Alternative Parser (a | b)

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##### Alternative Parser (`a | b`)

###### Description

The alternative operator, `a | b`, matches one of two or more operands (`a`, `b`, ... etc.):

    a | b | ...

Alternative operands are tried one by one on a first-match-wins basis starting from the leftmost operand. After a successfully matched alternative is found, the parser concludes its search, essentially short-circuiting the search for other potentially viable candidates. This short-circuiting implicitly gives the highest priority to the leftmost alternative.

Short-circuiting is done in the same manner as C or C++'s logical expressions; e.g. `if (x < 3 || y < 2)` where, if `x < 3`, the `y < 2` test is not done at all. In addition to providing an implicit priority rule for alternatives which is necessary, given its non-deterministic nature, short-circuiting improves the execution time. If the order of your alternatives is logically irrelevant, strive to put the (expected) most common choice first for maximum efficiency.

###### Header

    // forwards to <boost/spirit/home/qi/operator/alternative.hpp>
    #include <boost/spirit/include/qi_alternative.hpp>

Also, see Include Structure.

###### Model of

> `NaryParser`

Notation

`a`, `b`

  A `Parser`

###### Expression Semantics

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

Expression | Semantics
--- | ---
`a | b` | Match `a` or `b`.

###### Attributes

See Compound Attribute Notation.

Expression | Attribute
--- | ---
`a | b` |
    a: A, b: B --> (a | b): variant<A, B>
    a: A, b: Unused --> (a | b): optional<A>
    a: A, b: B, c: Unused --> (a | b | c): optional<variant<A, B> >
    a: Unused, b: B --> (a | b): optional<B>
    a: Unused, b: Unused --> (a | b): Unused
    a: A, b: A --> (a | b): A

| Note
Alternative parsers do not roll back changes made to the outer attribute because of a failed alternative. If you need to enforce that only the succeeded alternative changes the outer attribute please utilize the directive `hold``[]`.

###### Complexity

> The overall complexity of the alternative parser is defined by the sum of the complexities of its elements. The complexity of the alternative parser itself is O(N), where N is the number of alternatives.

###### Example

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

Some using declarations:

    using boost::spirit::ascii::string;
    using boost::spirit::qi::int_;
    using boost::spirit::qi::_1;
    using boost::variant;

Simple usage:

    test_parser("Hello", string("Hello") | int_);
    test_parser("123", string("Hello") | int_);

Extracting the attribute variant (using Boost.Variant):

    variant<std::string, int> attr;
    test_parser_attr("Hello", string("Hello") | int_, attr);

This should print `"Hello"`. Note: There are better ways to extract the value from the variant. See Boost.Variant visitation. This code is solely for demonstration.

    if (boost::get<int>(&attr))
        std::cout << boost::get<int>(attr) << std::endl;
    else
        std::cout << boost::get<std::string>(attr) << std::endl;

Extracting the attributes using Semantic Actions with Boost.Phoenix (this should print `123`):

    test_parser("123", (string("Hello") | int_)[std::cout << _1 << std::endl]);