Re: C bug of the day

[James Rogers <jimmaureenrogers@att.net>]

kanze@gabi-soft.fr wrote in
news:d6652001.0306130749.5d9cd084@posting.google.com: 

> James Rogers <jimmaureenrogers@att.net> wrote in message
> news:<Xns9398C9067948jimmaureenrogers@204.127.36.1>...
> > That said, like C++, Ada does not prohibit garbage collection.
> 
> Perhaps a more relevant question would be: do comment implementations
> have it?  Or is it used?  (But as others have mentionned, it may not
> be used because typical Ada applications are in domains where it is
> NOT appropriate.)
> 

The only implementation I am aware of was the no longer supported
version of GNAT that targeted the JVM. Garbage collection was supported
because the JVM requires it.

Typical Ada applications are in real time safety critical domains.
Garbage collection is not heavily used in those domains. As others
have noted, some domains such as avionics systems do not even use
dynamic memory allocation other than stack-based allocation.

What all Ada compilers do support is the creation and use of storage
pools. Quoting from the Ada language reference manual:

"Each access-to-object type has an associated storage pool. The storage 
allocated by an allocator comes from the pool; instances of 
Unchecked_Deallocation return storage to the pool. Several access 
types can share the same pool."

"A storage pool is a variable of a type in the class rooted at 
Root_Storage_Pool, which is an abstract limited controlled type. By 
default, the implementation chooses a standard storage pool for each access 
type. The user may define new pool types, and may override the choice of 
pool for an access type by specifying Storage_Pool for the type."

When you define a storage pool you also must define implementations
for the abstract procedures Allocate and Deallocate.

When you define an access type you can define which storage pool
instances of that type uses. All allocations and deallocations 
relating to instances of that access type use the specified storage 
pool and are subject to the allocation and deallocation rules of that 
storage pool.

This is how you would define Ada equivalents to smart pointers or
even garbage collection. I like the idea that you can create special
characteristics for some access types without requiring that special
behavior for all access types.

Jim Rogers

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