Directive Testing if Parser Succeeded (matches[])

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##### Directive Testing if Parser Succeeded (`matches[]`)

###### Description

The `matches[]` directive executes the embedded parser and returns whether it succeeded matching.

###### Header

    // forwards to <boost/spirit/home/qi/directive/matches.hpp>
    #include <boost/spirit/include/qi_matches.hpp>

Also, see Include Structure.

###### Namespace

Name
`boost::spirit::matches // alias: boost::spirit::qi::matches`

###### 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
--- | ---
`matches[a]` | Execute the subject parser `a`, and return as its attribute whether it succeeded. The directive itself does always succeed.

###### Attributes

Expression | Attribute
--- | ---
`matches[a]` | `bool`

###### Complexity

> The complexity is defined by the complexity of the subject parser, `a`

###### Example

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

Some using declarations:

    using boost::spirit::qi::matches;
    using boost::spirit::qi::int_;

This parser tries to match an `int` and returns `true` a its attribute as it succeeded matching:

    bool result = false;
    test_parser_attr("345", matches[int_], result);
    std::cout << std::boolalpha << result << std::endl; // should print: true

This parser tries to match an `int` as well and returns `false` as its attribute as it fails matching:

    result = true;
    test_parser_attr("abc", matches[int_], result);
    std::cout << std::boolalpha << result << std::endl; // should print: false