How to determine if a string is a number with C++?

I've had quite a bit of trouble trying to write a function that checks if a string is a number. For a game I am writing I just need to check if a line from the file I am reading is a number or not (I will know if it is a parameter this way). I wrote the below function which I believe was working smoothly (or I accidentally edited to stop it or I'm schizophrenic or Windows is schizophrenic):

bool isParam (string line)
{
    if (isdigit(atoi(line.c_str())))
        return true;

    return false;
}