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

#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";
}

Возможный вывод:

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