semantics of attribute const on constructors

[David Brown]

On 25/02/2011 16:43, Matthias Kretz wrote:
> Hi,
>
> On Friday 25 February 2011 16:26:24 Richard Guenther wrote:
>> On Fri, Feb 25, 2011 at 4:19 PM, Matthias Kretz wrote:
>>> My expectation was, that, since the ctor has a constructed object as
>>> return value, the compiler is free, instead of calling a ctor twice for
>>> the case of e.g.
>>> Foo a(1);
>>> Foo b(1);
>>> , to do a memcopy of a to b after the a ctor is done.
>>>
>>> Instead the result of using __attribute__((const)) is, that GCC feels
>>> free to optimize the whole ctor away. Apparently the ctor is treated as
>>> a function that returns void.
>>
>> If you lie to the compiler weird things will happen ;)
>
> I fully understand why it happened. So I imply your answer is that ctors do
> not have a return value and my expectation, as explained above, is wrong.
>
>>> Do you rather want a bug report for the website documenting attribute
>>> const, or for how GCC implements it? :-)
>>
>> None of the two.
>
> Ah, but if the semantics are meant to be this way then at least add another
> warning statement so that nobody else has to do a 2 hour bug-search session
> like I just did.
>
> The website says:
> "Many functions do not examine any values except their arguments, and have no
> effects except the return value. Basically this is just slightly more strict
> class than the pure attribute below, since function is not allowed to read
> global memory.
> Note that a function that has pointer arguments and examines the data pointed
> to must not be declared const. Likewise, a function that calls a non-const
> function usually must not be const. It does not make sense for a const
> function to return void."
>
> All of these statements hold for my ctor in question:
> 1. only examines its arguments
> 2. no effects other than writing to its class members
> 1b. doesn't read global memory
> 3. has an implicit pointer argument (this), which points to the memory area
> that is basically the return value of a ctor.
> 4. doesn't call any non-const function
> 5. there's no void anywhere in the function :-P
>
> So please change e.g. the last sentence to: "It does not make sense to use
> const with a function that returns void or with constructors."
>
> Regards,
> 	Matthias
>

A constructor /does/ take a pointer argument (the implicit "this"), and 
it (normally) /does/ have side effects - it sets values pointed to be 
*this.  So it can't be __attribute__((const)), as it breaks two of the 
conditions.