class_eval vs instance_eval

Question

Is there any difference in how class_eval & instance_eval work except def ? Inside class_eval block def defines method to class itself (ie instance method) and inside instance_eval def defines method to the eigenclass of the class (ie class method). AFAIK all other features work identically in both cases (eg define_method , attr_accessor , class << self; end , defining constants). Is it true?

Answer is : def , undef and alias have different contexts for class_eval and instance_eval .

Answer 1

Long story short:

• Object.instance_eval &block sets:
  • self to Object
  • The "current class" to Object.singleton_class
• Object.class_eval &block sets:
  • self to Object
  • The "current class" to Object

The "current class" is used for def , undef and alias , as well as constant and class variable lookups.

---

Now, let's have a look at the implementation details.

Here's how module_eval and instance_eval are implemented in C:

VALUE rb_mod_module_eval(int argc, VALUE *argv, VALUE mod) {
    return specific_eval(argc, argv, mod, mod);
}

VALUE rb_obj_instance_eval(int argc, VALUE *argv, VALUE self) {
    VALUE klass;
    if (SPECIAL_CONST_P(self)) { klass = Qnil; }
    else { klass = rb_singleton_class(self); }
    return specific_eval(argc, argv, klass, self);
}

Both call specific_eval , which takes the following arguments: int argc , VALUE *argv , VALUE klass and VALUE self .

Note that:

• module_eval passes the Module or Class instance as both klass and self
• instance_eval passes the object's singleton class as klass

If given a block, specific_eval will call yield_under , which takes the following arguments: VALUE under , VALUE self and VALUE values .

if (rb_block_given_p()) {
    rb_check_arity(argc, 0, 0);
    return yield_under(klass, self, Qundef);
}

There are two important lines in yield_under :

1. block.self = self;

   This sets the self of the block to the receiver.

2. cref = vm_cref_push(th, under, NOEX_PUBLIC, blockptr);

   The cref is a linked list which specifies the "current class", which is used for def , undef and alias , as well as constant and class variable lookups.

   That line basically sets the cref to under .

   Finally:

   • When called from module_eval , under will be the Class or Module instance.

   • When called from instance_eval , under will be the singleton class of self .

Answer 2

instance_eval允许您直接访问实例的实例变量，并使用self作为实例的引用。