std::deque shrink_to_fit() 方法

自 C++11 起

// Non const version only
void shrink_to_fit();

请求将未使用的容量减少到 size()。

重要

容量是否会减少取决于实现。

失效

如果发生重新分配，所有迭代器（包括超出末端迭代器）以及对元素的所有引用都将失效。

如果没有发生重新分配，任何迭代器或引用都不会失效。

参数

(无)

类型要求

• T （容器的元素类型）必须满足 MoveInsertable 的要求。

返回值

(无)

复杂度

最多与容器大小呈线性关系 - O(size())。

异常

如果抛出异常，而不是由T的移动构造函数抛出，则此函数没有效果（强异常保证）。

示例

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";
}

可能输出

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