cannot call impure function ~this

monarch_dodra monarchdodra at gmail.com
Wed Oct 16 00:32:26 PDT 2013


On Wednesday, 16 October 2013 at 07:27:25 UTC, Namespace wrote:
> On Wednesday, 16 October 2013 at 07:23:45 UTC, monarch_dodra 
> wrote:
>> On Tuesday, 15 October 2013 at 21:37:40 UTC, Namespace wrote:
>>> I get this error:
>>> ----
>>> /d701/f223.d(11): Error: pure function 'f223.getA' cannot 
>>> call impure function 'f223.A.~this'
>>> ----
>>>
>>> with this code:
>>> ----
>>> import std.stdio;
>>>
>>> struct A {
>>> public:
>>> 	~this() {
>>> 		writeln("DTor");
>>> 	}
>>> }
>>>
>>> A getA() pure nothrow {
>>> 	return A();
>>> }
>>>
>>> void main()
>>> {
>>> 	A a = getA();
>>> 	
>>> 	writeln("end of main");
>>> }
>>> ----
>>>
>>> But without pure and nothrow I get this output:
>>> ----
>>> end of main
>>> DTor
>>> ----
>>>
>>> Why the compiler thinks that the function should/could call 
>>> A::~this?
>>
>> It could have something to do with the fact that RVO is an 
>> optimization *opportunity* that the compiler is allowed to go 
>> for, even if it changes the program output.
>>
>> Hoewever, being an *opportunity*, the compiler still has to 
>> make sure the code is valid without said optimization, which 
>> in this case, isn't: getA would destroy it's temporary after 
>> blitting it on the stac, leading to an impure call.
>
> So it _could_ be impure, but mostly it isn't, right?

I guess that's one way to put it. I'd say it *is* impure, but all 
its impure bits have been optimized out. That's my explanation 
anyways.

I'm curious: Is this a problem for you? The function calling getA 
*can't* be pure either, so marking getA as pure is ...

I was going to say useless, but I guess "pure" is always an 
optimization opportunity for the compiler.

I'd file an ER, you never know.


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