• rsw0x (28/29) May 03 2015 I remember reading that guaranteed RVO was part of the D
• ketmar (14/19) May 03 2015 actually, you'd better avoid code that makes assumptions about struct=20

"rsw0x" <anonymous anonymous.com> writes:

I remember reading that guaranteed RVO was part of the D
standard, but I am completely unable to find anything on it in
the specification.

I'm also unable to find anything in it that explicitly states the
lifetime of returning a stack-local struct from a function.

However, it does state
Destructors are called when an object goes out of scope.

So without guaranteed RVO I am quite confused.

I apologize because this code will likely be poorly formatted.


import std.stdio;
struct S{
	~this(){
		writeln("Goodbye!");
	}
}

S foo(){
	S s;
	return s;
}

void main()
{
	S s2 = foo();
}


This says "Goodbye!" exactly once, indicating(?) that S was
NRVO'd which means the scope of s went from foo to main.
However, is this a guarantee by the standard? Is an
implementation allowed to define foo such that it returns by copy
and calls a destructor on s, meaning "Goodbye!" would print out
twice?

ketmar <ketmar ketmar.no-ip.org> writes:

On Mon, 04 May 2015 02:29:19 +0000, rsw0x wrote:

 This says "Goodbye!" exactly once, indicating(?) that S was NRVO'd which
 means the scope of s went from foo to main. However, is this a guarantee
 by the standard? Is an implementation allowed to define foo such that it
 returns by copy and calls a destructor on s, meaning "Goodbye!" would
 print out twice?

actually, you'd better avoid code that makes assumptions about struct=20
copying/moving. compiler is free to do what it sees better. moreover,=20
phobos can use moving too. but it also can use copy/destroy.

what i want to say is that you should write your code with structure=20
copying in mind. remember that structure is value type, it can be freely=20
moved and copied.

you can also explicitly disable structure copying with

   disable this (this);

and if you concerned about code optimisation... well, NRVO is guaranteed=20
in DMD, and recent GDC got it too. don't know about LDC, though, but you=20
can expect that NRVO is guaranteed, yes.

if new compiler will appear (SDC, for example), i think that it will do=20
NRVO too (sooner or later).=