C ++ sizeof Vector是24？ [英] C++ sizeof Vector is 24?

问题描述

我只是搞乱和学习关于向量以及结构，一度，我试图输出一个向量的大小以字节为单位。这里是代码：

I was just messing around and learning about vectors as well as structs, and at one point, I tried outputting the size of a vector in bytes. Here's the code:

#include <iostream>
#include <vector>

struct Foo{
    std::vector<int> a;
};

int main()
{
    using std::cout; using std::endl;   

    Foo* f1 = new Foo;

    f1->a.push_back(5);
    cout << sizeof(f1->a) << endl;
    cout << sizeof(f1->a[0]) << endl;

    delete[] f1;
}

输出 24 和 4 。

显然第二行打印4，因为这是一个int的大小。但是为什么另一个值是24？向量占用24字节的内存吗？谢谢！

Obviously the second line printed 4, because that is the size of an int. But why exactly is the other value 24? Does a vector take up 24 bytes of memory? Thanks!

推荐答案

虽然 std :: vector 由标准定义，可以有不同的实现：换句话说， std :: vector 下可以从实现改为实现。

While the public interface of std::vector is defined by the standard, there can be different implementations: in other words, what's under the hood of std::vector can change from implementation to implementation.

即使在相同的实现中（例如：Visual C ++的给定版本附带的STL实现）， std： ：vector 可以从发布版本和调试版本中进行更改。

Even in the same implementation (for example: the STL implementation that comes with a given version of Visual C++), the internals of std::vector can change from release builds and debug builds.

你看到的24个大小可以解释为3个指针在64位体系结构上的大小;因此您有3 x 8 = 24字节）。这些指针可以是：

The 24 size you see can be explained as 3 pointers (each pointer is 8 bytes in size on 64-bit architectures; so you have 3 x 8 = 24 bytes). These pointers can be:

• 向量开头


• 向量结尾


• 向量的保留内存结尾（即向量的容量）

• begin of vector
• end of vector
• end of reserved memory for vector (i.e. vector's capacity)

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