STL源码剖析 [特殊的空间配置器]（机智的type_traits.h）



       Traits就是通过定义一些结构体或类，并利用模板类特化和偏特化的能力，给类型赋予一些特性，这些特性根据类型的不同而异。traits的编程技巧极度弥补了C++语言的不足 traits含义就是特性，应用Trait模板参数，使得我们的程序既保持灵活性，同时减少类型参数的数量。能够使得我们对函数进行更加细粒度的控制。traits技术依靠显示模版特殊化来把代码中因类型不同而发生变化的片段拖出来，用统一的接口来包装。这个接口可以包含一个C++类所能包含的任何东西，如内嵌类型、成员函数、成员变量。作为客户的模版代码，可以通过traits模版类所公开的接口来简洁访问。 STL中机智的部分就是type_traits.h对于traits的完美使用。不知道traits的机智在哪里可以详细读读这几篇文章。

http://www.cnblogs.com/pugang/archive/2012/10/17/2727378.html

http://www.cppblog.com/woaidongmao/archive/2008/11/09/66387.html

http://www.cnblogs.com/youthlion/archive/2012/01/19/2271939.html

http://www.boost.org/doc/libs/1_31_0/libs/type_traits/c++_type_traits.htm

type_traits.h的源码：

// Filename:    type_traits.h

// Comment By:  凝霜
// E-mail:      [email protected]
// Blog:        http://blog.csdn.net/mdl13412

/*
 *
 * Copyright (c) 1997
 * Silicon Graphics Computer Systems, Inc.
 *
 * Permission to use, copy, modify, distribute and sell this software
 * and its documentation for any purpose is hereby granted without fee,
 * provided that the above copyright notice appear in all copies and
 * that both that copyright notice and this permission notice appear
 * in supporting documentation.  Silicon Graphics makes no
 * representations about the suitability of this software for any
 * purpose.  It is provided "as is" without express or implied warranty.
 */

#ifndef __TYPE_TRAITS_H
#define __TYPE_TRAITS_H

#ifndef __STL_CONFIG_H
#include <stl_config.h>
#endif

// 这个头文件提供了一个框架, 允许编译器在编译期根据类型属性派发函数的机制
// 这个框架对于编写模板代码非常有用
// 例如, 当我们需要拷贝一个未知类型的array, 它能帮助我们得知这个未知类型
// 是否具有一个trivial copy constructor, 以帮助我们决定memcpy()是否能使用

// __type_traits模板类提供了一些列的typedefs, 其值是__true_type或者__false_type
// __type_traits模板类可以接受任何类型的参数
// 模板类中的typedefs可以通过以下手段获取其正确值
//   1. general instantiation, 对于所有类型都要有保守值
//   2. 经过特化的版本
//   3. 一些编译器(例如Silicon Graphics N32 and N64 compilers)
//      会自动给所有类型提供合适的特化版本
//
// 例子:
// Copy an array of elements which have non-trivial copy constructors
// template <class T> void copy(T* source,T* destination,int n,__false_type);
// Copy an array of elements which have trivial copy constructors. Use memcpy.
// template <class T> void copy(T* source,T* destination,int n,__true_type);
//
// Copy an array of any type by using the most efficient copy mechanism
// template <class T> inline void copy(T* source,T* destination,int n)
// {
//      copy(source,destination,n,
//              typename __type_traits<T>::has_trivial_copy_constructor());
// }

struct __true_type
{
};

struct __false_type
{
};

template <class type>
struct __type_traits
{
    // 不要移除这个成员
    // 它通知能自动特化__type_traits的编译器, 现在这个__type_traits template是特化的
    // 这是为了确保万一编译器使用了__type_traits而与此处无任何关联的模板时
    // 一切也能顺利运作
   typedef __true_type     this_dummy_member_must_be_first;

   // 以下条款应当被遵守, 因为编译器有可能自动生成类型的特化版本
   //   - 你可以重新安排的成员次序
   //   - 你可以移除你想移除的成员
   //   - 一定不可以修改下列成员名称, 却没有修改编译器中的相应名称
   //   - 新加入的成员被当作一般成员, 除非编译器提供特殊支持

   typedef __false_type    has_trivial_default_constructor;
   typedef __false_type    has_trivial_copy_constructor;
   typedef __false_type    has_trivial_assignment_operator;
   typedef __false_type    has_trivial_destructor;
   typedef __false_type    is_POD_type;
};


// 以下针对C++内置的基本数据类型提供特化版本, 使其具有trivial default constructor,
// copy constructor, assignment operator, destructor
// 并标记其为POD类型
//
// 特化类型:
//         char, signed char, unsigned char,
//         short, unsigned short
//         int, unsigned int
//         long, unsigned long
//         float, double, long double

// Provide some specializations.  This is harmless for compilers that
//  have built-in __types_traits support, and essential for compilers
//  that don't.

__STL_TEMPLATE_NULL struct __type_traits<char>
{
   typedef __true_type    has_trivial_default_constructor;
   typedef __true_type    has_trivial_copy_constructor;
   typedef __true_type    has_trivial_assignment_operator;
   typedef __true_type    has_trivial_destructor;
   typedef __true_type    is_POD_type;
};

__STL_TEMPLATE_NULL struct __type_traits<signed char>
{
   typedef __true_type    has_trivial_default_constructor;
   typedef __true_type    has_trivial_copy_constructor;
   typedef __true_type    has_trivial_assignment_operator;
   typedef __true_type    has_trivial_destructor;
   typedef __true_type    is_POD_type;
};

__STL_TEMPLATE_NULL struct __type_traits<unsigned char>
{
   typedef __true_type    has_trivial_default_constructor;
   typedef __true_type    has_trivial_copy_constructor;
   typedef __true_type    has_trivial_assignment_operator;
   typedef __true_type    has_trivial_destructor;
   typedef __true_type    is_POD_type;
};

__STL_TEMPLATE_NULL struct __type_traits<short>
{
   typedef __true_type    has_trivial_default_constructor;
   typedef __true_type    has_trivial_copy_constructor;
   typedef __true_type    has_trivial_assignment_operator;
   typedef __true_type    has_trivial_destructor;
   typedef __true_type    is_POD_type;
};

__STL_TEMPLATE_NULL struct __type_traits<unsigned short>
{
   typedef __true_type    has_trivial_default_constructor;
   typedef __true_type    has_trivial_copy_constructor;
   typedef __true_type    has_trivial_assignment_operator;
   typedef __true_type    has_trivial_destructor;
   typedef __true_type    is_POD_type;
};

__STL_TEMPLATE_NULL struct __type_traits<int>
{
   typedef __true_type    has_trivial_default_constructor;
   typedef __true_type    has_trivial_copy_constructor;
   typedef __true_type    has_trivial_assignment_operator;
   typedef __true_type    has_trivial_destructor;
   typedef __true_type    is_POD_type;
};

__STL_TEMPLATE_NULL struct __type_traits<unsigned int>
{
   typedef __true_type    has_trivial_default_constructor;
   typedef __true_type    has_trivial_copy_constructor;
   typedef __true_type    has_trivial_assignment_operator;
   typedef __true_type    has_trivial_destructor;
   typedef __true_type    is_POD_type;
};

__STL_TEMPLATE_NULL struct __type_traits<long>
{
   typedef __true_type    has_trivial_default_constructor;
   typedef __true_type    has_trivial_copy_constructor;
   typedef __true_type    has_trivial_assignment_operator;
   typedef __true_type    has_trivial_destructor;
   typedef __true_type    is_POD_type;
};

__STL_TEMPLATE_NULL struct __type_traits<unsigned long>
{
   typedef __true_type    has_trivial_default_constructor;
   typedef __true_type    has_trivial_copy_constructor;
   typedef __true_type    has_trivial_assignment_operator;
   typedef __true_type    has_trivial_destructor;
   typedef __true_type    is_POD_type;
};

__STL_TEMPLATE_NULL struct __type_traits<float>
{
   typedef __true_type    has_trivial_default_constructor;
   typedef __true_type    has_trivial_copy_constructor;
   typedef __true_type    has_trivial_assignment_operator;
   typedef __true_type    has_trivial_destructor;
   typedef __true_type    is_POD_type;
};

__STL_TEMPLATE_NULL struct __type_traits<double>
{
   typedef __true_type    has_trivial_default_constructor;
   typedef __true_type    has_trivial_copy_constructor;
   typedef __true_type    has_trivial_assignment_operator;
   typedef __true_type    has_trivial_destructor;
   typedef __true_type    is_POD_type;
};

__STL_TEMPLATE_NULL struct __type_traits<long double>
{
   typedef __true_type    has_trivial_default_constructor;
   typedef __true_type    has_trivial_copy_constructor;
   typedef __true_type    has_trivial_assignment_operator;
   typedef __true_type    has_trivial_destructor;
   typedef __true_type    is_POD_type;
};

#ifdef __STL_CLASS_PARTIAL_SPECIALIZATION

// 针对指针提供特化
template <class T>
struct __type_traits<T*>
{
   typedef __true_type    has_trivial_default_constructor;
   typedef __true_type    has_trivial_copy_constructor;
   typedef __true_type    has_trivial_assignment_operator;
   typedef __true_type    has_trivial_destructor;
   typedef __true_type    is_POD_type;
};

#else /* __STL_CLASS_PARTIAL_SPECIALIZATION */

// 针对char *, signed char *, unsigned char *提供特化

struct __type_traits<char*>
{
   typedef __true_type    has_trivial_default_constructor;
   typedef __true_type    has_trivial_copy_constructor;
   typedef __true_type    has_trivial_assignment_operator;
   typedef __true_type    has_trivial_destructor;
   typedef __true_type    is_POD_type;
};

struct __type_traits<signed char*>
{
   typedef __true_type    has_trivial_default_constructor;
   typedef __true_type    has_trivial_copy_constructor;
   typedef __true_type    has_trivial_assignment_operator;
   typedef __true_type    has_trivial_destructor;
   typedef __true_type    is_POD_type;
};

struct __type_traits<unsigned char*>
{
   typedef __true_type    has_trivial_default_constructor;
   typedef __true_type    has_trivial_copy_constructor;
   typedef __true_type    has_trivial_assignment_operator;
   typedef __true_type    has_trivial_destructor;
   typedef __true_type    is_POD_type;
};

#endif /* __STL_CLASS_PARTIAL_SPECIALIZATION */


#endif /* __TYPE_TRAITS_H */

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// mode:C++
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