About ref used for performance reasons with struct

[deadalnix]

On Monday, 11 February 2013 at 10:04:55 UTC, Namespace wrote:
> But what if we want to pass a struct with intent as value?
> With this solution, we don't have much control.
> In general, I like the idea, but we should mark such parameter
> with '&' or whatever, so at least we can still take some 
> influence.

That is the whole point, everything appear as if it is passed by 
value.

struct A {
     uint member;
     this(this) {
         // Very long but pure operation.
     }
}

void main() {
     A a;
     foo(a);

     assert(a.member = 0); // Pass.
}

void foo(A a) { // Compiler can choose to pass by ref here.
     bar(a);
}

void bar(A a) { // If compiler choose to pass by ref, then it is 
bar responsibility to create a copy. A smarter move from the 
compiler is to mark bar as non electable for such optimization.
     a.member = 5;
}

In the example above, the compiler is allowed to pass a by ref to 
foo. As a consequence, the very long operation within the 
postblit can only be executed once, instead of 2.