C++11老关键字的新含义(auto, using，extern)

缺乏、安全感 2022-09-28 11:40 99阅读 0赞

出处:http://blog.csdn.net/cnsword/article/details/8034947

C++11对关键字进行了修订，加入了nullptr、constexpr、decltype、default、static\_assert等，同时原有的关键字（auto，using，extern）含义和用途进行了修订。在这里主要了解一下对auto、using、extern这三个关键字的修订。

## auto ##

### 自动化变量 ###

auto a = 12;
    auto b = 12.0f;
    auto c = true;
    auto d = [] (int x)->int{ return 12;};
    auto e = std::bind(&func, _1);

### 延迟绑定 ###

template<typename T, typename L>
    auto fun(T x, L y)->decltype(x + y){
       return x;}

## using ##

### 定义别名 ###

template<typename T>
    using Tlist = std::list<T>;

using Tlist = std::list<char>;

using df = void(*)();//等价于typedef void(*df)()
    
    
     
      
       
      和typedef的区别:
    
      
       定义一般类型的别名没区别。 C++11标准7.1.3.2：( 
      
       https://isocpp.org/files/papers/N3690.pdf )
    
      
       2 A typedef-name can also be introduced by an alias-declaration. The identifier following the using keyword becomes a typedef-name and the optional attribute-specifier-seq following the identifier appertains to that typedef-name. It has the same semantics as if it were introduced by the typedef specifier. In particular, it does not define a new type and it shall not appear in the type-id. [Example: using handler_t = void (*)(int); extern handler_t ignore; extern void (*ignore)(int); // redeclare ignore using cell = pair<void*, cell*>; // ill-formed — end example ] 
      
       定义模板的别名，只能使用using。

### 使用外部构造 ###

using A::A;

### 引入外部类型 ###

using typename A;

## extern ##

### 外部模板 ###

extern template<typename T>void(T t);