Re: cannot call impure function ~this

[Namespace]

On Wednesday, 16 October 2013 at 07:32:27 UTC, monarch_dodra 
wrote:
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.
No, I was just curious what's behind it. So specifically why the 
compiler could call the destructor.