How can I rewrite foo() function as a free-function that won't cause struct copying? My simplified code is:
========
struct S
{
    ref S foo() return
    {
        return this;
    }
}

void main()
{
    S().foo().foo().foo();
}
========
If I write it like "auto foo(S s) { return s; }" then statement in main() will copy value of S three times and I want to avoid this.

On Fri, Sep 18, 2020 at 06:20:41PM +0000, Andrey Zherikov via Digitalmars-d-learn wrote:
> How can I rewrite foo() function as a free-function that won't cause
> struct copying? My simplified code is:
> ========
> struct S
> {
>     ref S foo() return
>     {
>         return this;
>     }
> }
> 
> void main()
> {
>     S().foo().foo().foo();
> }
> ========
> If I write it like "auto foo(S s) { return s; }" then statement in
> main() will copy value of S three times and I want to avoid this.

Why can't you return by ref, which would also avoid the copying?

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


T

-- 
Let's not fight disease by killing the patient. -- Sean 'Shaleh' Perry

On Friday, 18 September 2020 at 18:43:38 UTC, H. S. Teoh wrote:
> Why can't you return by ref, which would also avoid the copying?
>
> 	ref S foo(return ref S s) { return s; }

Compiler errors out:
onlineapp.d(9): Error: function onlineapp.foo(return ref S s) is not callable using argument types (S)
onlineapp.d(9):        cannot pass rvalue argument S() of type S to parameter return ref S s

On Friday, 18 September 2020 at 18:20:41 UTC, Andrey Zherikov wrote:
> How can I rewrite foo() function as a free-function that won't cause struct copying?

I found solution:
========
struct S
{
    int i = -1;
    this(int n) {i=n;writeln(&this," ",i," ",__PRETTY_FUNCTION__);}
    this(ref return scope inout S rhs) inout {i=rhs.i+1;writeln(&this," ",i," ",__PRETTY_FUNCTION__);}
    ~this() {writeln(&this," ",i," ",__PRETTY_FUNCTION__);}
    ref auto getRef() return
    {
	    writeln(&this," ",i," ",__PRETTY_FUNCTION__);
        return this;
    }
}

ref auto foo(return ref S s)
{
    writeln(&s," ",s.i," ",__PRETTY_FUNCTION__);
    return s;
}

void main()
{
    S(5).getRef().foo().foo().foo();
}
========

Output confirms that there is no copying happens:
========
7FFDE98BDDF0 5 S onlineapp.S.this(int n) ref
7FFDE98BDDF0 5 onlineapp.S.getRef() ref return
7FFDE98BDDF0 5 onlineapp.foo(return ref S s) ref @system
7FFDE98BDDF0 5 onlineapp.foo(return ref S s) ref @system
7FFDE98BDDF0 5 onlineapp.foo(return ref S s) ref @system
7FFDE98BDDF0 5 void onlineapp.S.~this()
========