如何知道向量的实际最大大小?(不使用 std::vector::max_size) [英] How can I know the real maximum size of a vector? (Not using std::vector::max_size)

问题描述

在一个在线课程中，我正在学习向量.在其中一个例子中，他们解释说: std::vector::max_size() 应该给我向量可以达到的最大尺寸.我决定测试一下:

On an online course I am learning about vectors. In one of the examples they explained that: std::vector::max_size() should give me the maximum size the vector can reach. I decided to test it:

#include <iostream>
#include <exception>
#include <vector>

int main(void) {
    std::vector <int> nums;
    int max = nums.max_size();
    std::cout << "Max: " << max << std::endl;
    for (int i = 0; i < max; i++) {
        try {
            nums.push_back(i);
        }
        catch (std::bad_alloc ex) {
            std::cerr << ex.what() << std::endl;
            std::cout << "Failed at: " << i << std::endl;
            break;
        }
    }

    return 0;
}

这是运行它的结果:

Max: 1073741823
bad allocation
Failed at: 204324850

短了 869416973 个整数.

It was 869416973 ints short.

所以我开始在谷歌上搜索它.这里 我读到它返回容器可以达到的最大潜在尺寸"，并补充说但绝不能保证容器能够达到那个大小".我会想象它会失败，但不会那么多.在失败之前，它只走了 1/5 的路.为什么 std::vector::max_size 这么差?我认为更重要的是，有没有办法真正知道向量的潜在大小?

So I started googling it. Here I read that it returns the "the maximum potential size the container can reach", and adds "but the container is by no means guaranteed to be able to reach that size". I would have imagined that it would fail, but not by that much. It just got 1/5 of the way before failing. Why is std::vector::max_size so off? And what I see of more importance, is there a way of really knowing the potential size of a vector?

推荐答案

注意 max_size 函数返回元素的理论最大数量，它没有说明所需的内存量.

Note that the max_size function returns a theoretical maximum number of elements, it doesn't say anything about the amount of memory needed.

如果我们假设 sizeof(int) == 4(很常见)那么 204324850 元素将需要 817299400 字节的 连续的内存(将近 780 MiB).

If we assume that sizeof(int) == 4 (pretty common) then 204324850 elements would need 817299400 bytes of contiguous memory (that's almost 780 MiB).

您会收到 bad_alloc 异常，因为向量根本无法分配足够的内存来容纳所有元素.

You get a bad_alloc exception because the vector simply can't allocate enough memory to hold all the elements.

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