Re: static const int problem
On May 15, 2:44 pm, Zeppe
<zeppe@.remove.all.this.long.comment.email.it> wrote:
Lionel B wrote:
Strange thing is my gcc 4.1.2 compiles the problem code ok:
g++-4.1.2 --version
g++-4.1.2 (GCC) 4.1.2
whereas the OP's doesn't (perhaps it uses a different stdc++ lib ?).
Might be worth asking on the gcc lists, I guess.
Well, that's strange indeed, because it seems that I'm using the same
version as you. Probably the problem relies in the type of v.begin()
that in different architectures has got different definitions. Anyway,
I'm using (g++ -v)
Using built-in specs.
Target: i486-linux-gnu
Configured with: ../src/configure -v
--enable-languages=c,c++,fortran,objc,obj-c++,treelang --prefix=/usr
--enable-shared --with-system-zlib --libexecdir=/usr/lib
--without-included-gettext --enable-threads=posix --enable-nls
--program-suffix=-4.1 --enable-__cxa_atexit --enable-clocale=gnu
--enable-libstdcxx-debug --enable-mpfr --enable-checking=release
i486-linux-gnu
Thread model: posix
gcc version 4.1.2 (Ubuntu 4.1.2-0ubuntu4)
and the command line to compile is
g++ -Wall teststatic.cpp -o teststatic
And you get the error, or no?
Formally, not providing the definition is undefined behavior.
Practically, in this particular case, I would not expect there
to be a problem, unless the compiler explicitly does something
special to make it one (so that a program with undefined
behavior doesn't compile). (I wonder if the
"--enable-checking=release" has something to do with it.)
Another issue could be the library. If the address of the
constant is ever taken (including implicitly, e.g. by using it
as an argument for a reference parameter), then it will almost
certainly need a definition. If vector<>::iterator is a class
type (normally the case today, I think, and certainly the case
with g++), and the operator+ takes a reference (which seems
surprising to me, but perhaps there are reasons), and ptrdiff_t
(the type which is added to an iterator) is int, then the
address will be taken. (If ptrdiff_t is not int, then his
static const will first be converted to a temporary ptrdiff_t;
this conversion almost surely doesn't involve taking the
address.)
A quick check in the sources of g++ 4.1.0 show:
__normal_iterator
operator+(const difference_type& __n) const
{ return __normal_iterator(_M_current + __n); }
The operator+ does take a reference, so on machines where
ptrdiff_t (the type of difference_type by default, with the
default allocator) is int, the address of his static const will
be taken, and he must define it. Unless the compiler actually
inlines the function---if he turns on optimization, he might not
need to define it:-). (On my machines, ptrdiff_t is almost
always a long, so I wouldn't have to define it either.)
But whatever: the rules say you have to define it, or you have
undefined behavior. And defining it always works. So it's
obvious what you have to do when writing code.
--
James Kanze (GABI Software) email:james.kanze@gmail.com
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