Determine if a Type Should be Treated as a String (Qi and Karma)

is_string

The is_string customization point is a template meta-function. It is used by Qi String Literals (lit(str)), Qi Character Literals (lit(c)), Karma String Literals (lit(str)), Karma Character Literals (lit(c)) and other Spirit components. It determines whether a supplied type can be treated as a string.

Module Headers

#include <boost/spirit/home/support/string_traits.hpp>

Also, see Include Structure.

	Note
	This header file does not need to be included directly by any user program as it is normally included by other Spirit header files relying on its content.

Namespace

Name
`boost::spirit::traits`

Synopsis

template <typename T>
struct is_string
{
    <unspecified>;
};

Template parameters

Parameter	Description	Default
`T`	The type, `T` which needs to be tested as a string	none

Notation

T: An arbitrary type.
N: An arbitrary integral constant.
Char: A character type.
Traits: A character traits type.
Allocator: A standard allocator type.

Expression Semantics

Expression	Semantics
`is_string<T>::type`	Result of the metafunction that evaluates to mpl::true_ if a given type, `T`, is to be treated as a string and mpl::false_ otherwise. Generally, any implementation of `is_string` needs to behave as if if was a MPL Boolean Constant.

Predefined Specializations

Type	Semantics
`T`	Returns `mpl::false_`.
`T const`	Returns `is_string<T>`.
`char const*`	Returns `mpl::true_`.
`wchar_t const*`	Returns `mpl::true_`.
`char*`	Returns `mpl::true_`.
`wchar_t*`	Returns `mpl::true_`.
`char[N]`	Returns `mpl::true_`.
`wchar_t[N]`	Returns `mpl::true_`.
`char const[N]`	Returns `mpl::true_`.
`wchar_t const[N]`	Returns `mpl::true_`.
`char(&)[N]`	Returns `mpl::true_`.
`wchar_t(&)[N]`	Returns `mpl::true_`.
`char const(&)[N]`	Returns `mpl::true_`.
`wchar_t const(&)[N]`	Returns `mpl::true_`.
`std::basic_string<Char, Traits, Allocator>`	Returns `mpl::true_`.

When to implement

This customization point needs to be implemented to use user-defined string classes that do not correspond to std::string syntax and semantics.

Related Attribute Customization Points

If this customization point is implemented, the following other customization points need to be implemented as well.

Name	When to implement
`traits::is_char`	For string types whose underlying character type is not `char` or `wchar_t`, `is_char` must be implemented.
`traits::char_type_of`	Whenever `is_string` is implemented.
`traits::extract_c_string`	Whenever `is_string` is implemented.