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Roman Numerals

This example demonstrates:

Symbol Table

The symbol table holds a dictionary of symbols where each symbol is a sequence of characters. The template class, can work efficiently with 8, 16, 32 and even 64 bit characters. Mutable data of type T are associated with each symbol.

Traditionally, symbol table management is maintained separately outside the BNF grammar through semantic actions. Contrary to standard practice, the Spirit symbol table class symbols is a parser. An object of which may be used anywhere in the EBNF grammar specification. It is an example of a dynamic parser. A dynamic parser is characterized by its ability to modify its behavior at run time. Initially, an empty symbols object matches nothing. At any time, symbols may be added or removed, thus, dynamically altering its behavior.

Each entry in a symbol table may have an associated mutable data slot. In this regard, one can view the symbol table as an associative container (or map) of key-value pairs where the keys are strings.

The symbols class expects one template parameter to specify the data type associated with each symbol: its attribute. There are a couple of namespaces in X3 where you can find various versions of the symbols class for handling different character encoding including ascii, standard, standard_wide, iso8859_1, and unicode. The default symbol parser type in the main x3 namespace is standard.

Here's a parser for roman hundreds (100..900) using the symbol table. Keep in mind that the data associated with each slot is the parser's attribute (which is passed to attached semantic actions).

struct hundreds_ : x3::symbols<unsigned>
            ("C"    , 100)
            ("CC"   , 200)
            ("CCC"  , 300)
            ("CD"   , 400)
            ("D"    , 500)
            ("DC"   , 600)
            ("DCC"  , 700)
            ("DCCC" , 800)
            ("CM"   , 900)

} hundreds;

Here's a parser for roman tens (10..90):

struct tens_ : x3::symbols<unsigned>
            ("X"    , 10)
            ("XX"   , 20)
            ("XXX"  , 30)
            ("XL"   , 40)
            ("L"    , 50)
            ("LX"   , 60)
            ("LXX"  , 70)
            ("LXXX" , 80)
            ("XC"   , 90)

} tens;

and, finally, for ones (1..9):

struct ones_ : x3::symbols<unsigned>
            ("I"    , 1)
            ("II"   , 2)
            ("III"  , 3)
            ("IV"   , 4)
            ("V"    , 5)
            ("VI"   , 6)
            ("VII"  , 7)
            ("VIII" , 8)
            ("IX"   , 9)

} ones;

Now we can use hundreds, tens and ones anywhere in our parser expressions. They are all parsers.


Up until now, we've been inlining our parser expressions, passing them directly to the phrase_parse function. The expression evaluates into a temporary, unnamed parser which is passed into the phrase_parse function, used, and then destroyed. This is fine for small parsers. When the expressions get complicated, you'd want to break the expressions into smaller easier-to-understand pieces, name them, and refer to them from other parser expressions by name.

A parser expression can be assigned to what is called a "rule". There are various ways to declare rules. The simplest form is:

rule<ID> const r = "some-name";
Rule ID

At the very least, the rule needs an identification tag. This ID can be any struct or class type and need not be defined. Forward declaration would suffice. In subsequent tutorials, we will see that the rule ID can have additional functionalities for error handling and annotation.

Rule Name

The name is optional, but is useful for debugging and error handling, as we'll see later. Notice that rule r is declared const. Rules are immutable and are best declared as const. Rules are lightweight and can be passed around by value. Its only member variable is a std::string: its name.

[Note] Note

Unlike Qi (Spirit V2), X3 rules can be used with both phrase_parse and parse without having to specify the skip parser

Rule Attributes

For our next example, there's one more rule form you should know about:

rule<ID, Attribute> const r = "some-name";

The Attribute parameter specifies the attribute type of the rule. You've seen that our parsers can have an attribute. Recall that the double_ parser has an attribute of double. To be precise, these are synthesized attributes. The parser "synthesizes" the attribute value. If the parser is a function, think of them as function return values.

Rule Definition

After having declared a rule, you need a definition for the rule. Example:

auto const r_def = double_ >> *(',' >> double_);

By convention, rule definitions have a _def suffix. Like rules, rule definitions are immutable and are best declared as const.


Now that we have a rule and its definition, we tie the rule with a rule definition using the BOOST_SPIRIT_DEFINE macro:


Behind the scenes, what's actually happening is that we are defining a parse_rule function in the client namespace that tells X3 how to invoke the rule. And so for each rule defined using BOOST_SPIRIT_DEFINE, there is an overloaded parse_rule function. At parse time, Spirit X3 recursively calls the appropriate parse_rule function.

[Note] Note

BOOST_SPIRIT_DEFINE is variadic and may be used for one or more rules. Example: BOOST_SPIRIT_DEFINE(r1, r2, r3);


Unlike Qi (Spirit V2), X3 discards the notion of a grammar as a concrete entity for encapsulating rules. In X3, a grammar is simply a logical group of rules that work together, typically with a single top-level start rule which serves as the main entry point. X3 grammars are grouped using namespaces. The roman numeral grammar is a very nice and simple example of a grammar:

namespace parser
    using x3::eps;
    using x3::lit;
    using x3::_val;
    using x3::_attr;
    using ascii::char_;

    auto set_zero = [&](auto& ctx){ _val(ctx) = 0; };
    auto add1000 = [&](auto& ctx){ _val(ctx) += 1000; };
    auto add = [&](auto& ctx){ _val(ctx) += _attr(ctx); };

    x3::rule<class roman, unsigned> const roman = "roman";

    auto const roman_def =
        eps                 [set_zero]
            -(+lit('M')     [add1000])
            >>  -hundreds   [add]
            >>  -tens       [add]
            >>  -ones       [add]


Things to take notice of:

x3::rule<class roman, unsigned> const roman = "roman";
Let's Parse!
bool r = parse(iter, end, roman, result);

if (r && iter == end)
    std::cout << "-------------------------\n";
    std::cout << "Parsing succeeded\n";
    std::cout << "result = " << result << std::endl;
    std::cout << "-------------------------\n";
    std::string rest(iter, end);
    std::cout << "-------------------------\n";
    std::cout << "Parsing failed\n";
    std::cout << "stopped at: \": " << rest << "\"\n";
    std::cout << "-------------------------\n";

roman is our roman numeral parser. This time around we are using the no-skipping version of the parse functions. We do not want to skip any spaces! We are also passing in an attribute, unsigned result, which will receive the parsed value.

The full cpp file for this example can be found here: roman.cpp