no need to redo what has already been done
grep -n word file
or to use it in that context once you got the string do

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char *arg[5] ;
arg[0] = "grep" ; arg[1] = "-n" ; arg[2] = string.c_str() ; arg[3] = fileName.c_str() ;
arg[4] = NULL ;
execvp(arg[0], arg) ;

char *arg[5] ; arg[0] = "grep" ; arg[1] = "-n" ; arg[2] = string.c_str() ; arg[3] = fileName.c_str() ; arg[4] = NULL ; execvp(arg[0], arg) ;

(Your not using unix, when do one ever use console apps in windows ? go fetch mingw and tools )