《STL原始碼剖析》-- type_traits.h

// Filename:    type_traits.h

// Comment By:  凝霜
// E-mail:      mdl2009@vip.qq.com
// 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

// 這個標頭檔案提供了一個框架, 允許編譯器在編譯期根據型別屬性派發函式的機制
// 這個框架對於編寫模板程式碼非常有用
// 例如, 當我們需要拷貝一個未知型別的array, 它能幫助我們得知這個未知型別
// 是否具有一個trivial copy constructor, 以幫助我們決定memcpy()是否能使用

// __type_traits模板類提供了一些列的typedefs, 其值是__true_type或者__false_type
// __type_traits模板類可以接受任何型別的引數
// 模板類中的typedefs可以通過以下手段獲取其正確值
//   1. general instantiation, 對於所有型別都要有保守值
//   2. 經過特化的版本
//   3. 一些編譯器(例如Silicon Graphics N32 and N64 compilers)
//      會自動給所有型別提供合適的特化版本
//
// 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
{
    // 不要移除這個成員
    // 它通知能自動特化__type_traits的編譯器, 現在這個__type_traits template是特化的
    // 這是為了確保萬一編譯器使用了__type_traits而與此處無任何關聯的模板時
    // 一切也能順利運作
   typedef __true_type     this_dummy_member_must_be_first;

   // 以下條款應當被遵守, 因為編譯器有可能自動生成型別的特化版本
   //   - 你可以重新安排的成員次序
   //   - 你可以移除你想移除的成員
   //   - 一定不可以修改下列成員名稱, 卻沒有修改編譯器中的相應名稱
   //   - 新加入的成員被當作一般成員, 除非編譯器提供特殊支援

   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;
};


// 以下針對C++內建的基本資料型別提供特化版本, 使其具有trivial default constructor,
// copy constructor, assignment operator, destructor
// 並標記其為POD型別
//
// 特化型別:
//         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

// 針對指標提供特化
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 */

// 針對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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