Chapter 9

Imperative and object-oriented languages

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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.

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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.

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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)

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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

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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.

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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.

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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));

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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.

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Record types• For example: struct example {

int member1; double member2;

};

is represented as follows:

member 1

member 2

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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

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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.m2the program should be aware of which one is present

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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

1.1 1.2 1.3

2.1 2.2 2.3

row

column

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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.

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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 = 3number of elements in dimension1.LEN1 = 2number of elements in dimension2.LEN2 = 3

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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

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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 integerassume base (A) = 0000, an integer takes 4 bytesA[2,2,3] = base (A) + [(2-1) * 3 * 4 + (2-1) * 4 + (3-1)] * 4

= 0 + 72 =7218

Array type

K dimensions:

base (A) + [(i1-LB1) * LEN2 * LEN3 * …LENk +

(i2-LB2) * LEN3 * …LENk +

…

(ik-1 – LBk-1) * LENk +

(ik – LBk) ] *element size

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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

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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()

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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);

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Object type

Target code of o1 is:

Method table for A:

a1

a2

A_A

m1_A

m2_A

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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.

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Inheritance

class B extends class A {

int b1;

method B;

method m3_B;

}

o2 = B()

o2 looks like:

b1

a2

a1

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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.

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Method overriding

class A {…method m1(); {…}method m2(); {…}

};

class B extends A {…method m2() {…}method m3() {…}

};

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Method overridingMethod table for A:

Method table for B:

a is an object of Ab is an object of Ba.m2() invokes m2_A_Ab.m2() invokes m2_A_B

m1_A_A

m2_A_A

m1_A_A

m2_A_B

m3_B_B

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