为什么写此二进制文件会切断文件的结尾？

因此，我要做的是创建一个结构数组。然后，我将该数组写入二进制文件。我认为寻求文件中的第二个结构并将其覆盖。然后，我将文件读回到单独的结构数组中，并将其打印到屏幕上。

写入完成后，文件大小将从24字节变为16字节，第一个结构损坏，最后一个结构从文件中丢失。

这是我的代码：

#include<fstream>
#include<iostream>
using namespace std;

struct Point
{
    int x, y;
};

void print(const Point& p)
{
    cout << "(" << p.x << ", " << p.y << ")" << endl;
}

int main()
{
    Point p[] = {{1, 2}, {3, 4}, {5, 6}};

    for(auto a : p)
        print(a);

    // write array p to file
    fstream f("points.dat", ios::binary | ios::out);
    if(f)
    {
        f.write(reinterpret_cast<char*>(p), 3 * sizeof(Point));
        f.close();
    }

    // read second struct from file and print it to screen
    f.open("points.dat", ios::binary | ios::in);
    if(f)
    {
        Point p;
        f.seekg( 1 * sizeof(Point), ios::beg);
        f.read(reinterpret_cast<char*>(&p), sizeof(Point));
        f.close();
        print(p);
    }

    // change ios::out to ios::ate and it works fine...
    f.open("points.dat", ios::binary | ios::out);
    if(f)
    {
        Point p ={-1, -2};
        f.seekp(1 * sizeof(Point), ios::beg);
        // attempting to overwrite the second struct in the file 
        // shrinks the file from 24 bytes to 16, losing the last
        // struct in the file and causing the first struct to have
        // 0's in it's fields.
        f.write(reinterpret_cast<char*>(&p), sizeof(Point));
        f.close();
    }


    Point q[3];
    // read the file back into q and print q for testing.
    f.open("points.dat", ios::binary | ios::in);
    if(f)
    {
        // fails because the file is now only 16 bytes, instead of 24 
        f.read(reinterpret_cast<char*>(q), 3 * sizeof(Point));      
        f.close();

        for(auto a : q)
            print(a);
    }   
}

烦人的部分是，当我将ios :: out更改为ios :: ate时，它工作得很好。