Chapter 9 Imperative and object-oriented languages 1

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. 2

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. 3

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. 4

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) 5

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 6

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. 7

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. 8

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)); 9

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. 10

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

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: member 1 member 2 gap 12

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 13

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 row column 14

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. 15

Array type Assume an n-dimensional array A with base address base (A), where dimension k has lower bound LB k and upper bound UB k. The number of elements along dimension k is then LEN k = UB k -LB k +1 dimension 1 (row) lower bound LB 1 = 1 upper bound UB 1 = 2 dimension 2 (column) LB 1 = 1 UB 2 = 3 number of elements in dimension1.LEN 1 = 2 number of elements in dimension2.LEN 2 = 3 16

Array type How to compute the location (address) of A[1, 3], i 1 =1, i 2 =3. base (A) + [(i 1 -LB 1 )*LEN 2 + (i 2 -LB 2 )] * element size = base (A) + [(1-1) * 3 + (3-1)] * element size = base (A) +2 * element size 17

Array type 3 dimensions: base (A) + [(i1-LB 1 ) * LEN 2 * LEN 3 +(i 2 -LB 2 ) * LEN 3 + (i 3 -LB 3 )] * 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 = =72 18

Array type K dimensions: base (A) + [(i 1 -LB 1 ) * LEN 2 * LEN 3 * …LEN k + (i 2 -LB 2 ) * LEN 3 * …LEN k + … (i k-1 – LB k-1 ) * LEN k + (i k – LB k ) ] *element size 19

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 ……….0 20

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() 21

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); 22

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

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, } 24

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. 25

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

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. 27

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

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 m1_A_A m2_A_A m1_A_A m2_A_B m3_B_B 29