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From: ytomino <aghia05@gmail.com>
Newsgroups: comp.lang.ada
Subject: Re: discriminant questions
Date: Sat, 17 Sep 2011 15:55:15 -0700 (PDT)
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Date: 2011-09-17T15:55:15-07:00
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>C++ does not have implicit dereferencing at all. Ada has it but only for
> attributes, array indexing, component members.

C++ has it.

struct reference_type {
	int *element;
	operator int & () { return *element; }
};

This code is same as:

type Reference_Type (Element : access Integer) is null record;

and replace int * to int ** and int & to int *&,
we can use reference_type where raw pointer is required.
(In fact, smart pointers in Boost and C++0x have not cast operator
but operator * and operator -> . However client codes are same.)

Therefore, there is no reason that we could not it with Ada.
...except assignment.

Two Ada variables having different discriminant are incompatible
and it raise Constraint_Error at runtime.
But if default value exists, we can assign these variables.

Return to the first question. An access discriminant can not have
default value.
So I can not make smart pointer like syntax of raw access types after
all.
I want to know the reason.