转自：http://gcc.gnu.org/ml/gcc-help/2007-09/msg00205.html
`gcc -print-prog-name=cc1plus` -v
`g++ -print-prog-name=cc1plus` -v
例如，CentOS 6.3 64bit 下，`g++ -print-prog-name=cc1plus` -v 的运行结果是：
ignoring nonexistent directory "/usr/lib/gcc/x86_64-redhat-linux/4.4.6/include-fixed"
ignoring nonexistent directory "/usr/lib/gcc/x86_64-redhat-linux/4.4.6/../../../../x86_64-redhat-linux/include"
#include "..." search starts here:
#include <...> search starts here:
/usr/lib/gcc/x86_64-redhat-linux/4.4.6/../../../../include/c++/4.4.6
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/usr/lib/gcc/x86_64-redhat-linux/4.4.6/../../../../include/c++/4.4.6/x86_64-redhat-linux
/usr/lib/gcc/x86_64-redhat-linux/4.4.6/../../../../include/c++/4.4.6/backward
/usr/local/include
/usr/lib/gcc/x86_64-redhat-linux/4.4.6/include
/usr/include
End of search list.
即，默认include路径是
/usr/include/c++/4.4.6
/usr/include/c++/4.4.6/x86_64-redhat-linux
/usr/include/c++/4.4.6/backward
/usr/local/include
/usr/lib/gcc/x86_64-redhat-linux/4.4.6/include
/usr/include