Cleaning variables for a CGI app (as a derived class?)

Alrighty chaps,

I think it's just best if I show you what I'm trying to do here rather than trying to explain it in words.

Basically I'm writing web apps. What I need to work on still is the cleaning of variables sent by the users. In my programs, user variables are accessed like below:

Note: user parameters are stored internally as a map<string,string>; CGI::param() retrieves them

Code:

#include <string>
#include <cgipm.h>

int main()
{
    CGI q;     // imitates CGI.pm
    
    string cmd = q.param("cmd");    // as it works atm (insecure though)

    // What I was hoping to be able to do is to have it set up
    // something like this instead:

    string cmd = q.param("cmd");   // illegal: return not of type string

    string cmdRaw = q.param("cmd").raw();   // ok - just returns parameter as is
    string cmdCleaned = q.param("cmd").clean("/[a-z]*/");  // returns string if matches RegExp

    return 0;
}

I was planning on using PCRE for the cleaning - that's not the issue here - but as to how I get the clean() function to work like that I'm stuck.

What I was thinking would be ideal is if the function CGI::param() could only return values either by calling raw() on it, or by clean()-ing it. Sort of like taint checking of user variables. That's why I'd rather do it like this than through seperate function calls.

Thing is I've not actually made a derived class before (I understand the basic setup of them though, I think) and although this isn't just asking for you to do it for me, I wanted to know what sort of effort this would take. Also where do I start? It seemed a lot more straight forward when I thought of it.

I'm not sure I understand you completely, but I would think you should use a non-member function to do the work. The function would take the raw string and the RegExp expression. The syntax would be a little different: clean(q.param("cmd").raw(), "/[a-z]*/"); but would still do the work just as well.

You could also just add a second argument to param() that determines whether you should clean the parameter or not.