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Boost Spirit X3 is an object-oriented, recursive-descent parser for C++. It allows you to write grammars using a format similar to Extended Backus Naur Form (EBNF)[1] directly in C++. These inline grammar specifications can mix freely with other C++ code and, thanks to the generative power of C++ templates, are immediately executable. Conventional compiler-compilers or parser-generators have to perform an additional translation step from the source EBNF code to C or C++ code.

Since the target input grammars are written entirely in C++ we do not need any separate tools to compile, preprocess or integrate those into the build process. Spirit allows seamless integration of the parsing process with other C++ code. This often allows for simpler and more efficient code.

The created parsers are fully attributed, which allows you to easily build and handle hierarchical data structures in memory. These data structures resemble the structure of the input data and can directly be used to generate arbitrarily-formatted output.

A simple EBNF grammar snippet:

group       ::= '(' expression ')'
factor      ::= integer | group
term        ::= factor (('*' factor) | ('/' factor))*
expression  ::= term (('+' term) | ('-' term))*

is approximated using facilities of Spirit's X3 as seen in this code snippet:

group       = '(' >> expression >> ')';
factor      = integer | group;
term        = factor >> *(('*' >> factor) | ('/' >> factor));
expression  = term >> *(('+' >> term) | ('-' >> term));

Through the magic of expression templates, this is perfectly valid and executable C++ code. The production rule expression is, in fact, an object that has a member function parse that does the work given a source code written in the grammar that we have just declared. Yes, it's a calculator. We shall simplify for now by skipping the type declarations and the definition of the rule integer invoked by factor. Now, the production rule expression in our grammar specification, traditionally called the start symbol, can recognize inputs such as:

1 + 2
1 * 2
1/2 + 3/4
1 + 2 + 3 + 4
1 * 2 * 3 * 4
(1 + 2) * (3 + 4)
(-1 + 2) * (3 + -4)
1 + ((6 * 200) - 20) / 6
(1 + (2 + (3 + (4 + 5))))

Certainly we have modified the original EBNF syntax. This is done to conform to C++ syntax rules. Most notably we see the abundance of shift >> operators. Since there are no juxtaposition operators in C++, it is simply not possible to write something like:

a b

as seen in math syntax, for example, to mean multiplication or, in our case, as seen in EBNF syntax to mean sequencing (b should follow a). Spirit.X3 uses the shift >> operator instead for this purpose. We take the >> operator, with arrows pointing to the right, to mean "is followed by". Thus we write:

a >> b

The alternative operator | and the parentheses () remain as is. The assignment operator = is used in place of EBNF's ::=. Last but not least, the Kleene star *, which in this case is a postfix operator in EBNF becomes a prefix. Instead of:

a* //... in EBNF syntax,

we write:

*a //... in Spirit.

since there are no postfix stars, *, in C/C++. Finally, we terminate each rule with the ubiquitous semi-colon, ;.