Question

I found a C++ source file which calculates expressions from a command line argument (argv[1]), however I now want to change it to read a file.

double Utvardering(char* s) {
srcPos = s;
searchToken();
return PlusMinus();
}

int main(int argc, char* argv[]) {
if (argc > 1) {
    FILE* fFile = fopen(argv[1], "r");
    double Value = Utvardering(fopen(argv[1], "r"));
    cout << Value << endl;
}else{
    cout << "Usage: " << argv[0] << " FILE" << endl;
}
cin.get();
return 0;
}

However the Utvardering function requires a char* parameter. How can I convert the data read from a file, fopen to a char*?

Answer 1

The function fopen just opens a file. To get a string from there, you need to read the file. There are different ways to to this. If you know the max size of your string in advance, this would do:

const int MAX_SIZE = 1024;
char buf[MAX_SIZE];
if (!fgets(buf, MAX_SIZE, fFile) {
  cerr << "Read error";
  exit(1);
}
double Value = Utvardering(buf);

Note: this method is typical for C, not for C++. If you want more idiomatic C++ code, you can use something like this (instead of FILE and fopen):

ifstream in;    
in.open(argv[1]); 
if (!in) { /* report an error */ }
string str;
in >> str;