std::deque shrink_to_fit() method

since C++11

// Non const version only
void shrink_to_fit();

Requests to reduce the unused capacity to size().

important

It is implementation-defined whether the capacity will be reduced.

Invalidation

If reallocation occurs, all iterators, including the past-the-end iterator, and all references to the elements are invalidated.

If no reallocation takes place, no iterators or references are invalidated.

Parameters

(none)

Type requirements

• T (the container's element type) must meet the requirements of MoveInsertable.

Return value

(none)

Complexity

At most linear in the size of the container - O(size()).

Exceptions

If an exception is thrown other than by T's move constructor, this function has no effect (strong exception guarantee).

Example

Main.cpp

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

// 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