How to edit a row in a text file with C++?

Question

I have a txt file like this:

"shoes":12
"pants":33
"jacket":26
"glasses":16
"t-shirt":182

I need to replace the number of jacket ( from 26 to 42 for example ). So, I have wrote this code, but I don't know how to edit a specific row where there is the word "jacket":

#include <iostream>
#include <fstream> 

using namespace std;

int main() {

    ifstream f("file.txt");
    string s;

    if(!f) {
        cout< <"file does not exist!";
        return -1;
    }

    while(f.good()) 
    {       
        getline(f, s);
        // if there is the "jacket" in this row, then replace 26 with 42.
    }


    f.close(); 
    return 0;
}

Solution

In order to modify data in a text file, you'll generally have to read the entire file into memory, make the modifications there, then rewrite it. In this case, I'd suggest defining a structure for the entries, with name and quantity entries, equality defined as equality of the names, and an overloaded operator>> and operator<< to read and write it from the file. You're overall logic would then use functions like:

void
readData( std::string const& filename, std::vector<Entry>& dest )
{
    std::ifstream in( filename.c_str() );
    if ( !in.is_open() ) {
        //  Error handling...
    }
    dest.insert( dest.end(),
                 std::istream_iterator<Entry>( in ),
                 std::istream_iterator<Entry>() );
}

void
writeData( std::string const& filename, std::vector<Entry> const& data )
{
    std::ifstream out( (filename + ".bak").c_str() );
    if ( !out.is_open() ) {
        //  Error handling...
    }
    std::copy( data.begin(), data.end(), std::ostream_iterator<Entry>( out ) );
    out.close();
    if (! out ) {
        //  Error handling...
    }
    unlink( filename.c_str() );
    rename( (filename + ".bak").c_str(), filename.c_str() );
}

(I'd suggest raising exceptions in the error handling, so that you don't have to worry about the else branches of the ifs. Except for the creation of the ifstream in the first, the error conditions are exceptional.)