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Optional (-a)
Description

The optional operator, -a, is a unary operator that matches its operand zero or one time.

Header
// forwards to <boost/spirit/home/qi/operator/optional.hpp>
#include <boost/spirit/include/qi_optional.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

Match a zero or one time.

Attributes

See Compound Attribute Notation.

Expression

Attribute

-a

a: A --> -a: optional<A>
a: Unused --> -a: Unused

Complexity

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

Example
[Note] Note

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

Some using declarations:

using boost::spirit::ascii::char_;
using boost::spirit::qi::lexeme;
using boost::spirit::qi::int_;
using boost::fusion::vector;
using boost::fusion::at_c;
using boost::optional;

Parse a person info with name (in quotes) optional age [9] and optional sex, all separated by comma.

vector<std::string, optional<int>, optional<char> > attr;

test_phrase_parser_attr(
    "\"James Bond\", M"
  , lexeme['"' >> +(char_ - '"') >> '"']    // name
        >> -(',' >> int_)                   // optional age
        >> -(',' >> char_)                  // optional sex
  , attr);

// Should print: James Bond,M
std::cout << at_c<0>(attr);                 // print name
if (at_c<1>(attr))                          // print optional age
    std::cout << ',' << *at_c<1>(attr);
if (at_c<2>(attr))                          // print optional sex
    std::cout << ',' << *at_c<2>(attr);
std::cout << std::endl;        



[9] James Bond is shy about his age :-)


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