type_traits技术与C++

最新推荐文章于 2025-02-09 14:55:45 发布

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最新推荐文章于 2025-02-09 14:55:45 发布

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分类专栏：【编程语言】文章标签： c++

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本文介绍了C++中的Type_traits技术，用于在编译期间根据数据类型特性选择不同处理方式。通过实例展示了如何利用type_traits简化模板代码，并讨论了函数调用过程、模板函数的声明与定义方式以及模板invoke的相关问题。提供了Trait技术实现迭代器的参考代码链接。

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引言

一个方法实现过程中，业务逻辑很多都是相似的，但是与具体的特化类型的不同有一定的差异。
这个时候可以采用特化模板的方式实现，不同的类型使用不同的特化实现。但是这种情况造成一定的业务逻辑的冗余。而trait技术可以将特化类型通过封装，以一个统一的调用方式实现相同的业务逻辑。

Type_traits技术

type_traits可以翻译为类型提取器或者类型萃取器，很直白的说就是通过这个机制可以获取被操作数据类型的一些特征。这个机制在编写模板代码的时候特别有用，可以在编译期间就根据数据类型的特征分派给不同的代码进行处理。

STL中关于copy的代码

// This header file provides a framework for allowing compile time dispatch
// based on type attributes. This is useful when writing template code.
// For example, when making a copy of an array of an unknown type, it helps
// to know if the type has a trivial copy constructor or not, to help decide
// if a memcpy can be used.
struct __true_type {
   
};
struct __false_type {
   
};
template <class _Tp>
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;
};
// The class template __type_traits provides a series of typedefs each of
// which is either __true_type or __false_type. The argument to
// __type_traits can be any type. The typedefs within this template will
// attain their correct values by one of these means:
//     1. The general instantiation contain conservative values which work
//        for all types.
//     2. Specializations may be declared to make distinctions between types.
//     3. Some compilers (such as the Silicon Graphics N32 and N64 compilers)
//        will automatically provide the appropriate specializations for all
//        types.
// EXAMPLE:
//Copy an array of elements which have non-trivial copy constructors
template <class T> void copy(T* source, T

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