const

[Michel Fortin]

On 2008-03-28 04:26:42 -0400, "Janice Caron" <caron800 at googlemail.com> said:

> On 27/03/2008, Bill Baxter <dnewsgroup at billbaxter.com> wrote:
>> That sounds almost completely wrong to me.  C++ programmers tend to use
>> 'const' in their function signatures to document the things that
>> shouldn't change.  That sounds like self-documentation to me.  What do
>> you mean by "rely on convention instead"? Please explain.
> 
>     class C
>     {
>         int ** p;
>     }
> 
>     void f(const C &c)
>     {
>         **c.p = 1; // Legal in C++
>     }

No, that's not legal in C++ because p is private (class members are 
private in C++ unless explicitly made public). But beside that, is 
legal.

> There is no way to express compiler-checked transitive constancy in C++.

There's a way if you really want it: keep the variable private and make 
sure the accessor functions return a pointer corresponding to the 
constancy you want, something like

	const int * const * getP() const { return p; }
	int ** getP() { return p; }

That's all a manual task to get there though (error prone?), and you 
have to know about the type you're dealing with (which can be 
complicated in templates).

-- 
Michel Fortin
michel.fortin at michelf.com
http://michelf.com/