...one of the most highly
regarded and expertly designed C++ library projects in the
world.
— Herb Sutter and Andrei
Alexandrescu, C++
Coding Standards
This example demonstrates:
The symbol table holds a dictionary of symbols where each symbol is a sequence
of characters (a char
, wchar_t
, int
,
enumeration etc.) . The template class, parameterized by the character
type, 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 has 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 two template parameters. The first parameter specifies the character type of the symbols. The second specifies the data type associated with each symbol: its attribute.
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_ : qi::symbols<char, unsigned> { hundreds_() { add ("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_ : qi::symbols<char, unsigned> { tens_() { add ("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_ : qi::symbols<char, unsigned> { ones_() { add ("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<Iterator> r;
At the very least, the rule needs to know the iterator type it will be
working on. This rule cannot be used with phrase_parse
.
It can only be used with the parse
function -- a version that does not do white space skipping (does not have
the skipper argument). If you want to have it skip white spaces, you need
to pass in the type skip parser, as in the next form:
rule<Iterator, Skipper> r;
Example:
rule<std::string::iterator, space_type> r;
This type of rule can be used for both phrase_parse
and parse
.
For our next example, there's one more rule form you should know about:
rule<Iterator, Signature> r;
or
rule<Iterator, Signature, Skipper> r;
Tip | |
---|---|
All rule template arguments after Iterator can be supplied in any order. |
The Signature specifies the attributes 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. Think of them as function
return values.
There's another type of attribute called "inherited" attribute. We won't need them for now, but it's good that you be aware of such attributes. You can think of them as function arguments. And, rightly so, the rule signature is a function signature of the form:
result(argN, argN,..., argN)
After having declared a rule, you can now assign any parser expression to it. Example:
r = double_ >> *(',' >> double_);
A grammar encapsulates one or more rules. It has the same template parameters as the rule. You declare a grammar by:
grammar
class template
The roman numeral grammar is a very nice and simple example of a grammar:
template <typename Iterator> struct roman : qi::grammar<Iterator, unsigned()> { roman() : roman::base_type(start) { using qi::eps; using qi::lit; using qi::_val; using qi::_1; using ascii::char_; start = eps [_val = 0] >> ( +lit('M') [_val += 1000] || hundreds [_val += _1] || tens [_val += _1] || ones [_val += _1] ) ; } qi::rule<Iterator, unsigned()> start; };
Things to take notice of:
unsigned()
. It has a synthesized attribute (return
value) of type unsigned
with no inherited attributes (arguments).
roman::base_type
is a typedef for grammar<Iterator,
unsigned()>
.
If roman
was not a
template, you could simply write: base_type(start)
_val
is another Phoenix placeholder
representing the rule's synthesized attribute.
eps
is a special spirit
parser that consumes no input but is always successful. We use it to
initialize _val
, the
rule's synthesized attribute, to zero before anything else. The actual
parser starts at +char_('M')
,
parsing roman thousands. Using eps
this way is good for doing pre and post initializations.
a ||
b
reads: match a or b and
in sequence. That is, if both a
and b
match, it must
be in sequence; this is equivalent to a
>> -b | b
, but more efficient.
bool r = parse(iter, end, roman_parser, result); if (r && iter == end) { std::cout << "-------------------------\n"; std::cout << "Parsing succeeded\n"; std::cout << "result = " << result << std::endl; std::cout << "-------------------------\n"; } else { std::string rest(iter, end); std::cout << "-------------------------\n"; std::cout << "Parsing failed\n"; std::cout << "stopped at: \": " << rest << "\"\n"; std::cout << "-------------------------\n"; }
roman_parser
is an object
of type roman
, 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: ../../example/qi/roman.cpp