C++ sizeof Vector 是 24?

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;
}

输出为24和4.

显然第二行打印了 4，因为那是 int 的大小.但为什么另一个值是 24?向量是否占用 24 字节的内存?谢谢！