Re: Variables in for loop (style issue)

Walter Bright wrote:


Ok.

It so happens that this use of const is one of my favourite C++
features.

I guess I should start with Turbo Pascal. This was the language I first
used at university, and I feel lucky I did not have to use C or C++ at
that time. Anyway, Pascal had this nice separation of procedures and
functions where procedures could not return anything (unless by a
reference parameter). Procedures were meant to modify state. Functions
were meant to compute stuff without causing unintentional side-effects.

When I later moved on to C++, I was very pleased to learn that I could
now let the compiler enforce this notion of precedures and functions.

I think std::vector<T> is a good example. Take the iterator interface:

template< ... >
class vector
{
    iterator begin();
    const_iterator begin() const;
    iterator end();
    const_iterator end() const;
};

because begin() and end() are overloaded on const, vector<T>
will propagate constness for me. As you know, the first
advantage is that I can have read-only parameters no functions
and that I can return a read-only vector<T> by const reference.
Neither of these interfaces can perform operations that will
break the object's invariant.

The second advantage is when I implement other member functions
in my class that holds a vector<T> data member. If that
member function is const, I know that I'm only reading data and that
I preserve all invariants. Otherwise the compiler will tell me.

There is another advantage which is not yet as obvious in C++ as it
is in D:

Code inside contracts should be const-correct. Currently you have no
way to enforce this in D.

-Thorsten

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