How to read and write in file with `fstream` simul

问题:

I have the following code

#include<iostream>
#include<fstream>
#include<string>

using namespace std;

int main(void) {
    fstream ofile;
    ofile.open("test.txt", ios::in | ios::out | ios::app);
    for(string line; getline(ofile, line) ; ) {
        cout << line << endl;
    }
    ofile << "stackexchnange" << endl;
    ofile.close();
    return 0;
}

test.txt contains

hello world!
stackoverflow

Above code outputs

hello world!
stackoverflow

And after running the code stackexchange is not appending at the end of test.txt. How to read and then write in file?

回答1:

Nawaz' comment is correct. Your read loop iterates until the fstream::operator bool (of ofile) returns false. Therefore, after the loop, either failbit or badbit must have been set. failbit is set when loop tries to read for the final time but only EOF is left to read. This is completely OK, but you must reset the error state flag before trying to use the stream again.

// ...
ofile.clear();
ofile << "stackexchnange" << endl;

回答2:

fstream has two positions : input and output. In your case they both are set to the beginning of the file when you open it.

So you have 4 methods:

tellp // returns the output position indicator 
seekp // sets the output position indicator 
tellg // returns the input position indicator
seekg // sets the input position indicator 

in your case you can use the following line to set output position to the end of the file

ofile.seekp(0, std::ios_base::end);

PS i missed ios::app flag. mea culpa. comment of @Nawaz gives the right answer: after reading the whole file it is necessary to call

ofile.clear(); //cleanup error and eof flags