And-Predicate Parser (&a)

And-Predicate Parser (`&a`)

Description

Syntactic predicates assert a certain conditional syntax to be satisfied before evaluating another production. Similar to semantic predicates, eps, syntactic predicates do not consume any input. The and-predicate, &a, is a positive syntactic predicate that returns a zero length match only if its predicate matches.

Header

// forwards to <boost/spirit/home/qi/operator/and_predicate.hpp>
#include <boost/spirit/include/qi_and_predicate.hpp>

Also, see Include Structure.

Model of

UnaryParser

Notation

a: A Parser

Expression Semantics

Semantics of an expression is defined only where it differs from, or is not defined in UnaryParser.

Expression	Semantics
`&a`	If the predicate `a` matches, return a zero length match. Otherwise, fail.

Attributes

See Compound Attribute Notation.

Expression	Attribute
`&a`	`unused_type`

Complexity

The complexity is defined by the complexity of the predicate, a

Example

	Note
	The test harness for the example(s) below is presented in the Basics Examples section.

Some using declarations:

using boost::spirit::lit;

Basic look-ahead example: make sure that the last character is a semicolon, but don't consume it, just peek at the next character:

test_phrase_parser("Hello ;", lit("Hello") >> &lit(';'), false);