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digitalmars.D.learn - Constructor called instead of opAssign()

reply frame <frame86 live.com> writes:
I have generic types that are initialized by a base class for 
each derived object:

struct S(T) {
     this(T val) {
         value = 100;
     }

     void opAssign(T val) {
         value = val;
     }

     T value;
}

class A {
     this(this T)() {
         foreach (property; __traits(allMembers, T)) {
             static if (property == "x") {
                 __traits(getMember, cast(T) this, property) = 50;
             }
         }
     }
}

class B : A {
     S!int x;

     this(int val) {
        assert(x.value == 50);
        x = val;
     }
}

auto b = new B(200);
assert(b.x.value == 200); // 100


Although x is well initialized by A by calling opAssign(), the 
compiler doesn't care and calls the S!T ctor() on B again and 
opAssign() is ignored. Is this a bug or a rule?
Feb 13 2021
parent reply Boris Carvajal <boris2.9 gmail.com> writes:
On Sunday, 14 February 2021 at 07:09:20 UTC, frame wrote:

 Although x is well initialized by A by calling opAssign(), the 
 compiler doesn't care and calls the S!T ctor() on B again and 
 opAssign() is ignored. Is this a bug or a rule?


It's a rule described here: 
https://dlang.org/spec/struct.html#field-init

"In a constructor body ... the first instance of field assignment 
is its initialization."

If you assign a second time, it will use the opAssign method.
Feb 14 2021
parent reply frame <frame86 live.com> writes:
On Sunday, 14 February 2021 at 08:38:49 UTC, Boris Carvajal wrote:

 On Sunday, 14 February 2021 at 07:09:20 UTC, frame wrote:

 Although x is well initialized by A by calling opAssign(), the 
 compiler doesn't care and calls the S!T ctor() on B again and 
 opAssign() is ignored. Is this a bug or a rule?


 It's a rule described here: 
 https://dlang.org/spec/struct.html#field-init

 "In a constructor body ... the first instance of field 
 assignment is its initialization."

 If you assign a second time, it will use the opAssign method.


The first instance is in A - and why opAssign then works there?
Feb 14 2021
parent reply Boris Carvajal <boris2.9 gmail.com> writes:
On Sunday, 14 February 2021 at 08:46:34 UTC, frame wrote:

 The first instance is in A - and why opAssign then works there?


Sorry I didn't pay too much attention.

It seems the detection of first assignment only happens when the 
field and constructor have the same parent, so it doesn't work 
either if the field is from a base or derived class (your case by 
means of casting 'this').

I don't think this is intended rather it appears to be a 
bug/deficiency in the constructor flow analysis of DMD, which 
from what I'm reading is very rudimentary.
Feb 16 2021
parent frame <frame86 live.com> writes:
On Tuesday, 16 February 2021 at 09:04:43 UTC, Boris Carvajal 
wrote:


 I don't think this is intended rather it appears to be a 
 bug/deficiency in the constructor flow analysis of DMD, which 
 from what I'm reading is very rudimentary.


If I'm using a delegate in B, supplied to super() and called in 
A, then it works :P
Feb 16 2021