Re: [boost] Re: Re: Re: why is operator() not const in random numbergenerators?

From: Matthias Troyer (troyer_at_[hidden])
Date: 2003-04-16 10:57:34

On Wednesday, April 16, 2003, at 04:25 PM, Thorsten Ottosen wrote:

> ----- Original Message -----
> From: "Matthias Troyer" <troyer_at_[hidden]>
>> On Tuesday, April 15, 2003, at 05:39 PM, Thorsten Ottosen wrote:
> [snip]
>>> 2) After all, a client don't care what random number he gets, only
>>> that he
>>> gets one. The right result of a call to rand() is not that it is a
>>> particular random number, but that it's a random number. If the order
>>> of the
>>> sequence is important, that is still achievable if operator() is
>>> const, it
>>> is just not enforced by the compiler.
>>
>> Here I disagree. I actually do care very much about what number I get.
>> There are several application where it is of essential importance to
>> get the identical sequence again. I thus want to have control over
>> where the internal state of the generator is changed - and that's 
>> where
>> const comes in.
>
> you still have control, but the compiler doesn't. The compiler cannot 
> warn
> you if you
> use the random number generator somewhere else. But the same situation
> arises if I declare
> my variable mutable. Thus the only effect I get from a non-const 
> operator is
> that I need to prefix my member variable
> with mutable. (This issue would disappear if I make a local variable 
> instead
> of a member variable, though)

The compiler can warn me. If I pass a const generator to some function 
then the function can only make a local copy of the generator and use 
that, but leaves the original one intact. That way I can make sure that 
the state of the generator is not changed.

>>> 3) so (as seen from the client in 99% of the time) what is really the
>>> benefit of having a non-const operator() .?
>>
>> et me ask differently: what is the benefit of making it const?
>
> no mutable.

I never not needed mutable yet in any code of mine using random number 
generators. Could you give an example that shows where it is needed?

Matthias