Re: Safety of the Booch Ada 95 Components

[Simon Wright <simon@pogner.demon.co.uk>]

herwin@gmu.edu (Harry Erwin) writes:

> Simon Wright <simon@pogner.demon.co.uk> wrote:
[...]
> > > Apparently in the Booch components, if the copy goes bad, the To
> > > container has been cleared and is partially written. Basic exception
> > > safety (== the container is still useable) is probably supported.
> > 
> > Yes, this is the case for the Copy operation (which is *not* the
> > standard assignment operation!).
> 
> What is the standard assignment operation, then?  I'm new to Ada 95.  In
> C++ 99, we rarely trust the compiler-defined default versions of copy
> constructors and assignment operations, because they do bit-by-bit
> copies. 

The standard Ada assignment operation is :=

However, it's not an operation that you have control over in the way
that C++ gives you with operator=.

What you do have is Controlled types (types that inherit from
Ada.Finalization.Controlled).

From ALRM,

7.6 User-Defined Assignment and Finalization

(1) Three kinds of actions are fundamental to the manipulation of
     objects: initialization, finalization, and assignment. Every
     object is initialized, either explicitly or by default, after
     being created (for example, by an object_declaration or
     allocator).  Every object is finalized before being destroyed
     (for example, by leaving a subprogram_body containing an
     object_declaration, or by a call to an instance of
     Unchecked_Deallocation). An assignment operation is used as part
     of assignment_statements, explicit initialization, parameter
     passing, and other operations.

(2) Default definitions for these three fundamental operations are
     provided by the language, but a controlled type gives the user
     additional control over parts of these operations. In particular,
     the user can define, for a controlled type, an Initialize
     procedure which is invoked immediately after the normal default
     initialization of a controlled object, a Finalize procedure which
     is invoked immediately before finalization of any of the
     components of a controlled object, and an Adjust procedure which
     is invoked as the last step of an assignment to a (nonlimited)
     controlled object.

[...]

(17) For an assignment_statement, after the name and expression have
     been evaluated, and any conversion (including constraint
     checking) has been done, an anonymous object is created, and the
     value is assigned into it; that is, the assignment operation is
     applied. (Assignment includes value adjustment.) The target of
     the assignment_statement is then finalized. The value of the
     anonymous object is then assigned into the target of the
     assignment_statement. Finally, the anonymous object is
     finalized. As explained below, the implementation may eliminate
     the intermediate anonymous object, so this description subsumes
     the one given in 5.2, ``Assignment Statements''.

I don't think there's a web version of the ALRM at
www.adapower.com. There is one at www.adahome.com (no longer
maintained, it seems, but still with goodies like this).