C++：SGI STL——

• type_traits概念
• 代码
• END

iterator_traits负责萃取迭代器的特性， __type_traits负责萃取型别（type）的特性。
此处我们所关注的型别特性是指：
这个型别是否具备non-trivial defalt ctor?
是否具备non-trivial copy ctor?
是否具备non-trivial assignmentoperator?
是否具备non-trivial dtor?
如果答案是否定的，我们在对这个型别进行构造、析构、拷贝、赋值等操作时，就可以采用最有效率的措施（例如根本不调用身居高位，不谋实事的那些constructor,destructor )，而采用内存直接处理操作如malloc ( )、memcpy ()等等，获得最高效率。这对于大规模而操作频繁的容器，有着显著的效率提升。

根据iterator_traits得来的经验，我们希望，程序之中可以这样运用__type_traits，T代表任意型别:

__type_traits::has_trivial_default_constructor
__type_traits::has_trivial_copy_constructor
__type_traits::has_trivial_assignment_operator
__type_traits::has_trivial_destructor
__type_traits::is_POD_type // POD : Plain Old Data

我们希望上述式子响应我们“真”或“假”（以便我们决定采取什么策略) 但其结果不应该只是个bool 值，应该是个有着真/假性质的“对象”，因为我们希望利用其响应结果来进行参数推导,而编译器只有面对class object形式的参数才会做参数推导。为此，上述式子应该传回这样的东西:

struct __true_type { };
struct __false_type { };

这两个空白 classes没有任何成员，不会带来额外负担，却又能够标示真假，满足我们所需。

为了达成上述五个式子，__type_traits内必须定义一些typedefs，其值不是_true_type 就是 __false_type。下面是SGI 的做法:

template 
struct __type_traits
{
	typedef __true_type this_dummy_member_must_be_first;
	

	

	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;
};

上述__type_traits 可以接受任何型别的参数，五个typedefs将经由以下管道获得实值:

1. 一般具现体( general instantiation)，内含对所有型别都必定有效的保守值。上述各个has_trivial_xxx型别都被定义为__false_type，就是对所有型别都必定有效的保守值。
2. 经过声明的特化版本，例如内对所有C++标量型别( scalar types）提供了对应的特化声明。
3. 某些编译器（如Silicon Graphics N32和 N64编译器）会自动为所有型别提供适当的特化版本。

对内置类型提供特化版本

#ifndef MY_TYPE_TRAITS_H
#define MY_TYPE_TRAITS_H

namespace Srh
{
	struct __true_type {};
	struct __false_type {};

	template 
	struct __type_traits
	{
		typedef __true_type  this_dummy_member_must_be_first;
		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; 
	};

	// 特化版本
	template<> struct __type_traits
	{
		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;  
	};

	template<> struct __type_traits
	{
		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;   
	};

	template<> struct __type_traits
	{
		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;  
	};

	template<> struct __type_traits
	{
		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;   
	};

	template<> struct __type_traits
	{
		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;   
	};
	template<> struct __type_traits
	{
		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;   
	};
	template<> struct __type_traits
	{
		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;   
	};
	template<> struct __type_traits
	{
		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;   
	};
	template<> struct __type_traits
	{
		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;   
	};

	template<> struct __type_traits
	{
		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;   
	};
	template<> struct __type_traits
	{
		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;   
	};
	template<> struct __type_traits
	{
		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;   
	};

	template<> struct __type_traits
	{
		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;   
	};

	template<> struct __type_traits
	{
		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;   
	};
	template
	struct __type_traits
	{
		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

参考自《STL源码剖析》