copyright © 2005 curt hill constants in c++ why and how

Copyright © 2005 Curt Hill

Constants in C++Why and How


Introduction• There are three kinds of constants in

C++– Literals– Define named constants– Variables with const prefix

• We will see the whys and wherefores now


Literal constants• We have seen the following fours

kinds of constants:– -45 (int)– 2.3 (float)– 1E4 (float)– ‘A’ (char)– “Hi” (string)

• Normally, merely inspecting the constant will tell you its type, as well as its value


Literals Continued• What do we do to set the constant to

something other than a standard int or float?

• We can use a suffix to tell it the type• The L or l indicates that it is long• The U or u indicates unsigned• Floating point constants are by default

double• Suffix of F or f makes it a float, L a long

double• You may also use binary, octal or

hexadecimal constants but we do not need to do that here


Literal Problems• However, literal constants have the

following problems that sometimes need to be dealt with

• We have to retype them each time– This is a problem with high precision constants

like PI– Opportunities for typing errors– Just remembering what the 14 digit value is

• We have to remember what they represent– Small integer constants tend to get confused

with one another


Example:• Suppose that I am a manager and I have 5

people who work 5 days a week and 8 hours a day

• Thus the number of manhours is:workhours = 5 * 5 * 8;

• Now we will try something new, four ten hour days per week

• I use my text editor and change all 5s to 4s and all 8s to 10s

• We now getworkhours = 4 * 4 * 10;

• The problem with the above expression is that the 5 and 8 have no inherent meaning


Alternative• It would be better to say:int people = 5, dayperweek = 5, hoursperday = 8;...workhours = people * dayperweek * hoursperday

• Unfortunately that leads to a problem: – An inadvertent assignment to one of the

variables• The solution is that we would like to be

able to name constants, just like we name variables but without possibility of assignment


The define preprocessor directive

• This is the only C way had to do this• Constants are set up using the define

directive• Format:#define name value

• Blanks separate the items• The value is usually a constant of

some sort• The name has the format of a C

variable


Example• Some constants

#define DAYSPERWEEK 5#define HOURSPERDAY 8#define EMPLOYEES 5

hours = DAYSPERWEEK * HOURSPERDAY * EMPLOYEES;


Discussion• The preprocessor searches the rest of the

text and replaces every occurrence of NAME with VALUE

• The name is not a variable, though it may look like one

• Examples:#define PI 3.141592653589793x = PI * r * r;

• Simply does macro text substitution so that statement becomes:x = 3.141592653589793 * r * r;


More Discussion• Preprocessor does not know C/C++• It has its own syntax• No = or ; is needed• So#define PI = 19/6 does not work, it becomes x = = 19/6 * r * r;

• Even without = the 19/6 would be evaluated as an integer


More Discussion• String constants are not changed

such as: “ The value of PI is ”– This would remain the same

• #define must occur before use, but customarily comes at front of program

• Used to parameterize a program• There is a convention that constants

should be in all caps so a constant is obvious in the text


#define problems• A problem with the #define

statement is that no type is associated with the value

• All that occurs is macro processing, where one string is found and replaced with another

• C++ accepts #defined constants but adds a new way to define a constant


Const Keyword• A feature that is new to C++ is the

const qualifier:– const float pi = 3.14159;

• Any attempt to assign to this will result in an error

• The float type determines the precision used


Discussion• The const is a keyword that prefixes

the declaration• This may be used anywhere a

variable may be declared• The declaration must have an

initialization

copyright © 2005 curt hill constants in c++ why and how

Documents

curt hillcopyright

curt hillthe

curt hillintroductionthere

curt hillconstants

curt hillalternativeit

curt hillexamplesome

curt hillliteral constantswe

hexadecimal constants