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From: Matthew Heaney <mheaney@on2.com>
Newsgroups: comp.lang.ada
Subject: Re: Containers - nontrivial element access
Date: Wed, 03 Oct 2007 12:09:30 -0700
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Date: 2007-10-03T12:09:30-07:00
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On Oct 3, 2:49 pm, "Jeffrey R. Carter"
<spam.jrcarter....@acm.nospam.org> wrote:
>
> People.Element (X).Salary := S;
>
> and have it modify the value in the data structure.
>
> C++ uses reference semantics,

No, C++ has explicit reference types.  Ada of course has "reference
semantics" too, for composite types (at the discretion of the
implementation), for tagged types, and for limited types that are
"inherently" limited.

The C++ STL declares the index operator to return a reference to the
element:

   value_type& operator[](size_type);
   const value_type& operator[](size_type) const;

Of course, the C++ has by-value passing too.  The C++ STL uses by-
value passing for some of its operations, e.g.

   iterator operator++(int);  //post-fix form of inc operator

In fact, any arithmetic operator would have to return its result by-
value.


> so that a modification of a field modifies
> the value in the data structure. Ada uses value semantics,

Ada has both value semantics and reference semantics.  The language
specifies value semantics for some parameter classes (scalar or access
values) and reference semantics for others (tagged, limited, etc).


> so that
> modification of a field does not modify the value in the data structure.

But reference passing mechanism is completely orthogonal to the issue
being able to modify --in situ-- the container element.