Parser directives have the form: directive[expression]
A directive modifies the behavior of its enclosed expression, essentially 'decorating' it. The framework pre-defines a few directives. Clients of the framework are free to define their own directives as needed. Information on how this is done will be provided later. For now, we shall deal only with predefined directives.
Turns off white space skipping. At the phrase level, the parser ignores white spaces and all things considered as white spaces, possibly including comments as parameterized by the scanner passed into the parser's parse member function.
Situations where we want to work at the character level instead of the phrase level call for a special construct. Rules can be directed to work at the character level by enclosing the pertinent parts of the grammar inside the lexeme_d directive. For example, let us complete the example presented in the Introduction. There, we skipped the definition of the integer rule. Although its definition is quite obvious, here's how it is actually defined in the context of the framework:
integer = lexeme_d[ !(ch_p('+') | '-') >> +digit ];
The lexeme_d directive forces the parser to work on the character level. Without it, the integer rule would have allowed erroneous embedded white spaces in inputs such as "1 2 345" which will be parsed as "12345".
There are times when we want to inhibit case sensitivity. The as_lower_d directive converts all characters from the input to lower-case.
It is important to note that only the input is converted to lower case. Any parser enclosed inside the as_lower_d directive that expects any upper case characters will fail to parse anything. Example: as_lower_d['X'] will never succeed because it expects an upper case 'X' that the as_lower_d directive will never supply.
For example, in Pascal, keywords and identifiers are case insensitive. Pascal ignores the case of letters in identifiers and keywords. Thus the Pascal identifiers Id, ID and id are identical. Without the nocase directive, it would be awkward to define a rule that recognizes this:
r = str_p("id") | "Id" | "iD" | "ID";
Now, try doing that with the case insensitive Pascal keyword "BEGIN". The as_lower_d directive makes this simple:
r = as_lower_d["begin"];
The astute reader will notice that we did not explicitly wrap "begin" inside an str_p. Whenever appropriate, directives should be able to allow primitive types such as char, int, wchar_t, char const*, wchar_t const* and so on. Examples:
Alternatives in the Spirit parser compiler are short-circuited (see Operators). Sometimes, this is not what is desired. The longest_d directive instructs the parser not to short-circuit alternatives enclosed inside this directive, but instead makes the parser try all possible alternatives and choose the one matching the longest portion of the input stream.
Consider the parsing of integers and real numbers:
number = real | integer;
A number can be a real or an integer. This grammar is ambiguous. An input "1234" should potentially match both real and integer. Recall though that alternatives are short-circuited . Thus, for inputs such as above, the real alternative always wins. If we swap the alternatives:
number = integer | real;
We still have a problem. Now, an input "123.456" will be partially matched by integer until the decimal point. This is not what we want. The solution here is either to fix the ambiguity by factoring out the common prefixes of real and integer or, if that is not possible nor desired, use the longest_d directive:
number = longest_d[ integer | real ];
Opposite of the longest_d directive.
The longest_d and shortest_d directives can accept two or more alternatives. Examples:
Ensures that the result of a parser is constrained to a given min..max range (inclusive). If not, then the parser fails and returns a no-match.
This directive is particularly useful in conjunction with parsers that parse specific scalar values (for example, numeric parsers). Here's a practical example. Although the numeric parsers can be configured to accept only a limited number of digits (say, 0..2), there are no means to limit the result to a range (say -1.0..1.0). This design is deliberate. Doing so would have undermined Spirit's design rule that "the client should not pay for features that she does not use". We would have stored the min, max values in the numeric parser itself, used or unused. Well, we could get by by using static constants configured by a non-type template parameter, but that's not acceptable because that way, we can only accomodate integers. What about real numbers or user defined numbers such as big-ints?
Example, parse time of the form HH:MM:SS:
uint_parser<int, 10, 2, 2> uint2_p; r = lexeme_d [ limit_d(0u, 23u)[uint2_p] >> ':' // Hours 00..23 >> limit_d(0u, 59u)[uint2_p] >> ':' // Minutes 00..59 >> limit_d(0u, 59u)[uint2_p] // Seconds 00..59 ];
Sometimes, it would be useful to unconstrain just the maximum limit. This will allow for an interval that's unbounded to one direction. The directive min_limit_d ensures that the result of a parser is not less than minimun. If not, then the parser fails and returns a no-match.
Example, ensure that a date is not less than 1900
Opposite of min_limit_d. Thus, limit_d is equivalent to:
Copyright © 1998-2003 Joel de Guzman
Distributed under the Boost Software License, Version 1.0. (See accompanying file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)