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本章描述C++的 Type support (basic types, RTTI, type traits)，查看原文可知，C++ type support library提供了丰富的API，我们首先需要建立高屋建瓴的视角来全面了解这些API，然后逐个深入。

cppreference Type support (basic types, RTTI, type traits)

NOTE: 概述:

章节 说明
Basic types
Runtime type identification dynamic
Type traits static

Basic types

Fundamental types defined by the language

NOTE: C++语言内置的type，可以直接使用，不需要使用std::。

Additional basic types and macros

NOTE: C++语言定义的类型，这些类型非常重要，往往是以standard library的方式提供，需要使用std::

Fixed width integer types (since C++11)

NOTE: 这是C++11新增的，对fixed width integer type进行了统一。

Numeric limits

简介
numeric_limits C++ trait 风格的API
C numeric limits interface 传统C风格的API

Runtime type identification

NOTE: RTTI特性

Type traits (since C++11)

NOTE: 关于trait，参见C++\Idiom\Generic-programming\SFINAE-trait-enable-if。

这些type trait的实现可以作为学习generic programming的素材；

Type traits defines a compile-time template-based interface to query or modify the properties of types.

Attempting to specialize a template defined in the <type_traits> header results in undefined behavior, except that std::common_type may be specialized as described in its description.

A template defined in the <type_traits> header may be instantiated with an incomplete type unless otherwise specified, notwithstanding(虽然) the general prohibition(禁止) against instantiating standard library templates with incomplete types.

NOTE: 没有读懂

Type properties

NOTE: 可以看到这些Type properties API都是以 is_打头的，显然它们都是提供给programmer进行查询的(参见C++\Language-reference\Basic-concept\Type-system\Type-operation)，它们基于了programmer static reflection的能力(C++\Language-reference\Basic-concept\Type-system\Type-operation\Reflection)。

property包括:

1) Type category

2) Type propertity

3) Supported operations

Primary type categories

NOTE: reflect/query type category

Composite type categories

NOTE: NOTE: reflect/query type category

Type properties

NOTE: reflect/query type property

Supported operations

NOTE: reflect/query operation supportted by the type

Property queries
Type relationships

NOTE: 非常重要

Type modifications

NOTE: type modification API是进行generic programming的重要功能；

从原文所总结的API来看，C++ standard library提供了add、remove一对相反的操作。

Type modification templates create new type definitions by applying modifications on a template parameter. The resulting type can then be accessed through type member typedef.

Const-volatility specifiers
References
Pointers
Sign modifiers
Arrays

Miscellaneous transformations

NOTE: 这些transformation是非常有用的；

Operations on traits

NOTE: variadic logical AND/OR/NOT metafunction

Helper classes

Constant evaluation context (since C++20)

Detects whether the call occurs within a constant-evaluated context;