Question

See below in comment.

int main(){
    //freopen("input.txt","r",stdin);//if I uncomment this line the console will appear and disappear immediately
    int x;
    cin>>x;
    cout<<x<<endl;
    system("pause");
    return 0;
}

How to make it work?

Answer 1

Solution 1: use cin.ignore instead of system:

...
cout<<x<<endl;
cin.ignore(1, '\n'); // eats the enter key pressed after the number input
cin.ignore(1, '\n'); // now waits for another enter key
...

Solution 2: if you are using MS Visual Studio, press Ctrl+F5

Solution 3: reopen con (will only work on Windows, seems your case)

...
cout<<x<<endl;
freopen("con","r",stdin);
system("pause");
...

If you use solution 3, don’t forget to add comments on what the code is doing and why 🙂