能否向编译后得到的类中增加实例变量？能否向运行时创建的类中添加实例变量？为什么

不能向编译后得到的类中增加实例变量！

能向运行时创建的类中添加实例变量！

因为编译后的类已经注册在runtime中，类结构体中的objc_ivar_list 实例变量的链表和instance_size实例变量的内存大小已经确定，同时runtime 会调用

class_setIvarLayout 或 class_setWeakIvarLayout来处理strong weak引用，所以不能向存在的类中添加实例变量。

运行时创建的类是可以添加实例变量，调用 class_addIvar 函数，但是得在调用 objc_allocateClassPair 之后，objc_registerClassPair 之前，原因同上。

/** 
 * Adds a new instance variable to a class.
 * 
 * @return YES if the instance variable was added successfully, otherwise NO 
 *         (for example, the class already contains an instance variable with that name).
 *
 * @note This function may only be called after objc_allocateClassPair and before objc_registerClassPair. 
 *       Adding an instance variable to an existing class is not supported.
 * @note The class must not be a metaclass. Adding an instance variable to a metaclass is not supported.
 * @note The instance variable's minimum alignment in bytes is 1<<align. The minimum alignment of an instance 
 *       variable depends on the ivar's type and the machine architecture. 
 *       For variables of any pointer type, pass log2(sizeof(pointer_type)).
 */
OBJC_EXPORT BOOL
class_addIvar(Class _Nullable cls, const char * _Nonnull name, size_t size, 
              uint8_t alignment, const char * _Nullable types) 
    OBJC_AVAILABLE(10.5, 2.0, 9.0, 1.0, 2.0);

已经编译好的类，调用 class_addIvar 函数，总是会返回 NO ，就是这个原因。

想返回 YES ，只能在运行时，重新创建一个新类，才可以。

  //在运行时创建继承自NSObject的People类
  Class People = objc_allocateClassPair([NSObject class], "People", 0);
  //添加_name成员变量
  BOOL flag1 = class_addIvar(People, "_name", sizeof(NSString*), log2(sizeof(NSString*)), @encode(NSString*));
  if (flag1) {
      NSLog(@"NSString*类型  _name变量添加成功");
  }
  //添加_age成员变量
  BOOL flag2 = class_addIvar(People, "_age", sizeof(int), sizeof(int), @encode(int));
  if (flag2) {
      NSLog(@"int类型 _age变量添加成功");
  }
  //完成People类的创建
  objc_registerClassPair(People);
  unsigned int varCount;
  //拷贝People类中的成员变量列表
  Ivar * varList = class_copyIvarList(People, &varCount);
  for (int i = 0; i<varCount; i++) {
      NSLog(@"%s",ivar_getName(varList[i]));
  }
  //释放varList
  free(varList);
  //创建People对象p1
  id p1 = [[People alloc]init];
  //从类中获取成员变量Ivar
  Ivar nameIvar = class_getInstanceVariable(People, "_name");
  Ivar ageIvar = class_getInstanceVariable(People, "_age");
  //为p1的成员变量赋值
  object_setIvar(p1, nameIvar, @"张三");
  object_setIvar(p1, ageIvar, @33);
  //获取p1成员变量的值
  NSLog(@"%@",object_getIvar(p1, nameIvar));
  NSLog(@"%@",object_getIvar(p1, ageIvar));

调用结果

2021-07-16 13:54:20.021525+0800 Test[6914:114952] NSString*类型  _name变量添加成功
2021-07-16 13:54:20.021655+0800 Test[6914:114952] int类型 _age变量添加成功
2021-07-16 13:54:20.021740+0800 Test[6914:114952] _name
2021-07-16 13:54:20.021810+0800 Test[6914:114952] _age
2021-07-16 13:54:20.021913+0800 Test[6914:114952] 张三
2021-07-16 13:54:20.022003+0800 Test[6914:114952] 33