c++ - C++ 中的名称和类型

Question

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Answer 1

声明可以引入类型、函数、变量。

class A;  // This declares a type, A
A* aPtr;  // This declares a variable, aPtr
          // The type of aPtr is A*
A foo();  // This declares a function, foo.

在你的情况下，

List<Observer*> *_Observers;  // Declares a variable, _observers.
                              // The type of _observers is List<Observer*>*.
                              // For this to be a valid declarion, the types List,
                              // a class template, and Observer must be known 
                              // (declared or defined) before the
                              // variable declaration.