objective-c - Objective-C中的id类型真的是动态的吗

Question

id 类型是真正动态的(在运行时后期绑定(bind))还是类似于 C# 中的“var”关键字(在编译时后期绑定(bind))？

Answer 1

来自 The Objective-C Programming Language :

id is defined as pointer to an object data structure:

typedef struct objc_object {
    Class isa;
} *id;

更重要的是

The id type is completely nonrestrictive. By itself, it yields no information about an object, except that it is an object. At some point, a program typically needs to find more specific information about the objects it contains. Since the id type designator can’t supply this information to the compiler, each object has to be able to supply it at runtime.

The isa instance variable identifies the object’s class—what kind of object it is. Objects with the same behavior (methods) and the same kinds of data (instance variables) are members of the same class.

Objects are thus dynamically typed at runtime. Whenever it needs to, the runtime system can find the exact class that an object belongs to, just by asking the object. (To learn more about the runtime, see Objective-C Runtime Programming Guide.) Dynamic typing in Objective-C serves as the foundation for dynamic binding, discussed later.