C++ 向量减少分配大小

Question

C++ 向量在实践中什么时候动态减少其分配的大小。

我知道分配的 space 在插入完整向量时加倍，但我不清楚分配何时减少。经典滞后是在从 1/4 满向量中移除时将分配大小减半。

Answer 1

如果没有明确指示，它永远不会缩小分配的内存。

在 C++11 中有一个 shrink_to_fit 调用会要求实现执行此操作，但它可能不会减少分配的内存。在以前的版本中，您必须创建一个新副本并交换旧副本。

Answer 2

至少在我的编译器中，向量似乎不会减少它们分配的空间 space。当我运行:

        std::vector<int> v;
        for(unsigned x=0;x<20;++x)
        {
            v.push_back(x);
            out << "elements: " << v.size() << ", capacity: " << v.capacity() << std::endl;
        }
        for(unsigned x=v.size();x>0;--x)
        {
            v.pop_back();
            out << "elements: " << v.size() << ", capacity: " << v.capacity() << std::endl;
        }

返回的是：

    elements: 1, capacity: 1
    elements: 2, capacity: 2
    elements: 3, capacity: 4
    elements: 4, capacity: 4
    elements: 5, capacity: 8
    elements: 6, capacity: 8
    elements: 7, capacity: 8
    elements: 8, capacity: 8
    elements: 9, capacity: 16
    elements: 10, capacity: 16
    elements: 11, capacity: 16
    elements: 12, capacity: 16
    elements: 13, capacity: 16
    elements: 14, capacity: 16
    elements: 15, capacity: 16
    elements: 16, capacity: 16
    elements: 17, capacity: 32
    elements: 18, capacity: 32
    elements: 19, capacity: 32
    elements: 20, capacity: 32
    elements: 19, capacity: 32
    elements: 18, capacity: 32
    elements: 17, capacity: 32
    elements: 16, capacity: 32
    elements: 15, capacity: 32
    elements: 14, capacity: 32
    elements: 13, capacity: 32
    elements: 12, capacity: 32
    elements: 11, capacity: 32
    elements: 10, capacity: 32
    elements: 9, capacity: 32
    elements: 8, capacity: 32
    elements: 7, capacity: 32
    elements: 6, capacity: 32
    elements: 5, capacity: 32
    elements: 4, capacity: 32
    elements: 3, capacity: 32
    elements: 2, capacity: 32
    elements: 1, capacity: 32
    elements: 0, capacity: 32

Answer 3

在 C++11 之前，您可以通过为它分配一个新值来清空向量：

vector<int> x;
x.resize(500);
x = vector<int>(); // assigning a copy of new empty vector will shrink memory usage

C++ 向量减少分配大小

C++ vector reduce allocation size

c++

memory-management