In commit c280e90, support for rvalue references was added. That same commit introduced a define "EIGEN_HAVE_RVALUE_REFERENCES". For gcc, this is set to true any time __GXX_EXPERIMENTAL_CXX0X__ is defined. This makes sense; rvalue references were introduced at the same time as that macro (in gcc 4.3). That said, there is code that is enabled by this macro that uses "nullptr"--a feature that wasn't introduced until gcc 4.6.
The upshot is that gcc 4.3, 4.4, and 4.5 can not #include <Eigen/Dense> when the -std=c++0x flag is in use.
My suggestion: replace the use of nullptr with 0 to retain compatibility with old compilers.
I don't really see the point in supporting experimental features of pretty old compiler versions, if you need C++11 features, then use a compiler that truly support it. (gcc 4.6 was released almost 5 years ago).
Today, replacing nullptr by 0 might be enough, but tomorrow Eigen will very likely rely on other non implemented C++11 features in such old compiler in C++11 mode.
(In reply to Gael Guennebaud from comment #1)
> I don't really see the point in supporting experimental features of pretty
> old compiler versions, if you need C++11 features, then use a compiler that
> truly support it. (gcc 4.6 was released almost 5 years ago).
> Today, replacing nullptr by 0 might be enough, but tomorrow Eigen will very
> likely rely on other non implemented C++11 features in such old compiler in
> C++11 mode.
It would be great if it were up to me, but the compiler versions I am required to support at work is not my decision.
It's not really that I want Eigen to support some C++11 features, it's that I do not want it to assume that C++11 is fully implemented when it detects experimental support via the aforementioned macro and then fail to compile.
One other way to do this would be to add a way to forcefully disable C++11 (e.g., EIGEN_NO_CXX11).
I guess we should simply remove the line
defined(__GXX_EXPERIMENTAL_CXX0X__) || \
from Macros.h (and perhaps any other reference to __GXX_EXPERIMENTAL_CXX0X__, if there are any?). We do claim to support GCC 4.1 and newer -- but we don't promise to support experimental extensions.
oh, right, those should definitely be removed!
Fixed in devel and 3.2:
There are two references left in unsupported/test/mpreal/mpreal.h which I did not touch, as they should not influence Eigen itself in any way.