为什么不能以这种方式清空文件？

#include <iostream>
#include <fstream>
#include <graphics.h>

using namespace std;

int c;
fstream file("happy.txt");

int main()
{
    file.open("happy.txt", fstream::out|fstream::in|fstream::trunc );
    file.close();  //to clear the file first
    initwindow(1000,600);
    while (true){
        c = getch();

        file.open("happy.txt");
        file.seekp(0,file.end);   
        int fileEndCursor = file.tellp(); 
        file << (char)c;  //store what the user typed into file
        file.close();
    }
}

但是事实证明，文件未清除，每次代码运行完毕，上次运行的内容仍在文件中。请帮忙。