Attribute Parser (attr)

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##### Attribute Parser (`attr`)

###### Description

The Attribute parser does not consume any input, for this reason it always matches an empty string and always succeeds. It's purpose is to expose its specified parameter as an attribute.

###### Header

    // forwards to <boost/spirit/home/qi/auxiliary/attr.hpp>
    #include <boost/spirit/include/qi_attr.hpp>

Also, see Include Structure.

###### Namespace

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

###### Model of

> `PrimitiveParser`

Notation

`a`

  A arbitrary typed constant value, e.g. 0.0, "Hello", or a variable of arbitrary type or a Lazy Argument that evaluates to an arbitrary type.

`A`

  The type of `a` or if it is a Lazy Argument, its return type.

###### Expression Semantics

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

Expression | Semantics
--- | ---
`attr(a)` | Create a pseudo parser exposing the current value of `a` as its attribute without consuming any input at parse time.

###### Attributes

Expression | Attribute
--- | ---
`attr(a)` | `A`

###### Complexity

> O(1)

The complexity is constant as no input is consumed and no matching is done.

###### Example

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

Some using declarations:

    namespace phx = boost::phoenix;
    using boost::spirit::qi::attr;

Using `attr` with literals:

    std::string str;
    test_parser_attr("", attr("boost"), str);
    std::cout << str << std::endl;            // will print 'boost'

    double d;
    test_parser_attr("", attr(1.0), d);
    std::cout << d << std::endl;              // will print '1.0'

Using `attr` with Boost.Phoenix function objects:

    d = 0.0;
    double d1 = 1.2;
    test_parser_attr("", attr(phx::ref(d1)), d);
    std::cout << d << std::endl;              // will print '1.2'