6.5.3 Automatic evaluation

When dealing with objects that are just a little more complicated than the simple string objects we have implemented, chances are that you will want to have some automatic simplifications or canonicalizations performed on them. This is done in the evaluation member function eval(). Let's say that we wanted all strings automatically converted to lowercase with non-alphabetic characters stripped, and empty strings removed:

class mystring : public basic
{
    ...
public:
    ex eval(int level = 0) const;
    ...
};

ex mystring::eval(int level) const
{
    string new_str;
    for (size_t i=0; i<str.length(); i++) {
        char c = str[i];
        if (c >= 'A' && c <= 'Z') 
            new_str += tolower(c);
        else if (c >= 'a' && c <= 'z')
            new_str += c;
    }

    if (new_str.length() == 0)
        return 0;
    else
        return mystring(new_str).hold();
}

The level argument is used to limit the recursion depth of the evaluation. We don't have any subexpressions in the mystring class so we are not concerned with this. If we had, we would call the eval() functions of the subexpressions with level - 1 as the argument if level != 1. The hold() member function sets a flag in the object that prevents further evaluation. Otherwise we might end up in an endless loop. When you want to return the object unmodified, use return this->hold();.

Let's confirm that it works:

ex e = mystring("Hello, world!") + mystring("!?#");
cout << e << endl;
 // -> "helloworld"

e = mystring("Wow!") + mystring("WOW") + mystring(" W ** o ** W");  
cout << e << endl;
 // -> 3*"wow"