Re: fstream
Gaijinco <gaijinco@gmail.com> wrote in message ...
Ok. I mess up with the << and >> operators using them in the wrong
places.
But, I want to have an fstream object to read something from a file,
and then write something back.
To my understanding that should be done with:
std::fstream filestr ("test.txt", std::fstream::in | std::fstream::out);
Right?
Not if you don't understand how file IO works, and what the 'seekg()' and
'seekp()' are used for. (...and understand that the file will be truncated
at the point of write (output). also look up 'std::ios_base').
So if I want to read something I would use the >> operator, like:
filestr >> x;
std::ifstream filestr("myfile.txt");
if( not filestr.is_open() ){ std::cerr<<"Error"; exit(1);}
int x(0);
filestr >> x;
if( not filestr ){ std::cerr<<"Error"; exit(1);}
filestr.close();
And to write something I should use:
filestr << x
std::ofstream filestr2("myfile.txt");
if( not filestr2.is_open() ){ std::cerr<<"Error"; exit(1);}
int x(50);
filestr2 << x;
if( not filestr2 ){ std::cerr<<"Error"; exit(1);}
filestr2.close();
Is there any other consideration?
Because when I use this:
#include <fstream>
#include <string>
int main(){
std::fstream filestr ("test.txt", std::fstream::in |
std::fstream::out);
std::string s="";
filestr >> s;
You can't read nothing into something. <G>
s = "Not " + s;
filestr << s;
When you read nothing above, you put the filestream at EOF or a fail-state.
Use 'filestr.clear()', then output your 's'. You should then end up with a
file which contains "Not " (unless the file already existed and had
content).
return 0;
}
The file is never opened (if it doesn't exist it doesn't create it
also), if I print "s" it's still empty, and by the end of the program
the file is never written on.
--
Bob R
POVrookie