www.digitalmars.com         C & C++   DMDScript  

digitalmars.D.learn - anyway to pass the context of an inner type to a template so it can be

reply aliak <something something.com> writes:
This currently fails unless you mark the class as static:

auto construct(T)() {
     return new T;
}
void main() {
     class C {}
     auto s = construct!C;
}

So wondering if there's anything that can be done to get the 
above working?


Or if there isn't then how could the compiler be enhanced to 
allow for something like this if possible?

===
The use case is for a non-nullable type, where I want to 
guarantee that the value inside will never be null. I can't do it 
for inner classes though. And I can't allow the user to do 
something like:

void main() {
     class C {}
     auto s = construct(new C);
}

Because I can't guarantee that's not null.


Cheers,
- Ali
Jun 27 2018
next sibling parent reply Alex <sascha.orlov gmail.com> writes:
On Wednesday, 27 June 2018 at 12:02:10 UTC, aliak wrote:

 ===
 The use case is for a non-nullable type, where I want to 
 guarantee that the value inside will never be null. I can't do 
 it for inner classes though. And I can't allow the user to do 
 something like:

 void main() {
     class C {}
     auto s = construct(new C);
 }

 Because I can't guarantee that's not null.


 Cheers,
 - Ali


Is there any reason, why you don't want to use a struct? An 
instance of such is never null, still having access to its 
context, if it is a function.
Jun 27 2018
parent aliak <something something.com> writes:
On Wednesday, 27 June 2018 at 14:01:03 UTC, Alex wrote:

 On Wednesday, 27 June 2018 at 12:02:10 UTC, aliak wrote:

 ===
 The use case is for a non-nullable type, where I want to 
 guarantee that the value inside will never be null. I can't do 
 it for inner classes though. And I can't allow the user to do 
 something like:

 void main() {
     class C {}
     auto s = construct(new C);
 }

 Because I can't guarantee that's not null.


 Cheers,
 - Ali


 Is there any reason, why you don't want to use a struct? An 
 instance of such is never null, still having access to its 
 context, if it is a function.


Sorry, by non-nullable I meant not null. It's that construct 
produces a wrapper type that has an internal value that I want to 
guarantee is not null. So whether T is a struct or class is 
dependent on the user of construct.

- Ali
Jun 27 2018
prev sibling parent reply Timoses <timosesu gmail.com> writes:
On Wednesday, 27 June 2018 at 12:02:10 UTC, aliak wrote:

 This currently fails unless you mark the class as static:

 auto construct(T)() {
     return new T;
 }
 void main() {
     class C {}
     auto s = construct!C;
 }

 So wondering if there's anything that can be done to get the 
 above working?


 Or if there isn't then how could the compiler be enhanced to 
 allow for something like this if possible?

 ===
 The use case is for a non-nullable type, where I want to 
 guarantee that the value inside will never be null. I can't do 
 it for inner classes though. And I can't allow the user to do 
 something like:

 void main() {
     class C {}
     auto s = construct(new C);
 }

 Because I can't guarantee that's not null.


 Cheers,
 - Ali


After a bit of experimentation

     import std.stdio;
     template construct1(T)
     {
         enum construct1 = "new " ~ T.stringof;
     }

     template construct2(T)
     {
         // Attempt 1
         // can't do this : /
         //alias construct2 = mixin(construct1!T);

         // Attempt 2
         T construct2()
         {
             // Error: outer function context of D main is needed 
to new nested class onlineapp.main.C
             //return new T;

             // Error: undefined identifier C
             //mixin("return new " ~ T.stringof ~ ";");

             return null; // ...
         }
     }

     mixin template construct3(string s, T)
     {
         mixin("auto " ~ s ~ " = new " ~ T.stringof ~ ";");
     }

     void main() {
         class C { int i = 4;}

         auto a = mixin(construct1!C);
         assert(a.i == 4);

         mixin construct3!("b", C);
         b.i = 3;	
         assert(b.i == 3);

         // trying to get around using mixin here...
         auto c = construct2!C;
     }


Can't seem to avoid using mixin in main..
Jun 27 2018
parent aliak <something something.com> writes:
On Wednesday, 27 June 2018 at 19:28:37 UTC, Timoses wrote:

 Can't seem to avoid using mixin in main..


hehe yeah I see, didn't think of trying mixins, worth a shot! It 
seems like you had fun at least ;)
Jun 27 2018