Simple calculator using command line with C++

Question

I\'m writing a project that we do simple calculator from command line. The users input in this format programname firstNumber operator secondNumber. Here what I

Answer 1

The other answers are correct, but I'm going to pitch using strtod instead of atof in the name of Eris and because it's easier to check the input for correctness.

char * endp;
double firstNumber;
if (argv[1][0] != '\0')
{ // input string is not empty 
    firstNumber = strtod (argv[1], &endp);
    if (*endp != '\0')
    { // number didn't end at the end of the string. String not a number.
        cout << "First number was not a number, dude." << endl;
        return 0;
    }
}
else
{
    cout << "Can't do much without a number, dude." << endl;
    return 0;
}

With atof, the program can be called with "I'm the very model of a modern major-general" and produce results. Bad results, but results all the same.

Oh, and warning on calling exit in C++. The program stops dead right there, and no destructors are called. No harm here, but not a good habit to get into.