Can I call the default opAssign after overloading opAssign?
Dan
dbdavidson at yahoo.com
Fri Nov 23 11:53:40 PST 2012
On Monday, 19 November 2012 at 12:10:32 UTC, Dan wrote:
Just following up to get confirmation. Hopefully Johnathan or
similar expert can follow up.
Here is a strong statement:
If for any struct S you implement a postblit then there is no
need to implement opAssign to get a working assignment operator
from a type S because by design postblit is already called by
default opAssign. This is the behavior I see, but I may be
missing something since the language specification does not
mention postblit, only blit.
Thanks
Dan
> On Monday, 19 November 2012 at 05:22:38 UTC, Jonathan M Davis
> wrote:
>> On Monday, November 19, 2012 06:01:55 Rob T wrote:
>
>> postblit constructors and opAssign aren't really related. The
>> postblit
>> constructor is used when a _new_ instance is being constructed
>> (it plays the
>> same role as a copy constructor in C++). opAssign overloads
>> the assignment
>> operator and is only used when the assignment operator is
>> used, which does
>> _not_ happen when contstructing a new instance but only when
>> replacing the
>> value of an instance with that of another.
>
> Is this correct? From a implementation point of view it looks
> like opAssign is related to postblit in that it does call
> postblit first.
> From the spec:
> Struct assignment t=s is defined to be semantically
> equivalent to:
> t = S.opAssign(s);
> where opAssign is a member function of S:
> S* opAssign(S s)
> { ... bitcopy *this into tmp ...
> ... bitcopy s into *this ...
> ... call destructor on tmp ...
> return this;
> }
>
> It does not say postblit as well, but it does call it.
> When assigning one object into another it will first blit, then
> custom postblit if you have written one. A benefit of this is,
> if you want deep copy semantics and postblit does the work to
> provide it - you do not need an opAssign at all, as your
> postblit will be called. I think this is a step up over C++.
>
> The example below prints:
> ----------------------------------------------
> Begin assign
> postblit A
> End assign
> ----------------------------------------------
>
> import std.stdio;
> import std.traits;
>
> struct A {
> this(this) { c = c.dup; writeln("postblit A"); }
> char[] c;
>
> }
> struct B { A a; }
> struct C { B b; }
> struct D { C c; }
>
> void main() {
> D d1, d2;
> d1.c.b.a.c = ['a','b','c'];
> writeln("Begin assign");
> d2 = d1;
> writeln("End assign");
> }
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