C++ - 结合 typedef 和 typename 的语句的含义
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C++ - meaning of a statement combining typedef and typename
提问by Arvind
In a C++ header file, I am seeing this code:
在 C++ 头文件中，我看到了以下代码：
typedef typename _Mybase::value_type value_type;
Now, as I understand, quoting from ? C++ the Complete Reference? by Schildt. typenamecan be substituted by keyword class, the second use of typenameis to inform the compiler that a name used in a template declaration is a type name rather than an object name.
现在，据我所知，引用自 ? C++ 完整参考？通过希尔特。typename可以用关键字 class 代替，第二个用途typename是通知编译器在模板声明中使用的名称是类型名称而不是对象名称。
Similarly, you can define new data type names by using the keyword typedef. You are not
actually creating a new data type, but rather defining a new name for an existing
type.
同样，您可以使用关键字定义新的数据类型名称typedef。您实际上并不是在创建新数据类型，而是为现有类型定义新名称。
However, can you explain exactly what is the meaning of the above line of code, where typedefand typenameare combined together. And what does the "::" in the statement imply?
不过，你能不能解释一下上面这行代码到底是什么意思，在哪里typedef和typename组合在一起。::声明中的“ ”是什么意思？
回答by pippin1289
typedef is defining a new type for use in your code, like a shorthand.
typedef 正在定义一种新类型以在您的代码中使用，就像速记一样。
typedef typename _MyBase::value_type value_type;
value_type v;
//use v
typename here is letting the compiler know that value_typeis a type and not a static member of _MyBase.
这里的 typename 是让编译器知道这value_type是一个类型而不是_MyBase.
the ::is the scope of the type. It is kind of like "is in" so value_type"is in" _MyBase. or can also be thought of as contains.
的::是该类型的范围。它有点像 "is in" 所以value_type"is in" _MyBase。或者也可以被认为是包含。
回答by Paul Evans
the typenameis saying that _Mybase::value_typeis the name of type so the typedefcan reley on that fact.
该typename是说，_Mybase::value_type是类型的名称，以便typedef可以对事实reley。
