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D - Some ideas and a bug in template compilation

reply "Daniel Yokomiso" <daniel_yokomiso yahoo.com.br> writes:
Hi,

    While I was trying to circumvent the current template semantics, I got
stuck in the following problem. If I define a template like this:


template A(T, U) {
    private U u = new U();
}


    The compiler says "non-constant expression new U". IIRC templates don't
have a constructor, but I think they should have a constructor to initialize
template variables (hint or forget about templates and let us have generic
modules ;-) ).
    I got around this problem creating a setter for u, so the template user
must call setU explicitly. But this led to another problem. Suppose I have a
set of templates like this:


template A(T) {
    public interface Init {
        public T init();
    }
}
template A(T : int) {
    public class Init {
        public T init() {
            return 42;
        };
    }
}
template A(T : float) {
    public class Init {
        public T init() {
            return 3.14159;
        };
    }
}

template TB(T, U) {
    private U initializer;
    private void setInitializer(U init) {
        initializer = init;
    }
    public class B {
        private T _value;
        public this() {
            this._value = initializer.init();
        }
        public T value() {
            return this._value;
        }
    }
}
template TB(T) {
    private instance TB(T, instance A(T).Init) tb;
    private void setInitializer(instance A(T).Init init) {
        tb.setInitializer(init);
    }
    public class B : tb.B {
    }
}

int main() {
    instance TB(int, instance A(int).Init) tb;
    tb.setInitializer(new instance A(int).Init());
    tb.B b = new tb.B();
    printf("b.value -> %d\r\n", b.value());

    instance TB(float) tb2;
    tb2.setInitializer(new instance A(float).Init());
    tb2.B b2 = new tb2.B();
    printf("b2.value -> %e\r\n", b2.value());

    return 0;
}


    In this case U is a initializer type, and the A template can predefine
some common template specializations, and let the user create their own,
using a subclass for the Init interface. This code compiles and runs
correctly. But what I really needed was a default parameters in templates or
a "subtemplate", instead of this delegation trick. This could be useful
sometimes, but I'm not sure about this.
    Anyway, while I was writing this code example I forgot to type the
instance in the second TB definition usage of template A:


    private instance TB(T, A(T).Init) tb;


    And this made the compiler crash when it was parsing the file. But if we
forgot the instance in the main function:


    instance TB(int, A(int).Init) tb;


the following message is given: "found 'int' when expecting ')'". I don't
think this is the correct error  message, but at least it didn't crash.

    Best regards,
    Daniel Yokomiso.

"Clothes make the man. Naked people have little or no influence in society."
- Mark Twain
Dec 02 2002
parent reply "Walter" <walter digitalmars.com> writes:
"Daniel Yokomiso" <daniel_yokomiso yahoo.com.br> wrote in message
news:asgeg7$1df$1 digitaldaemon.com...

 Hi,

     While I was trying to circumvent the current template semantics, I got
 stuck in the following problem. If I define a template like this:


 template A(T, U) {
     private U u = new U();
 }


     The compiler says "non-constant expression new U". IIRC templates


don't

 have a constructor, but I think they should have a constructor to


initialize

 template variables (hint or forget about templates and let us have generic
 modules ;-) ).


Try adding:
    this()
    {
        u = new U();
    }
to the body of the template.


     I got around this problem creating a setter for u, so the template


user

 must call setU explicitly. But this led to another problem. Suppose I have


a

 set of templates like this:


 template A(T) {
     public interface Init {
         public T init();
     }
 }
 template A(T : int) {
     public class Init {
         public T init() {
             return 42;
         };
     }
 }
 template A(T : float) {
     public class Init {
         public T init() {
             return 3.14159;
         };
     }
 }

 template TB(T, U) {
     private U initializer;
     private void setInitializer(U init) {
         initializer = init;
     }
     public class B {
         private T _value;
         public this() {
             this._value = initializer.init();
         }
         public T value() {
             return this._value;
         }
     }
 }
 template TB(T) {
     private instance TB(T, instance A(T).Init) tb;
     private void setInitializer(instance A(T).Init init) {
         tb.setInitializer(init);
     }
     public class B : tb.B {
     }
 }

 int main() {
     instance TB(int, instance A(int).Init) tb;
     tb.setInitializer(new instance A(int).Init());
     tb.B b = new tb.B();
     printf("b.value -> %d\r\n", b.value());

     instance TB(float) tb2;
     tb2.setInitializer(new instance A(float).Init());
     tb2.B b2 = new tb2.B();
     printf("b2.value -> %e\r\n", b2.value());

     return 0;
 }


     In this case U is a initializer type, and the A template can predefine
 some common template specializations, and let the user create their own,
 using a subclass for the Init interface. This code compiles and runs
 correctly. But what I really needed was a default parameters in templates


or

 a "subtemplate", instead of this delegation trick. This could be useful
 sometimes, but I'm not sure about this.
     Anyway, while I was writing this code example I forgot to type the
 instance in the second TB definition usage of template A:


     private instance TB(T, A(T).Init) tb;


     And this made the compiler crash when it was parsing the file. But if


we

 forgot the instance in the main function:


     instance TB(int, A(int).Init) tb;


 the following message is given: "found 'int' when expecting ')'". I don't
 think this is the correct error  message, but at least it didn't crash.
Dec 03 2002
parent reply "Daniel Yokomiso" <daniel_yokomiso yahoo.com.br> writes:
"Walter" <walter digitalmars.com> escreveu na mensagem
news:askkmp$1vqc$1 digitaldaemon.com...

[snip]


 Try adding:
     this()
     {
         u = new U();
     }
 to the body of the template.


    The compiler says: "constructor this constructors only are for class
definitions".
Dec 04 2002
parent "Walter" <walter digitalmars.com> writes:
I shall have to fix that. -Walter

"Daniel Yokomiso" <daniel_yokomiso yahoo.com.br> wrote in message
news:asl7ik$2lno$1 digitaldaemon.com...

 "Walter" <walter digitalmars.com> escreveu na mensagem
 news:askkmp$1vqc$1 digitaldaemon.com...

 [snip]


 Try adding:
     this()
     {
         u = new U();
     }
 to the body of the template.


     The compiler says: "constructor this constructors only are for class
 definitions".
Dec 08 2002