(v) is actually (*&v) since when?

Question

Could C++ standards gurus please enlighten me:

Since which C++ standard version has this statement failed because (v) seems to be equivalent to (*&v)?

I.e. for example the code:

 #define DEC(V) ( ((V)>0)? ((V)-=1) : 0 )
 ...{...
        register int v=1;
        int r = DEC(v) ;
 ...}...

This now produces warnings under -std=c++17 like:

cannot take address of register variable

left hand side of operand must be lvalue

Many C macros enclose ALL macro parameters in parentheses, of which the above is meant only to be a representative example.

The actual macros that produce warnings are for instance the RTA_* macros in /usr/include/linux/rtnetlink.h.

Short of not using/redefining these macros in C++, is there any workaround?

Answer 1

If you look at the revision summary of the latest C++1z draft, you'd see this in [diff.cpp14.dcl.dcl]

[dcl.stc]
Change: Removal of register storage-class-specifier.
Rationale: Enable repurposing of deprecated keyword in future revisions of this International Standard.
Effect on original feature: A valid C++ 2014 declaration utilizing the register storage-class-specifier is ill-formed in this International Standard. The specifier can simply be removed to retain the original meaning.

The warning may be due to that.