The BoostSpiritLibrary is one of the BoostLibraries. See http://www.boost.org/doc/libs/1_49_0/libs/spirit/doc/html/index.html

''Spirit is an object-oriented recursive-descent parser generator framework implemented using template meta-programming techniques. Expression templates allow us to approximate the syntax of Extended Backus-Normal Form (EBNF) completely in C++.''

''The Spirit framework enables a target grammar to be written exclusively in C++. Inline EBNF grammar specifications can mix freely with other C++ code and, thanks to the generative power of C++ templates, are immediately executable. In retrospect, conventional compiler-compilers or parser-generators have to perform an additional translation step from the source EBNF code to C or C++ code.''

It makes use of other BoostLibraries, including the BoostPhoenixLibrary.

See also CppTemplateMetaprogramming (book) which gives examples of its use.
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See BoostLibraryDiscussion
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CategoryBoost CategoryCpp CategoryMetaprogramming