| Introduction |

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.

A simple EBNF grammar snippet:

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

is approximated using Spirit's facilities 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`. The production rule `expression` in our grammar specification, traditionally called the start symbol, can recognize inputs such as:

       12345
       -12345
       +12345
       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 done some modifications to the original EBNF syntax. This is done so as to conform to C++ syntax rules. Most notably we see the abundance of shift `>>` operators. Since there are no 'empty' 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). The framework 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 used to be 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, "`;`".

Copyright © 1998-2003 Joel de Guzman
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