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digitalmars.D - Casting between Structs

reply Jonathan Marler <johnnymarler gmail.com> writes:
I reduced an issue in my project to this code snippet:

struct Struct1 { }
struct Struct2 { }

void func1(const Struct1);
void func2()
{
     Struct2 s;
     func1(cast(Struct1)s);
}


DMD compiles this just fine but according to the spec this should 
not compile. The spec 
(https://dlang.org/spec/expression.html#CastExpression section 8) 
says that "Casting a value v to a struct S, when value is not a 
struct of the same type, is equivalent to: S(v)", however, if I 
write Struct1(s) in this example, I get a compile error.

Is the spec wrong or is this a problem in the DMD compiler?

P.S. this snippet causes LDC to segfault 
(https://github.com/ldc-developers/ldc/issues/3314), but it's not 
clear whether it should be made to work like DMD, or if they 
should conform to the spec and make this a compile error
Feb 08 2020
parent reply jxel <jxel gmall.com> writes:
On Saturday, 8 February 2020 at 17:36:17 UTC, Jonathan Marler 
wrote:

 I reduced an issue in my project to this code snippet:

 struct Struct1 { }
 struct Struct2 { }

 void func1(const Struct1);
 void func2()
 {
     Struct2 s;
     func1(cast(Struct1)s);
 }


 DMD compiles this just fine but according to the spec this 
 should not compile. The spec 
 (https://dlang.org/spec/expression.html#CastExpression section 
 8) says that "Casting a value v to a struct S, when value is 
 not a struct of the same type, is equivalent to: S(v)", 
 however, if I write Struct1(s) in this example, I get a compile 
 error.

 Is the spec wrong or is this a problem in the DMD compiler?

 P.S. this snippet causes LDC to segfault 
 (https://github.com/ldc-developers/ldc/issues/3314), but it's 
 not clear whether it should be made to work like DMD, or if 
 they should conform to the spec and make this a compile error


The spec is wrong in many aspects. It was written then never 
followed. Not sure if it even gets updated.

This seems like it should be an error. I don't know why you would 
ever want to do a cast like that, especially with lvalues. And if 
you did want to there is an easy alternative *cast(S1*)&v.
Feb 08 2020
parent reply Jonathan Marler <johnnymarler gmail.com> writes:
On Saturday, 8 February 2020 at 19:28:26 UTC, jxel wrote:

 On Saturday, 8 February 2020 at 17:36:17 UTC, Jonathan Marler 
 wrote:

 [...]


 The spec is wrong in many aspects. It was written then never 
 followed. Not sure if it even gets updated.


I hope you're wrong about this.  Can someone in D leadership 
please prove this statement wrong by clarifying whether this is a 
bug in DMD or a missing feature in the spec?
Feb 08 2020
parent reply Jonathan Marler <johnnymarler gmail.com> writes:
On Sunday, 9 February 2020 at 01:54:36 UTC, Jonathan Marler wrote:

 On Saturday, 8 February 2020 at 19:28:26 UTC, jxel wrote:

 On Saturday, 8 February 2020 at 17:36:17 UTC, Jonathan Marler 
 wrote:

 [...]


 The spec is wrong in many aspects. It was written then never 
 followed. Not sure if it even gets updated.


 I hope you're wrong about this.  Can someone in D leadership 
 please prove this statement wrong by clarifying whether this is 
 a bug in DMD or a missing feature in the spec?


I've created a bug here: "Struct Cast Not Documented or 
Incorrectly Accepted" 
https://issues.dlang.org/show_bug.cgi?id=20570
Feb 08 2020
parent Walter Bright <newshound2 digitalmars.com> writes:
On 2/8/2020 6:05 PM, Jonathan Marler wrote:

 I've created a bug here: "Struct Cast Not Documented or Incorrectly Accepted" 
 https://issues.dlang.org/show_bug.cgi?id=20570


It shouldn't compile. I'd have to investigate with the implementation.
Feb 13 2020