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digitalmars.D - Template question about alias

reply Edward Diener <eddielee_no_spam_here tropicsoft.com> writes:
Another D template question.

I am assuming that one can not create a variable which refers to a 
template instance, but one can create an "alias" for a template 
instance, ie.

template MyTemplate(T) { T data; /* other decldefs ... */};
MyTemplate!(int) aname; // illegal
alias MyTemplate!(int) aname; // legal

Is this correct ? If it is so, why did D not just scrap the use of 
'alias' in this case and allow the first form, 'MyTemplate!(int) aname;' 
to represent an alias automatically, since it would save endlessly 
having to type 'alias' for a syntax which could not be anything else ?
Mar 22 2008
next sibling parent "Jarrett Billingsley" <kb3ctd2 yahoo.com> writes:
"Edward Diener" <eddielee_no_spam_here tropicsoft.com> wrote in message 
news:fs4a59$2e5v$1 digitalmars.com...

 Another D template question.

 I am assuming that one can not create a variable which refers to a 
 template instance, but one can create an "alias" for a template instance, 
 ie.

 template MyTemplate(T) { T data; /* other decldefs ... */};
 MyTemplate!(int) aname; // illegal
 alias MyTemplate!(int) aname; // legal

 Is this correct ? If it is so, why did D not just scrap the use of 'alias' 
 in this case and allow the first form, 'MyTemplate!(int) aname;' to 
 represent an alias automatically, since it would save endlessly having to 
 type 'alias' for a syntax which could not be anything else ?


template MyTemplate(T)
{
    alias T MyTemplate;
}

MyTemplate!(int) aname; // legal, typeof(aname) is int
alias MyTemplate!(int) foo; // legal, foo is int
foo bar; // bar is int

In this case, the template is being used as a type.

It's not legal _in some cases_.  It is in others.  It requires semantic 
analysis to know whether it is or not, so it's not something that can be 
restricted by syntax.
Mar 22 2008
prev sibling parent reply Bill Baxter <dnewsgroup billbaxter.com> writes:
Edward Diener wrote:

 Another D template question.
 
 I am assuming that one can not create a variable which refers to a 
 template instance, but one can create an "alias" for a template 
 instance, ie.
 
 template MyTemplate(T) { T data; /* other decldefs ... */};
 MyTemplate!(int) aname; // illegal


This looks like the declaration of a class template instantiated for 
int.  So I'm glad that doesn't create an alias.  It would be very confusing.


 alias MyTemplate!(int) aname; // legal
 
 Is this correct ? If it is so, why did D not just scrap the use of 
 'alias' in this case and allow the first form, 'MyTemplate!(int) aname;' 


Because 'alias' is how you create type aliases in D.  I don't get why 
you'd view dispensening of the 'alias' there a benefit.


 to represent an alias automatically, since it would save endlessly 
 having to type 'alias' for a syntax which could not be anything else ?


--bb
Mar 22 2008
parent reply Edward Diener <eddielee_no_spam_here tropicsoft.com> writes:
Bill Baxter wrote:

 Edward Diener wrote:

 Another D template question.

 I am assuming that one can not create a variable which refers to a 
 template instance, but one can create an "alias" for a template 
 instance, ie.

 template MyTemplate(T) { T data; /* other decldefs ... */};
 MyTemplate!(int) aname; // illegal


 
 This looks like the declaration of a class template instantiated for 
 int.  So I'm glad that doesn't create an alias.  It would be very 
 confusing.
 

 alias MyTemplate!(int) aname; // legal

 Is this correct ? If it is so, why did D not just scrap the use of 
 'alias' in this case and allow the first form, 'MyTemplate!(int) aname;' 


 
 Because 'alias' is how you create type aliases in D.  I don't get why 
 you'd view dispensening of the 'alias' there a benefit.


If the first syntax was normally illegal, then having to type alias 
would be redundant if the first syntax were to be automatically 
considered an alias and be made legal. However I am told by another 
replier that the first syntax can sometimes be considered legal.
Mar 22 2008
parent Bill Baxter <dnewsgroup billbaxter.com> writes:
Edward Diener wrote:

 Bill Baxter wrote:

 Edward Diener wrote:

 Another D template question.

 I am assuming that one can not create a variable which refers to a 
 template instance, but one can create an "alias" for a template 
 instance, ie.

 template MyTemplate(T) { T data; /* other decldefs ... */};
 MyTemplate!(int) aname; // illegal


 This looks like the declaration of a class template instantiated for 
 int.  So I'm glad that doesn't create an alias.  It would be very 
 confusing.


 alias MyTemplate!(int) aname; // legal

 Is this correct ? If it is so, why did D not just scrap the use of 
 'alias' in this case and allow the first form, 'MyTemplate!(int) aname;' 


 Because 'alias' is how you create type aliases in D.  I don't get why 
 you'd view dispensening of the 'alias' there a benefit.


 
 If the first syntax was normally illegal, then having to type alias 
 would be redundant if the first syntax were to be automatically 
 considered an alias and be made legal. However I am told by another 
 replier that the first syntax can sometimes be considered legal.


I'm not sure what you're talking about anymore.  But it sounds like 
maybe you figured it out for yourself in a later message.  If not please 
ask again!

--bb
Mar 22 2008