Re: [boost] Metaprogramming: Using static const or enum?

From: Gabriel Dos Reis (gdr_at_[hidden])
Date: 2003-01-08 22:05:08

David Abrahams <dave_at_[hidden]> writes:

| Terje Slettebø <tslettebo_at_[hidden]> writes:
| 
| > Static constant members are lvalues. So, if you have a declaration such as
| >
| > void foo(int const&);
| >
| > and you pass it the result of a metaprogram
| >
| > foo(Pow3<7>::result);
| >
| > a compiler must pass the address of Pow3<7>::result, which forces
| > the compiler to instantiate and allocate the definition for the
| > static member.  As a result, the computation is no longer limited to
| > a pure "compile-time" effect.
| 
| I buy everything but the last sentence.  What do they mean?  Does the
| thing go into the link map?  If not, what?

I'm not one the authors of the book mentioned but I will say what I
know (from experience both as a user and casual implementor).  The
issue of enum/static const is an endless debate. However, one thing is
sure:  Even with the amended paragraph in the Standard (redefinition of
"used"), passing an lvalue to a function that expects a const
reference more or less takes the referenced entity's address, and as
such makes the "static const object" used, therefore a definition is
required.  One looses the "purely compile-time constant" aspect. 
Yes, a smarter compiler may do better, but such smarter compilers are
quite rare :-) 
Yes, the thingy ends up in the link map (as a local symbol). 

-- Gaby