I only know about vanilla Mac OS X with Apple's developer tools, but I just ran some checks on several systems that have developer tools installed.
On Leopard (Darwin 9.6.0) Apple supports both gcc 4.0 and 4.2. Both compilers use the same stdc++ libraries with the same headers. The headers seem to be in /usr/include/c++/4.0.0:
$ find /usr/include -name iostream /usr/include/c++/4.0.0/iostream /usr/include/gcc/darwin/3.3/c++/iostream
The location of these headers seems to remain the same across the SDKs for Panther, Tiger, Leopard and all versions of the iPhone OS, so I would expect that path to be accurate for any version of Darwin up to 9.6.0 that supports gcc 4.x.
In Snow Leopard (Darwin 10.0.0) the situation seems somewhat different, as there are separate headers for gcc 4.0 and 4.2:
$ find /usr/include -name iostream /usr/include/c++/4.0.0/iostream /usr/include/c++/4.2.1/iostream
I hope this helps. Please let me know if I can provide additional information.
Thanks to Marshall and Stuart, I suspect based on the information you've provided that it's going to be next to impossible to configure TR1 correctly... but I wonder do you know: * Do people commonly install Boost into a system path such as /usr/include or /usr/local/include ? * Is it possible to detect the Darwin version number (to separate 10.0 from 9.x)? * Do you know what happens if someone installs gcc from source (ie not from Apples developer tools)? Thanks! John.