Libraries
Making Functions Globally Reusable
(§ 6.4)
1
Review
We have written a program that used functions to
compute the area and circumference of ellipses.
area  ab
a
b
b
a
a b
circumference  2
2
2
2
2
Function Prototypes (placed before main()):
double ellipseArea(double length, double width);
double ellipseCircumference(double length, double width);
Function Definitions & Documentation
(placed after main()):
const double PI = 3.14159;
/* Function to compute the area of an ellipse
Receive: two double values length, width, representing
the major axis and minor axis of an ellipse
Return: the area of the ellipse
-------------------------------------------------------*/
double ellipseArea(double length, double width)
{
double halfLength = length/2.0,
halfWidth = width/2.0;
}
return PI * halfLength * halfWidth;
3
/* Function to compute the circumference of an ellipse
Receive: two double values length, width, representing
the major axis and minor axis of an ellipse
Return: the circumference of the ellipse
---------------------------------------------------------*/
double ellipseCircumference(double length, double width)
{
double halfLength = length/2.0,
halfWidth = width/2.0;
return 2.0 * PI * sqrt(
(halfLength * halfLength + halfWidth * halfWidth) / 2.0);
}
Putting these prototypes, documentation, and definitions
along with main() in the same program makes it possible to
reuse them at several different places in the program.
However, suppose that in order to solve some other
problem, a different program requires computing the area
and circumference of an ellipse.
4
How can we reuse our functions
in a different program?
Options:
• Copy-and-paste ellipseArea()
and ellipseCircumference()
from our previous program into
the new program.
• Store ellipseArea() and
ellipseCircumference()in a
___________ so that programs
can share them.
Is there
automatic
updating
of a program
if the original
functions are
modified?
_____
_______
We also
have global
reusability.
5
Libraries
A library consists of three files:
A _____________ file (whose name has a
.h suffix) that contains shareable
Header
Files
function __________________ and ____________.
An ________________________ file (whose name
has a .cpp suffix) that contains shareable
function _____________________.
A ____________________ file (whose name has
a .txt (or .doc) suffix) that contains
documentation for the library.
Source
Files
Resource
Files
In Visual C++, put all 3 and
the program that uses the
library in the same project.
6
Example
Since we are creating a library to
share functions that describe an ellipse,
we might name our library ellipse,
with
must be
• header file ellipse.h,
the same
• implementation file ellipse.cpp, and
• documentation file ellipse.txt.
Other Examples:
Text: Temperature conversion
Project: Metric conversion
7
Header file: ellipse.h
In ellipse.h, we place the function________________:
Can use this same opening
documentation in all 3 files
but change
1st line
/*----- ellipse.h ----Library of functions for computing ellipse attributes.
L. Nyhoff
CS 104X
Jan 16, 2012
Functions provided:
Note the
"directory"
ellipseArea: compute area of an ellipse
ellipseCircumference: compute circumference of an ellipse
------------------------------------------------------------*/
double
double
// ...
// ...
ellipseArea(double length, double width);
ellipseCircumference(double length, double width);
plus any others we want to provide such as the
focal length, eccentricity, etc.
The corresponding implementation file ellipse.cpp and
any program that uses these functions must contain
___________________________
8
Implementation file: ellipse.cpp
Their _______________ are placed in ellipse.cpp:
/*----- ellipse.cpp ----Library of functions for computing ellipse attributes.
L. Nyhoff
CS 104X
Jan 16, 2012
Functions provided:
ellipseArea: compute area of an ellipse
ellipseCircumference: compute circumference of an ellips
------------------------------------------------------------*/
#include <cmath>
#include <cassert>
Put after
using namespace std;
using namespace std;
_______________________
const double PI = 3.14159;
// Could go in ellipse.h
double ellipseArea(double length, double width)
{
________________________________________
double halfLength = length/2.0,
halfWidth = width/2.0;
return PI * halfLength * halfWidth;
}
9
Implementation file (cont.)
double ellipseCircumference(double length, double width)
{
assert(length >= 0 && width >= 0);
double halfLength = length/2.0,
halfWidth = width/2.0;
return 2.0 * PI *
sqrt((halfLength * halfLength + halfWidth * halfWidth)/2.0);}
//... plus definitions of any others we provide ...
This file can be __________________ separately from any
program that uses it (called a _______________ program).
10
Documentation file: ellipse.txt
Our documentation file will be a copy of the
header file, with __________________________
added foradditional documentation:
/*----- ellipse.cpp ----Library of functions for computing ellipse attributes.
L. Nyhoff
CS 104X
Jan 17, 2011
Functions provided:
ellipseArea: compute area of an ellipse
ellipseCircumference: compute circumference of an ellips
------------------------------------------------------------*/
/*------------------------------------------------------Compute the area of an ellipse.
Receive: length, width, two double values.
Return: the area of the corresponding ellipse.
-------------------------------------------------------*/
double ellipseArea(double length, double width);
11
Documentation file (cont.)
/*------------------------------------------------------Compute the circumference of an ellipse.
Receive: length, width, two double values.
Return: the circumference of ellipse defined by
length and width.
-------------------------------------------------------*/
double ellipseCircumference(double length, double width);
// ... plus prototypes and specifications
// ... for any others we provide ...
By storing the documentation in a separate file,
we provide information on how to use the library
without cluttering the other library files.
But some programmers
do put the documentation
12
in the header file.
Program Translation
Translating a program into machine language
consists of two steps:
1. ________________, in which the syntax of the
main program and of the implementation files
of any included libraries are checked, and if
no errors are found, converts them into
the computer’s machine language.
2. ___________, in which any calls to functions
(from main() or from other functions) are
bound to the definitions of those functions.
13
Using a Library
To use a library:
A program must ________________________
(usually above the main function and after
using namespace std;).
When the ___________ (actually its
preprocessor) encounters this #include
directive, it must be able to find the header file
so it can open it and replace the #include
directive with its contents so they get compiled
along with the program.
When the file contains function prototypes, the
effect of the #include directive is to insert
those prototypes into the program.
In Visual C++:
Put it in the
project's header
files
14
In Visual C++:
It must also be able to find the
corresponding implementation file so
it can open it, insert the contents of
its header file into it, and then compile
it (separately from the program).
Put it in the
project's
source files
Once these compilations are successful, the ___________
combines these compiled files into one, connecting
(i.e., "linking") each function call to the compiled
code of that function's definition.
A failure in either stage is an error.
15
Compilation Errors
A program calling a function for which _
________________is given produces a
________________ error.
This can occur if you call a library function and
neglect to #include the header file containing its
prototype.
You’ll be trying to use a function that has not been
______________.
16
Linking Errors
A program calling a function for which the
linker cannot find a ____________________
produces a linker error.
This can occur if a program calls a function
but the linker is not told to use the library
implementation file or object file containing
that function’s definition.
How this is done varies from platform to
platform, but often involves a project file.
17
Example
#include <iostream>
using namespace std;
// cin, cout, <<, >>, ...
#include "ellipse.h"
// insert ellipse prototypes
Temperature
conversion
example in
the text
Put after
int main()
using namespace std;
{
cout << "Program to compute the area and circumference of ellipses.\n";
double majorAxis, minorAxis;
cout << "\nEnter major & minor axes (in meters) (0 0 to stop): ";
cin >> majorAxis >> minorAxis;
while (majorAxis > 0)
{
double area = ellipseArea(majorAxis, minorAxis);
double circumference = ellipseCircumference(majorAxis, minorAxis);
}
}
cout << "\nFor an ellipse with major axis " << majorAxis
<< " meters and minor axis " << minorAxis << " meters\n"
<< "\tarea = " << area << " sq. meters\n"
<< "\tcircumference = " << circumference << " meters\n";
cout << "\nEnter major & minor axes (in meters) (0 0 to stop): ";
cin >> majorAxis >> minorAxis;
18
1>------ Rebuild All started: Project: EllipseLib, Configuration: Debug
Win32 -----1> ellipse.cpp
1> driver.cpp
1> Generating Code...
1> EllipseLib.vcxproj -> E:\CS10412Int\classprogs\EllipseLib\Debug\EllipseLib.exe
========== Rebuild All: 1 succeeded, 0 failed, 0 skipped ==========
EXECUTION:
Program to compute the area and circumference of ellipses.
Enter major & minor axes (in meters) (0 0 to stop): 2 2
For an ellipse with major axis 2 meters and minor axis 2 meters
area = 3.14159 sq. meters
circumference = 6.28318 meters
Enter major & minor axes (in meters) (0 0 to stop): 0 0
Press any key to continue . . .
19
Compilation creates a ____________________file
(usually with a .o or .obj suffix) from a
.cpp file.
int main()
{
// ...
}
file.cpp
C++
Compiler
00100111010
10110001001
...
11101100100
file.obj
20
Linking
Linking binds multiple ______________files
into a single ______________________ file
that can be executed by the operating system.
file1.obj
file2.obj
fileN.obj
00100111010
10110001001
...
11101100100
11100101011
10010001000
...
10101101101
01101101011
11010101001
...
00101100100
C++
Linker
00100111010
10110001001
...
11101100100
11100101011
10010001000
...
10101101101
01101101011
11010101001
...
00101100100
file.exe
21
OCD with Libraries
1. Specify the desired behavior of the program.
2. Identify the objects needed.
3. Identify the operations.
a. If an operation is not predefined:
Extending
C++
Write a function to perform it.
b. If an operation is likely to be reusable someday:
Store its function in a library and access it from there.
4. Organize objects and operations into an algorithm.
22