How to fix opAssign signature

[Ali Çehreli]

On 11/02/2012 05:29 AM, Dan wrote:
 > The following works, but I want to make opAssign in D take const ref D.
 > It needs to still print "(dup here)". How can this be done?
 >
 > Thanks
 > Dan
 >
 > ----------------
 > import std.stdio;
 >
 > struct A {
 > char a[];
 > this(this) { a = a.dup; writeln("(dup here)"); }
 > }
 > struct B { A a; }
 > struct C { B b; }
 > struct D {
 > C c;
 > // How can I make this take const ref D other
 > ref D opAssign(ref D other) {
 > c = other.c;
 > return this;
 > }

Try the copy-then-swap idiom, which is both exception-safe and efficient:

import std.algorithm;

// ...

     ref D opAssign(D other) {
         swap(c, other.c);
         return this;
     }

There may be corner cases where this is not efficient, but considering 
that assignment involves two sub-operations (make a copy of the new 
state and destroy the old state), the above is doing exactly that. (It 
is the same idiom for strongly exception-safe operator= in C++.)

That has been the observation that led me to understand that by-value is 
the way to go with struct opAssign. Please let us know whether it has 
weaknesses. :)

 > }
 >
 > void main() {
 > D d, d2;
 > d2 = d;
 > }
 >

Ali