There are two licenses affecting GNU libstdc++: one for the code, and one for the documentation. Here we will describe both of them, and try to answer some of the widespread questions. If you have more questions, ask the FSF or the gcc mailing list; the person writing this page is a programmer, not a lawyer.
The source code of libstdc++ is distributed under version 2 of the GNU General Public License, with the so-called "runtime exception," as follows (or see any header or implementation file):
As a special exception, you may use this file as part of a free software library without restriction. Specifically, if other files instantiate templates or use macros or inline functions from this file, or you compile this file and link it with other files to produce an executable, this file does not by itself cause the resulting executable to be covered by the GNU General Public License. This exception does not however invalidate any other reasons why the executable file might be covered by the GNU General Public License.
Hopefully that text is self-explanatory. If it isn't, you need to speak to your lawyer, or the Free Software Foundation.
Q: So any program which uses libstdc++ falls under the GPL?
A: No. The special exception permits use of the
library in proprietary applications.
Q: How is that different from the GNU {Lesser,Library}
GPL?
A: The LGPL requires that users be able to replace the LGPL code with a
modified version; this is trivial if the library in question is a C
shared library. But there's no way to make that work with C++, where
much of the library consists of inline functions and templates, which
are expanded inside the code that uses the library. So to allow people
to replace the library code, someone using the library would have to
distribute their own source, rendering the LGPL equivalent to the GPL.
Q: I see. So, what restrictions are there on
programs that use the library?
A: None. We encourage such programs to be released as open source,
but we won't punish you or sue you if you choose otherwise.
The documentation shipped with the library and made available over the web, excluding the pages generated from source comments, are copyrighted by the Free Software Foundation, and placed under the GNU Free Documentation License version 1.1. There are no Front-Cover Texts, no Back-Cover Texts, and no Invariant Sections.
For documentation generated by doxygen or other automated tools via processing source code comments and markup, the original source code license applies to the generated files. Thus, the doxygen documents are licensed GPL.
If you plan on making copies of the documentation, please let us know. We can probably offer suggestions.
Comments and suggestions about this page are welcome, and may be sent to the libstdc++ mailing list. Comments or questions about the licenses themselves are also welcome, and should be directed to the GCC list as descibed above.