string - implicit file extension using fopen in c++ -

i'm working on project i'm required take input file extension ".input". when run, user gives filename without file extension command line argument. take argument, argv[1] , open file specified can't work without user typing in entire filename

for example: user enters> run file.input //"run" executable, "file.input" filename

user supposed enter> run file

how file extension implied when using code:

fopen(argv[1],"r"); 

i tried using string, setting argv[1] , appending ".input" fopen won't accept string.

without seeing code, can't went wrong, suspect did this:

string filename = argv[1]; filename += ".input";  file* f = fopen(filename, "r"); // <--- error here 

the issue here c++ std::string type not char *, what's expected fopen. fix this, can use .c_str() member function of std::string type, gives null-terminated c-style string:

file* f = fopen(filename.c_str(), "r"); // no more errors! 

as mentioned in comment, though, think you'd better off using ifstream:

string filename = argv[1]; filename += ".input"; ifstream input(filename); 

there's no longer need .c_str(), , don't need worry leaking resources. everything's managed you. plus, it's type-safe!