Re: opAssign for most struct assignment calls not executed
On Thursday, 23 November 2017 at 15:45:17 UTC, Rene Zwanenburg
wrote:
On Thursday, 23 November 2017 at 15:26:03 UTC, Timoses wrote:
A aaa = a;
That's initialization, not assignment, which is subtly
different. It will call the postblit on aaa instead of
opAssign. A postblit can be defined this way:
this(this)
{
}
There is no way to access the original a from the postblit. All
fields have already been copied, you can use it to duplicate
reference types, increment reference counts, and other things
like that.
Wow, that's great! Thanks for the clarification you two!
For reference:
https://dlang.org/spec/struct.html#struct-postblit
http://ddili.org/ders/d.en/special_functions.html
Re: opAssign for most struct assignment calls not executed
On Thursday, 23 November 2017 at 15:26:03 UTC, Timoses wrote:
A aaa = a;
That's initialization, not assignment, which is subtly different.
It will call the postblit on aaa instead of opAssign. A postblit
can be defined this way:
this(this)
{
}
There is no way to access the original a from the postblit. All
fields have already been copied, you can use it to duplicate
reference types, increment reference counts, and other things
like that.
Re: opAssign for most struct assignment calls not executed
On Thursday, 23 November 2017 at 15:26:03 UTC, Timoses wrote:
A member;
this(A a)
{
this.member = a; // specifically aiming towards this
assignment...
That's not assignment, that's construction.
opAssign is only called when you assign over a struct object that
already exists. If there isn't an existing object in the variable
already, it instead calls a constructor.
A a = A(); // construction, there is no existing a to call
A a; // construction done here...
a = x; // so this is now assignment over the existing object
Sometimes, they look the same, as in your case, but since you are
assigning a member for the first time inside a constructor, it
still counts as construction of the child object instead of
assignment over an existing object.
opAssign for most struct assignment calls not executed
What am I missing?
import std.stdio;
struct A
{
int value;
A opAssign(A a)
{
writeln(" Assigning");
return this; // I know this makes little
sense, just for illustration
}
}
class B
{
A member;
this(A a)
{
this.member = a; // specifically aiming towards this
assignment...
}
}
void main()
{
A a = A(1);
A aa = A(2);
A aaa = a;
auto clas = new B(a);
writeln("Only this works:");
aaa = a;
writeln("or");
A ;
= aa;
}
Output:
Only this works:
Assigning
or
Assigning
Note that before "Only this works:" nothing was written.
How can I overwrite the opAssign operator??
I'm sure I'm missing something...
