Chapter 9 Imperative and object-oriented languages

1. Chapter 9Imperative and object-oriented languages

2. Source language data representation and handling • In the source language, a data item has a type, which may be a basic, built-in type of the language, or a constructed type, built using one of the type constructions in the language. • The target language data types are usually limited to single bytes, integers of various sizes, address representations, and floating point umbers of several sizes.

3. Basic types • The usual basic types in a source language are characters, integers of several sizes, and floating point numbers of several sizes. • The source language operations on these typically are arithmetic operations, assignment and comparison • The arithmetic operations: addition, subtraction, multiplication, division, and remainder. • All these can be mapped directly to the target language data types and operations.

4. Basic types • Some source languages also have a void type, corresponding to no value at all. • A target language representation for a void type is not needed.

5. Enumeration types • An enumeration type defines a set of names to be the values of the new data type. • The run-time representation is an integer, with values usually ranging from 0 to the number of enumeration values minus 1. • example: months ={Jan, Feb, … , Dec} Jan – 0 Feb – 1 … Dec – 11 (12-1)

6. Enumeration types • An enumeration type which is available in many languages, including some that do not have explicit enumeration types, is the Boolean type, with false and true as enumeration literals. • example: Boolean type = {false, true} 0 1

7. Pointer type • Most imperative and object-oriented languages support a pointer type of some kind. • Pointer represent the addresses of source language data structures. • Its target type is an unsigned integer of a size large enough to represent an address.

8. Pointer type • Operation: copying, assignment, comparison, etc. • The one operation that is particular to pointers is dereferencing, which consists of obtaining the value of the data structure that the pointer refers to. • If the value is small to fit in a register: a single machine instruction. If the value is large: a pointer to a record or array.

9. Pointer type • For example: the assignment q= *p; in which q is the name of a record variable of type T and p is a pointer to a record of the same type, may be translated to a call of a library routine byte_copy() byte_copy (&q, p, sizeof(T));

10. Record types • A record, also called a structure, is a data item in which a fixed number of members of possibly different types are grouped together. • In the target language, these members are represented consecutively in a memory area that is large enough to contain all members.

11. member 1 member 2 Record types • For example: struct example { int member1; double member2; }; is represented as follows:

12. member 1 gap member 2 Record types • The Scalable Processor ARChitecture (SPARC) processor requires an int (a 4-byte quantity) to have an address that is a multiple of 4 (is 4-byte aligned), and a double (an 8-byte quantity) to be 8-byte aligned. is represented as follows:

13. Union type • A union type is a data type of which the values are of one of a set of types. • For example: a variable a of type union { int m1; float m2; }a,b; holds either int m1 or float m2 for example a.m1 b.m2 the program should be aware of which one is present

14. Array type • An array type describes data structures which consist of series of items (also called elements) of the same type. • An array can have one dimension (a vector), two dimensions (a matrix), or more. • For example: A: array [1..3] of integer; A[1] A[2] A[3] B: array [1..2, 1..3] of integer column row

15. Array type • The run-time representation of the array consists of a consecutive sequence of the representation of the elements; the address of the first element is called the base address of the array. • All languages that support arrays also support element selection, which is usually called indexing.

16. Array type • Assume an n-dimensional array A with base address base (A), where dimension k has lower bound LBk and upper bound UBk. • The number of elements along dimension k is then LENk= UBk-LBk+1 dimension 1 (row) lower bound LB1 = 1 upper bound UB1 = 2 dimension 2 (column) LB1 = 1 UB2 = 3 number of elements in dimension1.LEN1 = 2 number of elements in dimension2.LEN2 = 3

17. Array type • How to compute the location (address) of A[1, 3], i1=1, i2=3. base (A) + [(i1-LB1)*LEN2 + (i2-LB2)] * element size = base (A) + [(1-1) * 3 + (3-1)] * element size = base (A) +2 * element size

18. Array type 3 dimensions: base (A) + [(i1-LB1) * LEN2 * LEN3 +(i2-LB2) * LEN3 + (i3-LB3)] * element size Example: A: array [1...2, 1…3, 1…4] of integer assume base (A) = 0000, an integer takes 4 bytes A[2,2,3] = base (A) + [(2-1) * 3 * 4 + (2-1) * 4 + (3-1)] * 4 = 0 + 72 =72

19. Array type K dimensions: base (A) + [(i1-LB1) * LEN2 * LEN3 * …LENk + (i2-LB2) * LEN3 * …LENk + … (ik-1 – LBk-1) * LENk + (ik – LBk) ] *element size

20. Set type • If it is a small sub-range of integer, it can be implemented as bitset. e.g. The set of integers ranging 0-31 can be represented in a 32-bit word. 2nd element in the set, other elements not. • Operations: • Set union is implemented with a bitwise OR • Set intersection is implemented with a bitwise AND, • Symmetric set difference is implemented with a bitwise EXCLUSIVE OR, • Set difference is implemented with a bitwise NOT. 01000……….0

21. Object type • An object is a record with built-in methods, with some additional features. Its type is a class. • Operations: • field selection: o1.i1, o1.f1, o2.i1, o2.f2 • Copying: set o1 to o2 • Method invocation: o1.m1()

22. Object type • Object also have constructors and destructors, which are methods that are to be invoked on object creation and object removal, respectively. class A { int a1; int a2; method A(); method m1(); method m2(); }; Method of object constructor, destructor. o1 = A (); destruct-A(o1);

23. Object type Target code of o1 is: Method table for A: a1 a2 A_A m1_A m2_A

24. Object type Assume: m2_A() has one integer parameter and returns no value and where Class_A is the C type name for class A Target code for m2_A: void m2_A (class_A *this, int i) { Body of m2_A, }

25. Inheritance • Inheritance allows the programmer to base a class B on a class A, so that B inherits the methods and fields of A, in additional to its own fields and methods. • This feature is also known as type extension: class B extends class A with zero or more fields and methods. • Class A is the parent class of class B, and class B is a subclass of class A.

26. Inheritance class B extends class A { int b1; method B; method m3_B; } o2 = B() o2 looks like: a1 a2 b1

27. Method overriding • When a class B extends a class A, it may redefine one or more of A’s methods; this feature is called method overriding. • Method declaration: method m1(); • Method definition: method m1() {a=1;} • Method re-definition: in subclass, method m1() {a=2;} • Abstract class: a class in which at least one method is declared, but not defined. The actual methods must then be defined in classes that extend the abstract class.

28. Method overriding class A { … method m1(); {…} method m2(); {…} }; class B extends A { … method m2() {…} method m3() {…} };

29. m1_A_A m2_A_A m1_A_A m2_A_B m3_B_B Method overriding Method table for A: Method table for B: a is an object of A b is an object of B a.m2() invokes m2_A_A b.m2() invokes m2_A_B