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digitalmars.D - Bug? Const Class Makes Constructor Const

reply Vijay Nayar <madric gmail.com> writes:
void main()
{
   const class Bob {
     this() {
       bob = null;
     }
     Bob bob;
   }

   Bob b = new Bob();
}

Compiling this program gives the following error:

onlineapp.d(10): Error: constructor `onlineapp.main.Bob.this() 
const` is not callable using argument types `()`

I know that declaring a 'const class' adds 'const' to every 
member of the class, but does that even include constructors?  
I'm not sure how one could use a class with a const constructor.  
Is this a bug?
Oct 25 2018
parent reply Steven Schveighoffer <schveiguy gmail.com> writes:
On 10/25/18 3:23 PM, Vijay Nayar wrote:

 void main()
 {
    const class Bob {
      this() {
        bob = null;
      }
      Bob bob;
    }
 
    Bob b = new Bob();
 }
 
 Compiling this program gives the following error:
 
 onlineapp.d(10): Error: constructor `onlineapp.main.Bob.this() const` is 
 not callable using argument types `()`
 
 I know that declaring a 'const class' adds 'const' to every member of 
 the class, but does that even include constructors? I'm not sure how one 
 could use a class with a const constructor. Is this a bug?


Generally:

const(Bob) /*or auto*/ b = new const(Bob)();

But the error message there looks really confusing to me.

-Steve
Oct 25 2018
parent reply Vijay Nayar <madric gmail.com> writes:
On Thursday, 25 October 2018 at 19:51:45 UTC, Steven 
Schveighoffer wrote:

 On 10/25/18 3:23 PM, Vijay Nayar wrote:

 void main()
 {
    const class Bob {
      this() {
        bob = null;
      }
      Bob bob;
    }
 
    Bob b = new Bob();
 }
 
 Compiling this program gives the following error:
 
 onlineapp.d(10): Error: constructor `onlineapp.main.Bob.this() 
 const` is not callable using argument types `()`
 
 I know that declaring a 'const class' adds 'const' to every 
 member of the class, but does that even include constructors? 
 I'm not sure how one could use a class with a const 
 constructor. Is this a bug?


 Generally:

 const(Bob) /*or auto*/ b = new const(Bob)();

 But the error message there looks really confusing to me.

 -Steve


Yeah it's a bit odd.  The error message isn't super clear, the 
only reason I think it's because "const" is applying to the 
constructor, is because I get the same error by manually applying 
const to all members like below.

void main()
{
   class Bob {
     this() const {
       bob = null;
     }
     const Bob bob;
   }

   auto b = new Bob();
}

But if the "this() const {" is changed to "this() {", then the 
error goes away.
Oct 25 2018
parent reply Steven Schveighoffer <schveiguy gmail.com> writes:
On 10/25/18 4:14 PM, Vijay Nayar wrote:

 On Thursday, 25 October 2018 at 19:51:45 UTC, Steven Schveighoffer wrote:

 On 10/25/18 3:23 PM, Vijay Nayar wrote:

 void main()
 {
    const class Bob {
      this() {
        bob = null;
      }
      Bob bob;
    }

    Bob b = new Bob();
 }

 Compiling this program gives the following error:

 onlineapp.d(10): Error: constructor `onlineapp.main.Bob.this() const` 
 is not callable using argument types `()`

 I know that declaring a 'const class' adds 'const' to every member of 
 the class, but does that even include constructors? I'm not sure how 
 one could use a class with a const constructor. Is this a bug?


 Generally:

 const(Bob) /*or auto*/ b = new const(Bob)();

 But the error message there looks really confusing to me.


 
 Yeah it's a bit odd.  The error message isn't super clear, the only 
 reason I think it's because "const" is applying to the constructor, is 
 because I get the same error by manually applying const to all members 
 like below.
 
 void main()
 {
    class Bob {
      this() const {
        bob = null;
      }
      const Bob bob;
    }
 
    auto b = new Bob();
 }
 
 But if the "this() const {" is changed to "this() {", then the error 
 goes away.


No, I mean the correct way to construct an object with a const 
constructor is `new const(Bob)`. The error message seems to suggest you 
just aren't calling it with the right parameters, but subtly it is 
conveying the error:

Error: constructor `onlineapp.main.Bob.this() const` is not callable 
using argument types `()`

See, you should be calling it with argument types `() const`, but you 
are using `()`.

It's a terrible way to say "you can only call this on a const object", 
but constructors are special, so I think a special error message should 
be used.

To answer your other question -- yes it is correct that const is 
applying to the constructor when you apply it to the type.

-Steve
Oct 25 2018
parent reply Vijay Nayar <madric gmail.com> writes:
On Thursday, 25 October 2018 at 20:27:43 UTC, Steven 
Schveighoffer wrote:

 See, you should be calling it with argument types `() const`, 
 but you are using `()`.

 It's a terrible way to say "you can only call this on a const 
 object", but constructors are special, so I think a special 
 error message should be used.

 To answer your other question -- yes it is correct that const 
 is applying to the constructor when you apply it to the type.

 -Steve


Oh, I see now.
   auto b = new const(Bob)();

I suppose const(Bob) is a type in this case.  I guess I'm not 
sure what the purpose of having a const constructor is. I see the 
benefit of having const members, to assure that the object is not 
modifed after creation, but when the constructor is const as 
well, that enforces head-const behavior as well, so that whenever 
a reference variable is initialized, that it cannot later 
reference something else.

Maybe it's just best that I avoid this feature and don't put 
const in front of the class.
Oct 25 2018
parent Steven Schveighoffer <schveiguy gmail.com> writes:
On 10/25/18 4:39 PM, Vijay Nayar wrote:

 I suppose const(Bob) is a type in this case.  I guess I'm not sure what 
 the purpose of having a const constructor is. 


The main purpose is if you have const members that have to be assigned 
from const data.

For example:

class C
{
    int *ptr;
    this(int *p) { ptr = p; }
}

If you want to construct a const(C) with a const(int)*, you need a const 
constructor:

this(const(int *)p) const { ptr = p; }

I don't see how else you would do it.


 I see the benefit of 
 having const members, to assure that the object is not modifed after 
 creation, but when the constructor is const as well, that enforces 
 head-const behavior as well, so that whenever a reference variable is 
 initialized, that it cannot later reference something else.


Then you want a class that has tail-const data, not a const class.


 Maybe it's just best that I avoid this feature and don't put const in 
 front of the class.


Yeah, a const class is existentially unchangable after construction.

-Steve
Oct 25 2018