[3542] in java-interest
Re: declaration v.s. definition ambiguity
daemon@ATHENA.MIT.EDU (Michael Daconta)
Wed Nov 15 21:11:47 1995
Date: Wed, 15 Nov 1995 16:09:32 -0700 (MST)
To: garya@village.org (Gary Aitken), java-interest@java.sun.com
From: daconta@PrimeNet.Com (Michael Daconta)
At 09:47 PM 11/13/95 -0700, Gary Aitken wrote:
>>declaration: Only an announcement to the compiler of an identifier to be
>>reserved in the current name space. In ANSI C, accomplished with the
>>extern keyword. For functions accomplished by omitting the function body.
>>For structures and classes accomplished with type identifier and no body.
>
>>definition: An announcement to the compiler of an identifier to be
>>reserved in the current name space. For primitive types, accompanied
>>with a reservation of storage space. For user-defined types,
>>accomplished by specifying the structure or class body. For functions
>>accomplished by specifying the function body.
>
>If I read the above correctly, the following is a declaration of Foo,
>not a definition:
>
> public static void main ( String argv[] ) {
> MyClass foo;
> }
>
>But it seems to me this is a definition.
>Also, abstract methods cannot have a body (unfortunately; should be may not)
Hi Gary,
In the above, the
MyClass foo;
is only a definition if MyClass has already been defined which
I'm sure is what you meant.
>
>As I understand the current syntax and semantics,
>it seems to me that a declaration is:
>
> An announcement to the compiler of an identifier to be reserved
> in the current name space, with no storage allocation.
>
>and a definition is:
>
> An announcement to the compiler of an identifier to be reserved
> in the current name space, together with the reservation of
> storage for the identifier.
>
>If we remember to take into account the difference between a class, and
>an instance of a class, the above is all consistent. In a class definition,
>the storage is allocated for the class itself -- the stuff which implements
>the class; not an instance of the class.
I think you are close; however, I don't think there is actually any storage
allocated for "the stuff which implements the class." Not at runtime anyway.
At compile time there is of course symbol table entries. However,
I think the ANSI C distinction of storage allocation was referring to
runtime allocation.
However, I think you are correct in that you need to take into an account
that a class is different than a primitive type. A class definition is
much closer to a function definition than a primitive type definition.
Using the analogy of a function definition seems to clear it up for
me.
>
> A class, in java, is never just declared. It is declared
> and defined in one operation. The definition amounts to allocating
> storage for all aspects of the class which do not require additional
> storage allocation when an object of the class is instantiated.
>
> A class instantiation, such as the "MyClass foo;" above, is a
> definition, as it reserves the storage associated with the instance.
>
> An interface, in java, is always declared, but never defined,
> since the definition of an interface is dependent on the class
> which implements the interface; a definition for an interface
> effectively occurs in the context of a class which implements
> the interface.
>
>Does this make sense?
>And please remember I'm no expert on this -- Arthur or somebody else
>should pass judgement on any statements like the ones I made above.
>
>Gary Aitken garya@village.org
>
Gary I think your definitions are good. Of course, when you say,
"It is declared and defined in one operation," that is not technically
correct because a definition is a declaration. That is why they are
so often used interchangeable. It is perfectly legal to say that for
int a;
the identifier a is declared in this namespace.
int a is a definition but a definition is a declaration so,
(using logic) if A=B=C then A=C.
I have it down now. Thanks for your help.
- Mike
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