DIP 1018--The Copy Constructor--Community Review Round 1

[Kagamin]

On Wednesday, 19 December 2018 at 13:40:41 UTC, RazvanN wrote:
> All this copies become useless when using the copy constructor.

A copy constructor is very verbose though, imagine a copy 
constructor for a struct with 30 fields, where it copies 29 
fields and sets the 30th to null. Postblit is observation of 
pattern that copy constructor usually changes only a small part 
of the struct. In this synthetic example copying wouldn't make 
much difference:
for this code:
---
struct A
{
     int b;
     void inc()
     {
         const int shadow_b=b;
         //b++
         const int t=shadow_b;
         b=t+1;
     }
}

A f(ref A a)
{
     A b=a;
     b.inc();
     return b;
}
---
`ldc -Os` generates this code for function f:
---
	movl	(%rdi), %eax
	addl	$1, %eax
	retq
---