Reading from file right after Writing in it - c++

Question

I wanna write in file and then read from it and print the result
Here is my code
#include<iostream>
#include<fstream>
using namespace std;
int main(int argc,char* argv[]){
int x,y;
ofstream fd1(argv[1]);
ifstream fd2(argv[1]);
cin>>x;
fd1<<x;
fd2>>y;
cout<<"just read "<<y<<endl;
fd1.close();
fd2.close();
return 0;
}
What's wrong with it? I input 123 it outputs "just read -1078463800"

Answer 1

Even if you can open both in read & write, this write operation is buffered, which means that it may not be written to disk unless you flush the stream (or you close the file).
Of course the code below works perfectly:
#include<iostream>
#include<fstream>
using namespace std;
int main(int argc,char* argv[]){
int x,y;
ofstream fd1(argv[1]);
cin>>x;
fd1<<x;
fd1.close();
ifstream fd2(argv[1]);
if (fd2.good())
{
cout << "read OK" << endl;
}
fd2>>y;
cout<<"just read "<<y<<endl;
fd2.close();
return 0;
}

Answer 2

The fd2>>y statement is failing because there is nothing to read from the file yet. std::ofstream buffers its output, and you haven't flushed the buffer to the file on disk before trying to then read from the file.
std::ofstream flushes its buffer when:
a new line is written.
its flush() method is called, either directly or when std::flush or std::endl is streamed to it.
Try this:
fd1 << x << flush;
Or:
fd1 << x;
fd1.flush();
On a side note, you really should be checking for errors along the way. The std::ofstream and std::ifstream constructors could fail to create/open the file. The << and >> operators could fail to write/read values. All of those operations can report errors that you can check for. For example:
#include <iostream>
#include <fstream>
int main(int argc, char* argv[])
{
int x, y;
std::ofstream fd1(argv[1]);
std::ifstream fd2(argv[1]);
if (!fd1.is_open())
{
std::cout << "cannot create output file" << std::endl;
}
else if (!fd2.is_open())
{
std::cout << "cannot open input file" << std::endl;
}
else if (!(std::cin >> x))
{
std::cout << "invalid input" << std::endl;
}
else if (!(fd1 << x << std::flush))
{
std::cout << "cannot write to output file" << std::endl;
}
else if (!(fd2 >> y))
{
std::cout << "cannot read from input file" << std::endl;
}
else
{
std::cout << "just read " << y << std::endl;
}
return 0;
}
Alternatively:
#include <iostream>
#include <fstream>
int main(int argc, char* argv[])
{
int x, y;
std::ofstream fd1;
std::ifstream fd2;
fd1.exceptions(std::ofstream::failbit);
fd2.exceptions(std::ifstream::failbit);
std::cin.exceptions(std::ifstream::failbit);
try
{
fd1.open(argv[1]);
fd2.open(argv[1]);
std::cin >> x;
fd1 << x << std::flush;
fd2 >> y;
std::cout << "just read " << y << std::endl;
}
catch (const std::ios_base::failure &e)
{
std::cout << "error! " << e.what() << std::endl;
}
return 0;
}