类是结构体还是结构体指针

如果我没记错的话，结构意味着对象，结构指针意味着指向对象的指针，对吗？在一篇文章中，它说类是结构，这意味着它们是对象。

Filter the list of all classes
The traditional definition of a class in Objective-C looks like this:

struct objc_class {
    Class isa;
    Class super_class;
    const char *name;
    long version;
    long info;
    long instance_size;
    struct objc_ivar_list *ivars;
    struct objc_method_list **methodLists;
    struct objc_cache *cache;
    struct objc_protocol_list *protocols;
    };

因此，如果它说类是结构，那么它们将如何适合以下论证：objc_msgSend(id self, SEL_cmd)这需要一个类型为的结构指针id?

类是结构体，但是Class是一个指针类型，定义为

typedef struct objc_class *Class;

这回答了问题的第一部分。

现在，如果你看一下<objc/objc.h>你会找到

struct objc_class {
    Class isa  OBJC_ISA_AVAILABILITY;

 #if !__OBJC2__
    Class super_class                                        OBJC2_UNAVAILABLE;
    const char *name                                         OBJC2_UNAVAILABLE;
    long version                                             OBJC2_UNAVAILABLE;
    long info                                                OBJC2_UNAVAILABLE;
    long instance_size                                       OBJC2_UNAVAILABLE;
    struct objc_ivar_list *ivars                             OBJC2_UNAVAILABLE;
    struct objc_method_list **methodLists                    OBJC2_UNAVAILABLE;
    struct objc_cache *cache                                 OBJC2_UNAVAILABLE;
    struct objc_protocol_list *protocols                     OBJC2_UNAVAILABLE;
#endif

} OBJC2_UNAVAILABLE;

and in <obj/runtime.h>你会找到

/// Represents an instance of a class.
struct objc_object {
    Class isa  OBJC_ISA_AVAILABILITY;
};

这意味着在 Objective-C 2.0 中objc_object and objc_class是相同的结构，都有一个isa field.

这就是为什么你可以通过Class其中一个id是必须的：类毕竟是对象.

You would normally expect a warning about the incompatible pointers type, but apparently the Obj-C compiler uses an ad-hoc treatment this specific case. I have no references to support this, though.

EDIT

最后在clang源码中找到了参考：

In ASTContext.cpp，这是Class宣言

TypedefDecl *ASTContext::getObjCClassDecl() const {
  if (!ObjCClassDecl) {
    QualType T = getObjCObjectType(ObjCBuiltinClassTy, 0, 0);
    T = getObjCObjectPointerType(T);
    TypeSourceInfo *ClassInfo = getTrivialTypeSourceInfo(T);
    ObjCClassDecl = TypedefDecl::Create(const_cast<ASTContext &>(*this),
                                        getTranslationUnitDecl(),
                                        SourceLocation(), SourceLocation(),
                                        &Idents.get("Class"), ClassInfo);
  }
  
  return ObjCClassDecl;
}

这是id宣言

TypedefDecl *ASTContext::getObjCIdDecl() const {
  if (!ObjCIdDecl) {
    QualType T = getObjCObjectType(ObjCBuiltinIdTy, 0, 0);
    T = getObjCObjectPointerType(T);
    TypeSourceInfo *IdInfo = getTrivialTypeSourceInfo(T);
    ObjCIdDecl = TypedefDecl::Create(const_cast<ASTContext &>(*this),
                                     getTranslationUnitDecl(),
                                     SourceLocation(), SourceLocation(),
                                     &Idents.get("id"), IdInfo);
  }
  
  return ObjCIdDecl;
}

在这两种情况下，类型都设置为以下结果getObjCObjectPointerType。这会导致编译器同时考虑Class and id作为指向 Objective-C 对象的指针。