value_traits.qbk

来自「Boost provides free peer-reviewed portab」· QBK 代码 · 共 195 行

[/ 
  Copyright 2007 John Maddock.
  Distributed under the Boost Software License, Version 1.0.
  (See accompanying file LICENSE_1_0.txt or copy at
  http://www.boost.org/LICENSE_1_0.txt).
]

[section:value_traits Type Traits that Describe the Properties of a Type]

These traits are all /value traits/, which is to say the traits classes all
inherit from __integral_constant, and are used to access some numerical
property of a type.  Often this is a simple true or false Boolean value, 
but in a few cases may be some other integer value (for example when dealing
with type alignments, or array bounds: see `__alignment_of`, `__rank` and `__extent`).

[section:primary Categorizing a Type]

These traits identify what "kind" of type some type `T` is.  These are split into
two groups: primary traits which are all mutually exclusive, and composite traits
that are compositions of one or more primary traits.

For any given type, exactly one primary type trait will inherit from
__true_type, and all the others will inherit from __false_type, in other
words these traits are mutually exclusive.
 
This means that `__is_integral<T>::value` and `__is_floating_point<T>::value` 
will only ever be true for built-in types; if you want to check for a 
user-defined class type that behaves "as if" it is an integral or floating point type, 
then use the `std::numeric_limits template` instead.

[*Synopsis:]

   template <class T>
   struct __is_array;
     
   template <class T>
   struct __is_class;
   
   template <class T>
   struct __is_complex;
     
   template <class T>
   struct __is_enum;
     
   template <class T>
   struct __is_floating_point;
     
   template <class T>
   struct __is_function;

   template <class T>
   struct __is_integral;
     
   template <class T>
   struct __is_member_function_pointer;
     
   template <class T>
   struct __is_member_object_pointer;
     
   template <class T>
   struct __is_pointer;
     
   template <class T>
   struct __is_reference;
     
   template <class T>
   struct __is_union;
     
   template <class T>
   struct __is_void;
     
The following traits are made up of the union of one or more type
categorizations. A type may belong to more than one of these categories, 
in addition to one of the primary categories.

   template <class T>
   struct __is_arithmetic;
   
   template <class T>
   struct __is_compound;
   
   template <class T>
   struct __is_fundamental;
   
   template <class T>
   struct __is_member_pointer;
   
   template <class T>
   struct __is_object;
   
   template <class T>
   struct __is_scalar;
   
[endsect]

[section:properties General Type Properties]

The following templates describe the general properties of a type.

[*Synopsis:]

   template <class T>
   struct __alignment_of;

   template <class T>
   struct __has_nothrow_assign;

   template <class T>
   struct __has_nothrow_constructor;

   template <class T>
   struct __has_nothrow_default_constructor;

   template <class T>
   struct __has_nothrow_copy;

   template <class T>
   struct __has_nothrow_copy_constructor;

   template <class T>
   struct __has_trivial_assign;

   template <class T>
   struct __has_trivial_constructor;

   template <class T>
   struct __has_trivial_default_constructor;

   template <class T>
   struct __has_trivial_copy;

   template <class T>
   struct __has_trivial_copy_constructor;

   template <class T>
   struct __has_trivial_destructor;

   template <class T>
   struct __has_virtual_destructor;

   template <class T>
   struct __is_abstract;

   template <class T>
   struct __is_const;

   template <class T>
   struct __is_empty;

   template <class T>
   struct __is_stateless;

   template <class T>
   struct __is_pod;

   template <class T>
   struct __is_polymorphic;
   
   template <class T>
   struct __is_signed;
   
   template <class T>
   struct __is_unsigned;

   template <class T>
   struct __is_volatile;

   template <class T, std::size_t N = 0>
   struct __extent;

   template <class T>
   struct __rank;

[endsect]

[section:relate Relationships Between Two Types]

These templates determine the whether there is a relationship 
between two types:

[*Synopsis:]

   template <class Base, class Derived>
   struct __is_base_of;

   template <class From, class To>
   struct __is_convertible;

   template <class T, class U>
   struct __is_same;
   
[endsect]

[endsect]