How do "pure" member functions work?

[Steven Schveighoffer]

On Sat, 20 Aug 2011 12:43:29 -0400, Timon Gehr <timon.gehr at gmx.ch> wrote:

> On 08/20/2011 06:24 PM, Sean Eskapp wrote:
>> == Quote from David Nadlinger (see at klickverbot.at)'s article
>>> On 8/20/11 5:13 PM, Sean Eskapp wrote:
>>>> Does marking a member function as pure mean that it will return the  
>>>> same
>>>> result given the same parameters, or that it will give the same  
>>>> result, given
>>>> the same parameters and given the same class/struct members?
>>> The second one, the implicit this parameter is just considered a normal
>>> argument as far as purity is concerned.
>>> David
>>
>> Wait, references and pointers are now valid for pure function arguments?
>
> There are different forms of pure functions:
>
> weakly pure:
> no mutable globals are read or written. the function may however change  
> its arguments. weakly pure functions are useful mainly for the  
> implementation of functions with stronger purity guarantees.

I have to just interject real quick:  One aspect of pure functions that  
usually is forgotten (probably because it's seldom used) is that pure  
functions are not allowed to accept or return shared data.

In other words:

pure int foo(ref shared(int) i); // compiler error.

I think even without the ref, it's a compiler error, although I don't see  
what the point of a shared non-reference type is...

Other than that, you got everything right.

-Steve