Address of register variable
c, c++, keyword, register-allocation
Solution
Here's an excerpt from Section 6.7.1 (footnote 101) of the C99 standard (pdf):
The implementation may treat any `register` declaration simply as an `auto` declaration. However, whether or not addressable storage is actually used, the address of any part of an object declared with storage-class specifier register cannot be computed, either explicitly (by use of the unary `&` operator as discussed in 6.5.3.2) or implicitly (by converting an array name to a pointer as discussed in 6.3.2.1). Thus, the only operator that can be applied to an array declared with storage-class specifier `register` is `sizeof`.
And from Section 7.1.1, Paragraph 3 of the C++ standard (pdf):
A `register` specifier has the same semantics as an `auto` specifier together with a hint to the implementation that the object so declared will be heavily used. [Note: the hint can be ignored and in most implementations it will be ignored if the address of the object is taken. —end note]
Fun tidbits about `register`
The C++ group (WG21) wants to deprecate `register`:
The `register` keyword serves very little function, offering no more than a hint that a note says is typically ignored. It should be deprecated in this version of the standard, freeing the reserved name up for use in a future standard, much like `auto` has been re-used this time around for being similarly useless.
Notes from the March, 2009 meeting:
The consensus of the CWG was in favor of deprecating `register`.
Look what the C99 group (WG14) said about `register` (pdf) at a meeting:
General agreement to deprecate the “`auto`” keyword. Should we ask WG21 to go back to the previous use of “`register`” (no address)? No, this will not fly with WG21.
Problem
In C, we cannot use & to find out the address of a register variable but in C++ we can do the same. Why is it legal in C++ but not in C? Can someone please explain this concept in-depth.