Boolean Generators (bool_)

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##### Boolean Generators (`bool_`)

###### Description

As you might expect, the `bool_generator` can generate output from boolean values. The `bool_generator` generator can be used to generate output from ordinary primitive C/C++ `bool` values or user defined boolean types if the type follows certain expression requirements (for more information about the requirements, see below)). The `bool_generator` is a template class. Template parameters fine tune its behavior.

###### Header

    // forwards to <boost/spirit/home/karma/numeric/bool.hpp>
    #include <boost/spirit/include/karma_bool.hpp>

Also, see Include Structure.

###### Namespace

Name
`boost::spirit::lit // alias: boost::spirit::karma::lit`
`boost::spirit::bool_ // alias: boost::spirit::karma::bool_`
`boost::spirit::true_ // alias: boost::spirit::karma::true_`
`boost::spirit::false_ // alias: boost::spirit::karma::false_`

| Note
`lit` is reused by the String Generators, the Character Generators, and the Numeric Generators. In general, a char generator is created when you pass in a character, a string generator is created when you pass in a string, and a numeric generator is created when you use a numeric (boolean) literal.

###### Synopsis

    template <
        typename B
      , unsigned Policies>
    struct bool_generator;

###### Template parameters

Parameter | Description | Default
--- | --- | ---
`B` | The boolean base type of the boolean generator. | `bool`
`Policies` | The policies to use while converting the boolean. | `bool_policies<B>`

###### Model of

> `PrimitiveGenerator`

Notation

`b`

  Boolean literal, or a Lazy Argument that evaluates to a boolean value of type `B`

`B`

  Type of `b`: any type usable as a boolean, or in case of a Lazy Argument, its return value

###### Expression Semantics

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

Expression | Semantics
--- | ---
`lit(b)` | Generate the boolean literal `b` using the default formatting (`false` is generated as `"false"`, and `true` is generated as `"true"`). This generator never fails (unless the underlying output stream reports an error).
`bool_` | Generate the boolean value provided by a mandatory attribute using the default formatting (`false` is generated as `"false"`, and `true` is generated as `"true"`). This generator never fails (unless the underlying output stream reports an error).
`bool_(b)` | Generate the booelan value provided by the immediate literal value the generator is initialized from using the default formatting (`false` is generated as `"false"`, and `true` is generated as `"true"`). If this generator has an associated attribute it succeeds only if the attribute is equal to the immediate literal (unless the underlying output stream reports an error). Otherwise this generator fails and does not generate any output.
`true_` | Generate `"true"`. If this generator has an associated attribute it succeeds only if the attribute is `true` as well (unless the underlying output stream reports an error).
`false_` | Generate `"false"`. If this generator has an associated attribute it succeeds only if the attribute is `false` as well (unless the underlying output stream reports an error).

All generators listed in the table above (except `lit(num)`) are predefined specializations of the `bool_generator<B, Policies>` basic boolean generator type described below. It is possible to directly use this type to create boolean generators using a wide range of formatting options.

Expression | Semantics
--- | ---

    bool_generator<
        B, Policies
    >()

Generate the boolean of type `B` provided by a mandatory attribute using the specified `Policies` This generator never fails (unless the underlying output stream reports an error).

    bool_generator<
        B, Policies
    >()(b)

Generate the boolean of type `B` provided by the immediate literal value the generator is initialized from, using the specified `Policies`. If this generator has an associated attribute it succeeds only if the attribute is equal to the immediate literal (unless the underlying output stream reports an error). Otherwise this generator fails and does not generate any output.

| Note
All boolean generators properly respect the `upper` and `lower` directives.

###### Additional Requirements

The following lists enumerate the requirements which must be met in order to use a certain type `B` to instantiate and use a `bool_generator<B, Policies>`.

The type `B`:

- must be (safely) convertible to `bool`

###### Attributes

Expression | Attribute
--- | ---
`bool_(b)` | `unused`
`bool_` | `bool`, attribute is mandatory (otherwise compilation will fail)
`bool_(b)` | `bool`, attribute is optional, if it is supplied, the generator compares the attribute with `b` and succeeds only if both are equal, failing otherwise.

    bool_generator<
        B, Policies
    >()

`B`, attribute is mandatory (otherwise compilation will fail)

    bool_generator<
        B, Policies
    >()(b)

`B`, attribute is optional, if it is supplied, the generator compares the attribute with `b` and succeeds only if both are equal, failing otherwise.

| Note
In addition to their usual attribute of type `B` all listed generators accept an instance of a `boost::optional<B>` as well. If the `boost::optional<>` is initialized (holds a value) the generators behave as if their attribute was an instance of `B` and emit the value stored in the `boost::optional<>`. Otherwise the generators will fail.

###### Boolean Formatting Policies

If special formatting of a boolean is needed, overload the policy class `bool_policies<B>` and use it as a template parameter to the `bool_generator<>` boolean generator. For instance:

    struct special_bool_policy : karma::bool_policies<>
    {
        template <typename CharEncoding, typename Tag
          , typename OutputIterator>
        static bool generate_false(OutputIterator& sink, bool b)
        {
            // we want to spell the names of false as eurt (true backwards)
            return string_inserter<CharEncoding, Tag>::call(sink, "eurt");
        }
    };

    typedef karma::bool_generator<special_bool_policy> backwards_bool_type;
    backwards_bool_type const backwards_bool;

    karma::generate(sink, backwards_bool, true);    // will output: true
    karma::generate(sink, backwards_bool(false));   // will output: uert

The template parameter `B` should be the type to be formatted using the overloaded policy type. At the same time `B` will be used as the attribute type of the created real number generator. The default for `B` is `bool`.

###### Booelan Formatting Policy Expression Semantics

A boolean formatting policy should expose the following:

Expression | Description
--- | ---

    template <typename Inserter
      , typename OutputIterator
      , typename Policies>
    bool call (OutputIterator& sink, Num n
      , Policies const& p);

This is the main function used to generate the output for a boolean. It is called by the boolean generator in order to perform the conversion. In theory all of the work can be implemented here, but the easiest way is to use existing functionality provided by the type specified by the template parameter `Inserter`. The default implementation of this functions is:

    template <typename Inserter, typename OutputIterator
      , typename Policies>
    static bool
    call (OutputIterator& sink, B b, Policies const& p)
    {
        return Inserter::call_n(sink, b, p);
    }

`sink` is the output iterator to use for generation

`b` is the boolean to convert

`p` is the instance of the policy type used to instantiate this real number generator.

    template <typename CharEncoding,
        typename Tag, typename OutputIterator>
    bool generate_false(
        OutputIterator& sink, B b);

This function is called to generate the boolean if it is `false`.

`sink` is the output iterator to use for generation

`b` is the boolean to convert (the value is `false`).

The template parameters `CharEncoding` and `Tag` are either of the type `unused_type` or describe the character class and conversion to be applied to any output possibly influenced by either the `lower[]` or `upper[]` directives.

The return value defines the outcome of the whole generator.

    template <typename CharEncoding,
        typename Tag, typename OutputIterator>
    bool generate_true(
        OutputIterator& sink, B b);

This function is called to generate the boolean if it is `true`.

`sink` is the output iterator to use for generation

`b` is the boolean to convert (the value is `true`).

The template parameters `CharEncoding` and `Tag` are either of the type `unused_type` or describe the character class and conversion to be applied to any output possibly influenced by either the `lower[]` or `upper[]` directives.

The return value defines the outcome of the whole generator.

###### Complexity

> O(N), where `N` is the number of characters needed to represent the generated boolean.

###### 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::bool_;
    using boost::spirit::karma::lit;

Basic usage of an `bool_` generator:

    test_generator("true", lit(true));
    test_generator("false", bool_(false));
    test_generator_attr("true", bool_(true), true);
    test_generator_attr("", bool_(true), false);    // fails (as true != false)!
    test_generator_attr("false", bool_, false);