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From: shang@corp.mot.com (David L. Shang)
Subject: Re: Types, types: static vs. dynamic -- classification
Reply-To: shang@corp.mot.com
Organization: MOTOROLA 
Date: Thu, 30 Nov 1995 15:59:29 GMT
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In article <49i9un$nd0@stc06.ctd.ornl.gov> mbk@jt3ws1.etd.ornl.gov (Kennel)  
writes
> 
> I'd like to start a discussion:
> 
> 	What is the essential difference between 'type' and 'value'??
> 
> In quite static systems (say Fortran 77) there is little problem, because
> what people see as 'type' is for the convenience of the programmer, but 
> little sign of it continues to exist at run-time, the assembly code is
> ignorant of explict type. 
> 
> In something like C++ and Eiffel & Ada95, 'objects' carry around some datum 
> that is called 'a type tag' or some such, but acts a bit like an auxiliary
> value.   There is still 'type information' as in Fortran that was used
> in the human-readable source code which is not present in the executable.
> 
> The existence "RTTI" reflects this fact: types can be used as values.
> 

Type tags are not the real value of a type. They are references to the
information about type descriptions which are necessary for runtime
identicification and other type related handling. For example, the
id for the type, the supertype of the type, the size of the object,
the virtual entry, etc. none of them is the state of a type. So we
can hardly say that they consitute a value of a type.

So, many languages use types as pseudo values, not the real value.

David Shang
