ada_adt.html

来自「Data Structure Ebook」· HTML 代码 · 共 106 行

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106 行

<HTML><HEAD>
<TITLE>Data Structures and Algorithms - Ada ADTs</TITLE>
</HEAD>
<BODY BGCOLOR="#ffffff">
<H1>Data Structures and Algorithms</H1>
<HR>

<H3>6.2 ADTs in Ada</H3>

Ada was designed in the late 70's - just before object
orientation was "discovered". However at that time the
value of abstract data types was well understood and Ada
provides good support for this concept.

<P>
Two Ada constructs are needed for defining an ADT:
the data type and its methods are placed in an Ada <TT>package</TT>.
For safety and information hiding, the data type is made
<TT>private</TT>. Although a package's "client" can see the
structure of the data type, the compiler prevents access to 
individual attributes of the type: thus effectively implementing
the information hiding principle.
The client can see the information, but can't do anything with it!
(I believe that the reason for exposing the structure of the 
private type is purely pragmatic:
compilers and linkers know how much space an ADT in a separately compiled
package - for which only the specification might be available -
requires.)
<P>
An Ada package for complex numbers would be implemented:
<P><FONT COLOR=green><PRE>
PACKAGE complex_numbers IS
    TYPE complex IS PRIVATE;
    I : CONSTANT complex;    -- 'i'

    FUNCTION "-"( complex a ) RETURNS complex; -- Unary minus

    FUNCTION "+"( complex a; complex b ) RETURNS complex;
    FUNCTION "-"( complex a; complex b ) RETURNS complex;
    FUNCTION "*"( complex a; complex b ) RETURNS complex;
    FUNCTION "="( complex a; complex b ) RETURNS boolean;

PRIVATE
    TYPE complex IS RECORD
        real, imag : FLOAT;
        END RECORD;
    I : CONSTANT complex := (0.0, 1.0);
END complex_numbers;
</PRE></FONT>
The <TT>body</TT> or implementation would usually be placed in 
a separate file and compiled separately:
<P><FONT COLOR=green><PRE>
PACKAGE BODY complex_numbers IS

    FUNCTION "-"( complex a ) RETURNS complex IS -- Unary minus
	RETURN complex'(-a.real,-a.imag);
	END "-";

    FUNCTION "+"( complex a; complex b ) RETURNS complex IS
        RETURN complex'(a.real+b.real,a.imag+c.imag);
        END "+";

    FUNCTION "-"( complex a; complex b ) RETURNS complex IS
        RETURN complex'(a.real-b.real,a.imag-c.imag);
        END "-";

    FUNCTION "*"( complex a; complex b ) RETURNS complex IS
	RETURN complex'(a.real*b.real - a.imag*b.imag,
                        a.real*b.imag + a.imag*b.real );
	END "*";

    FUNCTION "="( complex a; complex b ) RETURNS boolean IS
	RETURN (a.real = b.real) AND (a.imag = b.imag);
	END "=";

END complex_numbers;
</PRE></FONT>
<P>
Note that Ada provides excellent operator overloading capabilities,
which enable us to write mathematically "natural" code: <I>e.g.</I>
<P><FONT COLOR=green><PRE>
    complex a, b, c, z;

    IF a = b THEN
	c := z - a;
        z := -z;
    END IF;
</PRE></FONT>
<P>
You can also observe that Ada is an extreme case of the
principle that programs are read many times
more often than they are read (for modern languages
at least - nothing is ever likely to match the verbosity of COBOL!).
Keywords appear everywhere:
<TT><FONT COLOR=green>IS, RETURNS,</FONT></TT> <I>etc.</I>.
<BR>
<HR>
<A HREF="sorting.html" tppabs="http://www.ee.uwa.edu.au/~plsd210/ds/sorting.html">Sorting</A><BR>
<A HREF="ds_ToC.html" tppabs="http://www.ee.uwa.edu.au/~plsd210/ds/ds_ToC.html">Table of Contents</A>
<HR>
<SMALL>
&copy; <A HREF=mailto:morris@ee.uwa.edu.au>John Morris</A>, 1996
</SMALL>
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