdd 300 elements:\n";
for (int i = 1000; i < 1300; ++i)
deq.push_back(i);

std::cout << "\nPop 100 elements:\n";
for (int i = 0; i < 100; ++i)
deq.pop_front();

std::cout << "\nRun shrink_to_fit:\n";
deq.shrink_to_fit();

std::cout << "\nDestroy deque as it goes out of scope:\n";
}

Possible output:¶

Default-construct deque:
allocating 64 bytes
allocating 512 bytes

Add 300 elements:
allocating 512 bytes
allocating 512 bytes

Pop 100 elements:

Run shrink_to_fit:
allocating 64 bytes
allocating 512 bytes
allocating 512 bytes
deallocating 512 bytes
deallocating 512 bytes
deallocating 512 bytes
deallocating 64 bytes

Destroy deque as it goes out of scope:
deallocating 512 bytes
deallocating 512 bytes
deallocating 64 bytes

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 850 C++98 std::deque lacked explicit shrink-to-fit provided
operations
LWG 2033 C++11 1. T was not required to be MoveInsertable 1. required
2. the complexity requirement was missing 2. added

See also¶

size returns the number of elements
(public member function)