BigMan@[EMAIL PROTECTED]
wrote:
> I see, the problem seems to come from 8.5.3/5...
> 1. I'm curious about why the compiler should want to make a
> copy of an rvalue and bind a const reference to it instead of
> binding the const reference directly to the rvalue. Can anyone
> explain this?
The compiler doesn't want to. For some reason, the standard
says that it has to be able to, however.
> 2. Why I need a copy ctor taking a non-const reference?
> Because I'm trying to create a type (e.g. class) that
> describes a resource wrapper. The resource is a non-copyable
> one (it can be duplicated, but one should avoid doing so for
> performance reasons). Examples of such resources are: Windows
> HANDLE's, HKEY's, HMODULE's, HINTERNET's, MPI communicators
> and datatypes, and last but not least - huge amounts of
> dynamically allocated memory. I need to use the type that
> describes the resource wrapper (let's name it RW) in
> expressions such as:
> RW a = CreateRW( ); // 1: create a wrapper around a mutable
> resource;
> RW const a = CreateRW( ); // 2: create a wrapper around a
> non-mutable resource;
> a = b + c + d; // 3: use wrappers in arithmetic expressions, just
> like built-in types; all identifiers here stand for objects of type
RW
> or const RW;
> CreateRW( ) = a; // 4: disallow assignment to rvalues and
> tem****aries - it is pointless;
> Is it possible to create a type (or two - one for const
> wrappers, and one for non-const ones) that meets all 4
> requirements? If so, how?
The usual solution is to use some sort of reference counting,
with the visible class acting as a handle for the resource, and
forwarding to it (a bit like in the letter/envelope idiom). The
wrapper class has full copy semantics, but the actual copy is
only shallow. (Depending on the desired semantics, you might
implement copy on write, or you might simply do***ent the
reference semantics.)
--
James Kanze GABI Software http://www.gabi-soft.fr
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