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

void shrink_to_fit();

(since C++11)

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 and references are invalidated. end() iterator is also invalidated.

Parameters

(none).

Type requirements
-`T` must meet the requirements of MoveInsertable.

Return value

(none).

Complexity

At most linear in the size of the container.

Notes

If an exception is thrown other than by T's move constructor, there are no effects.

Example

#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: