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digitalmars.D.learn - Different visibility in template for class and struct?

reply Shigeki Karita <shigekikarita gmail.com> writes:
Why is local struct visible in this outer template, while local 
class is not?

https://wandbox.org/permlink/MfsDa68qgaMSIr4a

https://dlang.org/spec/template.html#instantiation_scope

---

enum p(T) = __traits(compiles, new T());

class GlobalC {}
struct GlobalS {}

void main()
{
     class LocalC {}
     static assert(p!GlobalC);
     static assert(!p!LocalC);

     struct LocalS {}
     static assert(p!GlobalS);
     static assert(p!LocalS); // ??
}
May 11 2020
parent reply Steven Schveighoffer <schveiguy gmail.com> writes:
On 5/11/20 11:11 AM, Shigeki Karita wrote:

 Why is local struct visible in this outer template, while local class is 
 not?
 
 https://wandbox.org/permlink/MfsDa68qgaMSIr4a
 
 https://dlang.org/spec/template.html#instantiation_scope
 
 ---
 
 enum p(T) = __traits(compiles, new T());
 
 class GlobalC {}
 struct GlobalS {}
 
 void main()
 {
      class LocalC {}
      static assert(p!GlobalC);
      static assert(!p!LocalC);
 
      struct LocalS {}
      static assert(p!GlobalS);
      static assert(p!LocalS); // ??
 }
 


First, it actually does compile, I think because the compiler recognizes 
that LocalS is POD (plain old data), without methods, so it doesn't need 
a context pointer.

e.g.:

auto make(T)() { return new T(); }

...

auto x = make!LocalS; // ok

If you add a method to LocalS, then make!LocalS fails to compile. 
However, strangely, p!LocalS still returns true, I think that is an error.

-Steve
May 11 2020
parent reply Shigeki Karita <shigekikarita gmail.com> writes:
On Monday, 11 May 2020 at 15:29:53 UTC, Steven Schveighoffer 
wrote:

 On 5/11/20 11:11 AM, Shigeki Karita wrote:

 [...]


 First, it actually does compile, I think because the compiler 
 recognizes that LocalS is POD (plain old data), without 
 methods, so it doesn't need a context pointer.

 e.g.:

 auto make(T)() { return new T(); }

 ...

 auto x = make!LocalS; // ok

 If you add a method to LocalS, then make!LocalS fails to 
 compile. However, strangely, p!LocalS still returns true, I 
 think that is an error.

 -Steve


Thanks for your answer. Now I understand that POD matters here.

When I add a dtor: `struct LocalS { ~this() {} }`, the `p!LocalS` 
got false.
May 11 2020
parent reply Steven Schveighoffer <schveiguy gmail.com> writes:
On 5/11/20 11:40 AM, Shigeki Karita wrote:

 On Monday, 11 May 2020 at 15:29:53 UTC, Steven Schveighoffer wrote:

 On 5/11/20 11:11 AM, Shigeki Karita wrote:

 [...]


 First, it actually does compile, I think because the compiler 
 recognizes that LocalS is POD (plain old data), without methods, so it 
 doesn't need a context pointer.

 e.g.:

 auto make(T)() { return new T(); }

 ...

 auto x = make!LocalS; // ok

 If you add a method to LocalS, then make!LocalS fails to compile. 
 However, strangely, p!LocalS still returns true, I think that is an 
 error.


 
 Thanks for your answer. Now I understand that POD matters here.
 
 When I add a dtor: `struct LocalS { ~this() {} }`, the `p!LocalS` got 
 false.


There is still a bug though. if p!T returns true, but make!T doesn't 
compile, then something is inconsistent.

-Steve
May 11 2020
parent Shigeki Karita <shigekikarita gmail.com> writes:
On Monday, 11 May 2020 at 16:10:36 UTC, Steven Schveighoffer 
wrote:

 On 5/11/20 11:40 AM, Shigeki Karita wrote:

 On Monday, 11 May 2020 at 15:29:53 UTC, Steven Schveighoffer 
 wrote:

[...]


 
 Thanks for your answer. Now I understand that POD matters here.
 
 When I add a dtor: `struct LocalS { ~this() {} }`, the 
 `p!LocalS` got false.


 There is still a bug though. if p!T returns true, but make!T 
 doesn't compile, then something is inconsistent.

 -Steve


I agree. This should be a bug 
https://wandbox.org/permlink/5YoKKt8xjdexzJHp
May 11 2020