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

#include <cstddef>
#include <deque>
#include <iostream>
#include <new>
// 带调试输出的最小C++11分配器
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没有capacity()函数（与std::vector不同）
    // 因此我们使用自定义分配器来展示shrink_to_fit的工作原理
    std::cout << "默认构造deque：\n";
    std::deque<int, NAlloc<int>> deq;
    std::cout << "\n添加300个元素：\n";
    for (int i = 1000; i < 1300; ++i)
        deq.push_back(i);
    std::cout << "\n弹出100个元素：\n";
    for (int i = 0; i < 100; ++i)
        deq.pop_front();
    std::cout << "\n运行shrink_to_fit：\n";
    deq.shrink_to_fit();
    std::cout << "\n当deque超出作用域时销毁：\n";
}