Instance_eval, class_eval, and eval IN RUBY

Ruby has the function facility to execute code saved as strings at runtime, the eval family method. Including kernel # Eval, object # instance_eval, Module # class_eval.

　　Kernel # eval

It is the most direct method

As follows:

1 p eval("2+2")2 3 eval("def m;p ‘hello world‘;end")4 eval("m")

Output

4
"Hello World"

Eval is powerful, dangerous, and vulnerable to code injection. Although Ruby has a global variable $ safe to control it, it is best not to use it.

 

　　Object # instance_eval

This method changes self to the receiver of the instance_eval call and evaluates the string or code block.

As follows:

1 p self2 3 a = []4 a.instance_eval {p self} 

Output

Main
[]

Instance_eval is often used to access private data of other objects-especially instance variables

As follows:

1 class C2     def initialize3         @x = 14     end5 end6     7 c = C.new8 c.instance_eval {puts @x}

Output

1

Instance_eval can also accept strings to access local variables.

As follows:

1 arr = [‘a‘,‘b‘,‘c‘]2 ch = ‘d‘3 arr.instance_eval "self[1] = ch"4 p arr

Output

["A", "D", "C"]

　　

The method defined in instance_eval is a singleton method.

As follows:

1 obj = Object.new2 3 obj.instance_eval do4     def method5         p "hello world"6     end7 end8 p obj.singleton_methods

Output

[: Method]

 

　　Module # class_eval

Class_eval is the same as module_eval. It can enter the class definition body.

1 C = Class.new2 C.class_eval do3     def method4         p "hello world"5     end6 end7 c = C.new8 c.method

Output

"Hello World"

　　

Class_eval can do something that class keywords cannot do.

1. Evaluate the string in the context of the Class Definition
2. Open the class definition for an anonymous class (but does not contain a singleton class) (that is, open a class without knowing the class name)
3. Get access to variables in the peripheral Scope

1 def add_method_to(a_class)2     a_class.class_eval do3         def method ; p "hello world";end4     end5 end6 add_method_to String7 "abc".method

Output

"Hello World"

 

1 var = "hello world"2 3 class C4 end5 C.instance_eval {puts var}

Output

"Hello World"

　　

　　Summary: The instance_eval () method only changes self, while class_eval () modifies self and the current class at the same time. By modifying the current class, class_eval () actually re-opens the class, just like the class keyword.

Module # class_eval () is more flexible than the class keyword. Class keywords can only use constants, while class_eval () can use the class_eval () method for any variables that represent classes. Class opens a new scope, but class_eval () is actually a flat scope.