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## Introduction

Boost Spirit is an object oriented, recursive-descent parser and output generation library for C++. It allows to write grammars and format descriptions using a format very similar to EBNF (Extended Backus Naur Form, see [4]) 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. 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.

The syntax and semantics of the libraries API directly form domain specific embedded languages (DSEL). In fact, Spirit exposes 3 different DSEL's to the user:

- one for creating parser grammars,
- one for the specification of the required tokens to be used for parsing,
- and one for the description of the required output formats.

Since the target input grammars and output formats 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 and output generation process with other C++ code. Often this allows for simpler and more efficient code.

Both, the created parsers and generators, are fully attributed which allows 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.

The figure below depicts the overall structure of the Boost Spirit library. The library consists out of 4 major parts:

- Spirit.Classic: This is the almost unchanged code base taken from the former Boost Spirit V1.8 distribution. It has been moved into the namespace boost::spirit::classic. A special compatibility layer has been added to ensure complete compatibility with existing code using Spirit V1.8.
- Spirit.Qi: This is the parser library allowing to build recursive descent parsers. The exposed domain specific language can be used to describe the grammars to implement, and the rules for storing the parsed information.
- Spirit.Lex: This is the library usable to create tokinizers (lexers). The domain specific language exposed by Spirit.Lex
- Spirit.Karma: This is the generator library allowing to create code for recursive descent, data type driven output formatting. The exposed domain specific language is almost equivalent to the parser description language used in Spirit.Qi, except that it is used to describe the required output format to generate from a given data structure.

Figure 1. The overall structure of the Boost Spirit library

The separate sublibraries Spirit.Qi, Spirit.Karma and Spirit.Lex are well integrated with any of the other parts. Because of their similar structure and identical underlying technology these are usable either separately or together at the same time. For instance is it possible to directly feed the hierarchical data structures generated by Spirit.Qi into output generators created using Spirit.Karma; or to use the token sequence generated by Spirit.Lex as the input for a parser generated by Spirit.Qi.

The figure below shows the typical data flow of some input being converted to some internal representation. After some (optional) transformation this data is converted back into some different, external representation. The picture highlights Spirit's the place in this data transformation flow.

Figure 2. The place of Spirit.Qi and Spirit.Karma in a data transformation flow of a typical application

### A quick overview about Parsing with Spirit.Qi

Spirit.Qi is Spirit's sublibrary dealing with generating parsers based on a given target grammar (essentially a format description of the input data to read).

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 Qi sublibrary 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:

    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 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). Spirit 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, `;`.

### A quick overview about Output Generation with Spirit.Karma

Spirit not only allows to describe the structure of the input. Starting with Version 2.0 it enables the specification of the output format for your data in a very similar way, and based on a single syntax and compatible semantics.

Let's assume we need to generate a textual representation from a simple data structure as a `std::vector<int>`. Conventional code probably would look like:

    std::vector<int> v (initialize_and_fill());
    std::vector<int>::iterator end = v.end();
    for (std::vector<int>::iterator it = v.begin(); it != end; ++it)
        std::cout << *it << std::endl;

which is not very flexible and quite difficult to maintain when it comes to changing the required output format. Spirit's sublibrary Karma allows to specify output formats for arbitrary data structures in a very flexible way. following snippet is the Karma format description used to create the very The same output as the traditional code above:

    *(int_ << eol)

Here are some more examples of format descriptions for different output representations of the same `std::vector<int>`:

Table 2. Different output formats for `std::vector<int>`

Format | Example | Description
--- | --- | ---
`'[' << *(int_ << ',') << ']'` | `[1,8,10,]` | Comma separated list of integers
`*('(' << int_ << ')' << ',')` | `(1),(8),(10),]` | Comma separated list of integers in parenthesis
`*hex` | `18a` | A list of hexadecimal numbers
`*(double_ << ',')` | `1.0,8.0,10.0,` | A list of floating point numbers

The syntax is very similar to Qi with the exception that we use the `<<` operator for output concatenation. This should be easy to understand as it follows the conventions used in the Standard's I/O streams.

Another important feature of karma is to allow to fully decouple the data type from the output format. You can use the same output format with different data types as long as these conforma conceptually. The next table gives some related examples.

Table 3. Different data types usable with the output format `(*int_ << eol)`

Data type |
`int i[4]` | C style arrays
`std::vector<int>` | Standard vector
`std::list<int>` | Standard list
`boost::array<long, 20>` | Boost array