STL — SGT STL的私房菜: __type_traits

心已赠人 2022-06-03 08:35 175阅读 0赞

# SGT STL的私房菜: \_\_type\_traits #

traits的编程技法非常的6，我们在前面的[迭代器设计思维][Link 1]的时候已经领教过了，它适当的弥补了C++语言本身的不足. STL只对迭代器加

以规范，制定 出 iterator\_traits. SGI把这种技法进一步扩大到迭代器以外的世界上，于是有了所谓的\_\_type\_traits. 双底线前缀词意

指 这是SGI STL内部所用的东 西 ，不在STL标准规范之内.

iterator\_traits负责萃取迭代器的特性，\_\_type\_traits则负责萃取型别的特性. 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(在我这里理解->非常规默认构造函数)? 是否具备non-trivial copy ctor()?是否具备 non-trivial

assignment operator? 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，赋值等操作的时候，

就可以采用最有效率的措施，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，

有这显著的效率提升.

定义于SGI<type\_traits.h>中的\_\_type\_traits，提供了一种机制，允许针对不同的型别属性，在编译时期完成函数派送决定。 这对于撰

写template很 有 帮助，例如，当我们准备对一个"元素型别未知"的数组执行copy操作时，如果我们能事先知道其元素型别是否有一个

trivial copy constructor，便 能够 帮助我们决定是否可使用快速的memcpy()或memove().

根据我们的iterator\_traits得来的经验，我们希望，程序之中可以这样运用\_\_type\_traits<T>,T代表任意型别:

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

所以现在已经很容易了，我们只需要判断上面这几项是真是假就结束了. 但是上面传输的结果不能单纯是一个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，上述式子应该传会这样的东 西:

struct __true_type {
    };
    
    struct __false_type {
    };

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

为了达到上述的五个式子，\_\_type\_traits内定义一些typedefs，其值不是\_\_true\_type就是\_\_false\_type。 下面看看SGI STL源代码:

template <class type>
    struct __type_traits {
    	typedef __true_type     this_dummy_member_must_be_first;
    	/* Do not remove this member. It informs a compiler which
    	automatically specializes __type_traits that this
    	__type_traits template is special. It just makes sure that
    	things work if an implementation is using a template
    	called __type_traits for something unrelated. */
    
    	/* The following restrictions should be observed for the sake of
    	compilers which automatically produce type specific specializations
    	of this class:
    	- You may reorder the members below if you wish
    	- You may remove any of the members below if you wish
    	- You must not rename members without making the corresponding
    	name change in the compiler
    	- Members you add will be treated like regular members unless
    	you add the appropriate support in the compiler. */
    
    
    	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;
    };

因为我们的内置类型就那么几个，而用户自定义类型的可能多的数不过来. 自定义类型可能大部分都是\_\_false\_type. 所以我们

的SGI先把所有的类型 的五种萃取型别的特性定义为\_\_false\_type. 但是内置类型的五种萃取型别的特性基本为\_\_true\_type. 所以呢

，再对内置类型特化就解决掉这个问题.

// 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
    
    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 */
    
    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;
    };

\_\_type\_traits在SGI STL应用简直不要太广泛，我们单单的看源代码没有办法更好的理解它. 必须加一点实例. STL定义了5个函

数，他们内部都使用 了\_\_type\_traits. 我有一篇博客有源代码加图示来解释这5个函数，相信你了解懂这5个函数的时候，那么你一定

就深刻理解\_\_type\_traits的思想.

博客链接： [内存基本处理工具][Link 2]！！！！！！！！！！！！！！

[Link 1]: http://blog.csdn.net/dawn_sf/article/details/78841389
[Link 2]: http://blog.csdn.net/dawn_sf/article/details/78867864