Subject: [sv-ec] Protection of class types
From: Arturo Salz (Arturo.Salz@synopsys.com)
Date: Thu Mar 20 2003 - 17:32:51 PST
Hi All,
I looked at the issue of protecting types (or nested classes) in C++ and
whether we should enable that feature in SystemVerilog-3.1.
The first question is straightforward. C++ does allow nested classes and
other type declarations to be declared private/protected/public. Unlike
data members, which are private by default, types are public by default.
After examining the issue closer, the ability to protect nested classes does
create some undesirable complexity. Consider the following C++ class:
--------------------------------------------------------------------------------
class Goo
{
private:
class Nested { ... };
Nested *foo();
};
If you don't include the implementation of foo in the declaration, and try to
define it out-of-body, like this:
Nested *foo() { .... }
Generates a compiler error since Nested is not visible at the top level.
If we write:
Goo::Nested *foo() { .... }
That doesn't work either since foo is private to Goo.
If we write:
Goo::Nested *Goo::foo() { ... }
That doesn't work!
The problem is that class Nested is now private to Goo, thus, only members
of Goo can access the type! Thus, this function can only be defined inside
the declaration of Goo, but not anywhere else outside.
In C++, the only way one can get around these restrictions is by changing foo
from a function to a task that takes a reference to a Nested pointer as a parameter:
class Goo
{
private:
class Nested { ... };
void *foo(Nested *&);
};
Now, foo can be declared out-of-body, thus:
void Goo::foo(Nested *& p) { p = new Nested; }
which, is roughly equivalent to:
Goo::Nested* Goo::foo() { return new Nested; }
The difference is that Goo::Nested is no longer at file scope, but inside the
scope of a member of class Goo .
--------------------------------------------------------------------------------
It is this kind of complexity that caused the gnu folks to relax the type checking
in gcc and break with the standard by simply making all class-nested types public.
Therefore, I recommend that class-nested types in SystemVerilog-3.1 remain
public. No decision that we make today will limit in any way our ability to add this
feature in a future release. Right now we can choose to make it an error to prefix a
type declaration with the protected or local keywords (the current proposal), or we
can allow these keywords and ignore them (as gcc has been doing).
Arturo
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