Not Predicate (!a)

Not Predicate (`!a`)

Description

The not predicate generator is used to test, whether the embedded generator fails, without generating any output. It succeeds if the embedded generator fails.

Header

// forwards to <boost/spirit/home/karma/operator/not_predicate.hpp>
#include <boost/spirit/include/karma_not_predicate.hpp>

Also, see Include Structure.

Model of

UnaryGenerator

Expression Semantics

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

Expression	Semantics
`!a`	The generator `a` is executed for the sole purpose of testing whether it succeeds. The not predicate generator succeeds if its embedded generator fails (unless the underlying output stream reports an error). The not predicate never produces any output.

The not generator is implemented by redirecting all output produced by its embedded generator into a discarding device.

Attributes

See Compound Attribute Notation.

Expression	Attribute
`!a` (not predicate, unary `!`)	`a: A --> !a: A`

	Note
	The attribute of the not predicate is not always `unused_type`, which is different from Qi's not predicate. This is necessary as the generator the and predicate is attached to most of the time needs an attribute.

Complexity

The overall complexity of the not predicate generator is defined by the complexity of its embedded generator. The complexity of the not predicate generator itself is O(1).

Example

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

Some includes:

#include <boost/spirit/include/karma.hpp>
#include <boost/spirit/include/phoenix_core.hpp>
#include <boost/spirit/include/phoenix_operator.hpp>
#include <boost/fusion/include/std_pair.hpp>
#include <iostream>
#include <string>

Some using declarations:

using boost::spirit::karma::double_;
using boost::spirit::karma::ascii::char_;
using boost::spirit::karma::ascii::string;
using boost::phoenix::ref;

Basic usage of a not predicate generator:

test_generator_attr("c", !char_('a') << 'b' | 'c', 'a');
test_generator_attr("b", !char_('a') << 'b' | 'c', 'x');

test_generator_attr("def", !string("123") << "abc" | "def", "123");
test_generator_attr("abc", !string("123") << "abc" | "def", "456");