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digitalmars.D - Concerns about struct initialization

reply Dru <Dru notreal.com> writes:
Hello,
I have two concerns about struct initialization
It would be nice to hear your opinion

1)
would like to override default copy initialization
code example:

import std.stdio;
void main(){
	struct A{
		this(int n) {
			writeln("construct int");
		}
		this(ref const A other) {
			writeln("construct ref A");
		}
		ref A opAssign(ref const A other) {
			writeln("opAssign");
			return this;
		}
	}
	A a1 = A(1);
	A a2 = A(a1);
	A a3 = 1;
	A a4;
	a4 = a1;
	
	A a5 = a1; //does not call constructor or opAssign
}


2)
I was reading this: 
https://dlang.org/spec/struct.html#dynamic_struct_init
It's pretty weird that a static opCall works as a constructor
Problems are:
- it doesn't work when ANY constructor is defined
- it blocks calling to default constructor

code example:

import std.stdio;
struct A{
	static A opCall(int n) {
		writeln("opCall");
		A a;
		return a;
	}

	//adding a constructor will error
	//this(string str) {}
}

void main(){
	A a1 = 1; //calls opCall

	//calling default constructor will error
	//A a2 = A();
}
Dec 22 2018
parent reply Johannes Loher <johannes.loher fg4f.de> writes:
On Saturday, 22 December 2018 at 08:53:20 UTC, Dru wrote:

 1)
 would like to override default copy initialization


In order to do this, you need to use a postblit 
(https://dlang.org/spec/struct.html#struct-postblit):

```
struct A {
     this(this) {
         writeln("postblit called");
     }
}

void main() {
     A a;
     A b = a; // prints "postblit called"
}
```

See also DIP1018 
(https://github.com/dlang/DIPs/blob/07da1f2cabc8b1bc3ad66034598a133e5ad13
56/DIPs/DIP1018.md) which proposes that we switch to regular copy constructors
for this.
Dec 22 2018
parent reply Dru <Dru notreal.com> writes:
Thanks didn't know about postblits =)

That DIP looks good, except that they used "ref" instead of "ref 
const"
which would be safer

I hope it gets implemented
Dec 22 2018
parent Johannes Loher <johannes.loher fg4f.de> writes:
On Saturday, 22 December 2018 at 11:47:31 UTC, Dru wrote:

 Thanks didn't know about postblits =)

 That DIP looks good, except that they used "ref" instead of 
 "ref const"
 which would be safer

 I hope it gets implemented


DIP1018 is currently in community review. If you have any 
feedback, please voice it here: 
https://forum.dlang.org/thread/eoqddfqbjtgfydajozsn forum.dlang.org
Dec 22 2018