MIT CS143 lecture 02 Overview of COOL

Cool Overview

Classroom Object Oriented Language

Designed to：

• Be implementable in a short time
• Give a taste of implementation of modern（Abstraction、Static typing、Reuse (inheritance)、Memory management、etc）

But many things are left out

一个简单的例子

class Point {
 	x : Int ← 0;
 	y : Int ← 0;
};
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Cool programs are sets of class definitions：

• A special class Main with a special method main
• All Cool code lives inside classes

A class is a collection of attributes and methods

Instances of a class are objects

Cool 对象

class Point {
 	x : Int ← 0;
 	y : Int; (* use default value *)
};
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The expression new Point creates a new object of class Point

An object can be thought of as a record with a slot for each attribute：

方法

A class can also define methods for manipulating the attributes：

class Point {
 	x : Int ← 0;
 	y : Int ← 0;
	movePoint(newx : Int, newy : Int): Point {
 		{ 
 			x ← newx; 
 			y ← newy; 
 			self; 
 		} -- close block expression
 	}; -- close method
 }; -- close class
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Methods can refer to the current object using self

Information Hiding in Cool

• Methods are global
• Attributes are local to a class（They can only be accessed by the class’s methods）

class Point {
 	... 
 	x () : Int { x };
 	setx (newx : Int) : Int { x ← newx }; 
};
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Each object knows how to access the code of a method. As if the object contains a slot pointing to the code

In reality implementations save space by sharing these pointers among instances of the same class

Inheritance

We can extend points to colored points using subclassing（class hierarchy）

class ColorPoint inherits Point {
 	color : Int ← 0;
 	movePoint(newx : Int, newy : Int): Point {{
 		color ← 0;
 		x ← newx;
 		y ← newy; 
 		self;
 	}}; 
};
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Cool Types

Every class is a type

Base classes：

• Int：for integers
• Bool：for boolean values: true, false
• String ：for strings
• Object：root of the class hierarchy

All variables must be declared（compiler infers types for expressions）

Cool Type Checking

x : A;
x ← new B;
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Is well typed if A is an ancestor of B in the class hierarchy（Anywhere an A is expected a B can be used）

Type safety：A well-typed program cannot result in runtime type errors

Method Invocation and Inheritance

• Methods are invoked by dispatch
• Understanding dispatch in the presence of inheritance is a subtle aspect of OO languages

方法调用的一个例子（调用一个单参数方法m(x)）

Other Expressions

Expression language

every expression has a type and a value：

• Loops：while E loop E pool
• Conditionals：if E then E else E fi
• Case statement：case E of x : Type ⇒ E; … esac
• Arithmetic：+, -, …
• Logical operations <, =, …
• Assignment x ← E
• Primitive I/O out_string(s), in_string(), …

Missing features

• arrays, floating point operations, exceptions, …

Cool 内存管理

• Memory is allocated every time new is invoked
• Memory is deallocated automatically when an object is no longer reachable
• Done by the garbage collector (GC)（There is a Cool GC）