COMP345:
Advanced Program Design with C++
Lecture 1
Department of Computer Science and Software Engineering Concordia University
Contents
1. C++ program structure
➢ Free function and main function
2. Component and compilation unit ➢ header file and compilation file
3. Namespaces
Structure of C++ programs
❑ C++ is a superset of C
❖Main function is the program driver ❖Free functions
❖Data structures
❖A C program is a valid C++ program
❑ Classes encapsulate other functions: Object-oriented
❑ The main function is a free function, not a class member
Simple C++ program
Some compilers will allow you to omit the int or replace it with void
ends the invocation of main and return 0 as the function’s value. This statement is not required
Program structure: multiple files programs
▪ Only simplistic programs are self-contained into one file
▪ C++ is a language that specifies programming structures and includes only
basic data structures and operators
▪ Most programs require the use of libraries
▪ Complier will compile the library’s code and the program’s code, then the linker will link them
▪ Unlike Java, a single C++ file can contain many classes/functions/data structures.
▪ This leads to what is called physical design, i.e. decisions as to what entities are grouped within/across files.
Program structure: multiple files programs
▪ Aside from having to use libraries, large programs need to be split into different files for various reasons:
▪ Speed up compilation: Upon changes to the code, the compiler will recompile only the files that had a change.
▪ Increase organization, decrease browsing time: Physically splitting your code along logical lines will make it easier to browse through the code to locate classes, functions, etc.
▪ Facilitate code reuse: Modular physical design allows for grouping related entities and separating them from less related ones. Each group can then be logically designed to be reusable across different projects. Reused code can be fixed, fixing all projects that use it.
▪ Split coding responsibilities among programmers: For really large projects, several programmers are involved. The larger are the program files, the more likely it is that several programmers are changing the same file simultaneously.
Contents
1. C++ program structure
2. Component and compilation unit
➢ header file and compilation file 3. Namespaces
4. Basic terms & concepts
Program structure: compilation unit
▪ In C++, a compilation unit is a file (.cpp)
▪ A file may contain several functions, data
structures, or classes (unlike Java)
▪ Each compilation unit is compiled individually into an object file
▪ The object files typically have cross- references to other object files
▪ The linker then attempts to resolve cross- references between the object files to form the unique executable file
Handle some textual manipulation before the text is given to the compiler (looks at directive #)
e.g. provides the ability for the inclusion of header files, macro expansions, conditional compilation, and line control.
Compilation unit and linkage: Example
file: one.h
class A {
int Af(int);
… }
int onef(int);
file: one.cpp
#include one.h #include two.h int A::Af(int){
B myB;
… myB.Bf()…;
}
int onef()int{ …
}
file: one.o
A::Af …
… B.Bf() …
onef()
compile unit one.cpp
file: two.h
class B { int Bf();
… }
class C{ …
}
file: two.cpp
#include two.h int B::Bf(){
… }
…
file: two.o
B::Bf …
…
compile unit two.cpp
Program structure: compilation units as program parts
▪ Compilation units are separate program parts, or components
▪ Kept in separate files
▪ Compiled separately and linked together before the program runs
▪ Compared to Java, C++ provides much more freedom to define what is a “part”, as it can be composed of a group of functions, data structures and classes
▪ There is no such thing as Java-like “packages” in C++.
▪ In C++, there is a separation between the interface and the
implementation of a component
▪ Each component is a group of highly cohesive and highly coupled elements
Program structure: cohesion
▪ Cohesion: The degree to which the elements of a module belong together in the achievement of a common goal.
▪ Cohesion is increased if:
▪ The functionalities embedded in a module have much in common.
▪ The composing elements carry out a small number of related activities, by avoiding coarsely grained and/or unrelated sets of data.
▪ Advantages of high cohesion:
▪ Increased understandability of modules (simpler, fewer operations).
▪ Increased maintainability, because changes in one module require fewer changes in other modules.
▪ Increased reusability, because application developers will find the component they need more easily among the cohesive set of operations provided by the module.
Program structure: coupling
▪ Coupling: the degree to which an element relies on other elements for its operation
▪ Coupling is not a desired quality, but is a side-effect of defining different elements
to carry a common task
▪ Much related to cohesion: good modules exhibit high cohesion and high coupling
▪ Coupling between modules is what really needs to be avoided
▪ Disadvantages of high coupling:
▪ A change in one module forces a ripple effect of changes in other modules
▪ Assembly of modules requires more effort due to the increased inter-module dependency
▪ A module might be harder to reuse and/or test because dependent modules must be included
Contents
1. C++ program structure
2. Component and compilation unit
➢ header file and compilation file 3. Namespaces
4. Basic terms & concepts
Program structure: header files and implementation files
▪ header File (.h file)
▪ Contains class declaration with free functions and operators declarations
▪ Useful to programmers, as it is overview of a component that omits implementation details
▪ Source File (.cpp file)
▪ Contains free/member function definitions ▪ The .cpp file is a compilation unit
▪ General rules:
▪ There should be a one-to-one relationship between a given .ccp file and a
corresponding .h file, and they should have the same name.
▪ The .cpp file “#includes” its corresponding .h file.
▪ A .cpp file should never be “#included”, as it effectively merges two compilation units together.
Header file: example
//This is the header file dtime.h. This is the interface for the class DigitalTime.
//Values of this type are times of day. The values are input and output in
//24 hour notation as in 9:30 for 9:30 AM and 14:45 for 2:45 PM.
#ifndef
#define
DTIME_H
DTIME_H
#include
using
namespace
std;
class DigitalTime
{
:
public
DigitalTime(
int
DigitalTime( );
getHour( )
const
getMinute( )
const
void
advance(
int
minutesAdded);
void
advance(
int
hoursAdded, int
minutesAdded);
friend
bool
operator
==(const
DigitalTime& time1, const
DigitalTime& time2);
friend
istream& operator
>>(istream& ins, DigitalTime& theObject);
friend
ostream& operator
<<(ostream& outs, const
DigitalTime& theObject);
theHour, int theMinute); ;
;
private
:
int
hour;
int
minute;
static
void
readHour(int& theHour);
static
void
readMinute(int& theMinute);
};
static
int
digitToInt(char
c);
#endif
//DTIME_H
Concordia University Department of Computer Science and Software Engineering
Source file: example
//This is the implementation file: dtime.cpp of the class DigitalTime.
//It contains all the function definitions for the function declarations in the dtime.h file.
//The interface for the class DigitalTime is in the header file dtime.h.
//Note that there is no class declaration in this file.
#include
#include
#include
using
#include
//Uses iostream and cstdlib:
DigitalTime::DigitalTime(
int
theHour, int
theMinute) {
if
(theHour < 0 || theHour > 24 || theMinute < 0 || theMinute > 59){
}
{
}
}
}
}
}
{
{
cout <<
exit(1);
"Illegal argument to DigitalTime constructor."
;
else
hour = theHour;
minute = theMinute;
if
(hour == 24)
hour = 0;
DigitalTime::DigitalTime( ){
hour = 0;
minute = 0;
int
DigitalTime::getHour( )
const
return
return
namespace
“dtime.h”
hour;
std;
int
DigitalTime::getMinute( )
const
minute;
// All other member functions and operators definitions declared in dtime.h should be defined here
}
Concordia University Department of Computer Science and Software Engineering
Program structure: header files
▪ Typically, for each component there is a x.cpp and a corresponding x.h file
▪ Any source file generally #includes their own header file.
▪ Any source file x.cpp using entities declared/defined in another component generally will start with #include “otherComponent.h”
▪ This way, during the compilation of this x.cpp, the compiler first compiles the declarations of all the entities of this component as declared in otherComponent.h.
▪ Any file that refers to some names not declared internally file must #include the file in which these names are declared.
▪ If a function definition is compiled that contains a function call for a function that was properly declared, this function’s call needs to be resolved in order to be branched upon at runtime.
▪ If the function is defined within the current compilation unit, it is resolved at compile time.
▪ If the function is not defined in the current compilation unit, the linker will attempt to resolve it by
looking over what all the compilation units offer.
▪ In Java, much of this is solved by having files named after the single class they contain.
▪ The Java compilation model is much easier to use, though less flexible than the C++ compilation model.
Compilation unit and linkage: Example
file: one.h
class A {
int Af(int);
… }
int onef(int);
file: one.cpp
#include one.h #include two.h int A::Af(int){
B myB;
… myB.Bf()…;
}
int onef()int{ …
}
file: one.o
A::Af …
… B.Bf() …
onef()
compile unit one.cpp
file: two.h
class B { int Bf();
… }
class C{ …
}
file: two.cpp
#include two.h int B::Bf(){
… }
…
file: two.o
B::Bf …
…
compile unit two.cpp
Program structure: header files inclusion
▪ A component’s entities’ declarations (classes with that member functions and free functions declaration) should always be in its header file.
▪ Other components that use this component will “#include” it. There is different syntax to use if you are including a user-defined module, or an existing library module:
#include “mymodule.h”
▪ Quotes indicate a user-defined module
▪ The compiler will find it in your project directories
#include
▪ < > indicate predefined library header file
▪ The compiler will find it in the library directories ▪ Using different search paths
Program structure: implementation files
▪ A component’s implementation code should be in a .cpp file
▪ Give the header file and the implementation file the same name
▪ mymodule.h and mymodule.cpp
▪ Not enforced by the compiler, but failure to do so is confusing to other programmers
▪ A component is composed of classes and free functions
▪ Implementation file must #include the module’s header file, as it contains the module’s
classes and data structures declarations
▪ If it does not, the compiler will complain that its on entities are undeclared
▪ cpp files contain the executable code, i.e. function definitions ▪ Function definitions:
▪ Functions – main() function, free functions, member functions ▪ Operators – free operators, member operators
Program structure: redundant includes
▪ Header files are typically included multiple times in an application’s code by different components
e.g., class declaration included by class implementation and main program file
▪ Must only be compiled only once ▪ else, multiply defined names
▪ No guarantee which “#include” in which file the compiler might see first or how many files will end up including a particular header file
▪ Use preprocessor directives
▪ Instructs the compiler to read a header file only once
Program structure: compilation unit
▪ In C++, a compilation unit is a file (.cpp)
▪ A file may contain several functions, data
structures, or classes (unlike Java)
▪ Each compilation unit is compiled individually into an object file
▪ The object files typically have cross- references to other object files
▪ The linker then attempts to resolve cross- references between the object files to form the unique executable file
Handle some textual manipulation before the text is given to the compiler (looks at directive #)
e.g. provides the ability for the inclusion of header files, macro expansions, conditional compilation, and line control.
Program structure: redundant includes
▪ Header file fname.h structure:
#ifndef FNAME_H #define FNAME_H …
//Content of header file
… #endif
▪ FNAME typically name of file for consistency, readability
▪ This syntax avoids multiple definitions from compiling the same header file more than once ▪ May also use:
#pragma once …
//Content of header file
…
Program structure: redundant includes
▪ In fact, this is a specialized use of the conditional compilation preprocessor directive.
▪ Conditional compilation:
#ifdef x //or ifndef …
#else … #endif
▪ Can be used to switch between portions of code by switching on/off x, e.g. machine- dependent code.
▪ Variables (e.g. x) are macro variables used/maintained by the preprocessor.
▪ You may encounter some legacy code that makes very heavy use of preprocessor directives.
Contents
1. C++ program structure
2. Component and compilation unit
➢ header file and compilation file 3. Namespaces
Program structure: namespaces
▪ Namespace: Collection of name definitions inside of a program, potentially across different files
▪ For example, namespace “std” is common in libraries. Has all standard library definitions we need
#include
▪ Includes entire standard library of name definitions
▪ #include
using std::cout;
▪ Can specify just the objects we want
▪ Can be more efficient, as it avoids including things we don’t use
Program structure: namespaces
▪ Used as a solution to resolve potential name clashes ▪ Large programs use many classes and functions
▪ As a program re-uses many other files, it increases the possibility of encountering entities that have the same name
▪ Namespaces are meant to deal with this
▪ A namespace can be turned “on” with the using namespace directive
▪ But how to “switch it off” after it has been activated?
❖ You cannot, but conveniently, the using directive is effective only in the code block in which it is used (see next slide). Use different namespaces in separated code blocks. Though this solution has its limitations.
❖ This is one reason why the use of using NS::name directive is advocated over using namespace NS
Program structure: namespaces
▪ Given namespaces NS1 and NS2
▪ Both have void function myFunction()defined differently
▪ If we want to use either definitions at different places in our program, we may do the following: {
using namespace NS1;
myFunction(); }
{
using namespace NS2; myFunction();
}
OR
{
using NS1::MyFunction(); myFunction();
}
{
using NS2::MyFunction(); myFunction();
}
Program structure: global namespace
▪ All code goes in some namespace
▪ Unless specified, code belongs to the global namespace
▪ No need for using directive
▪ Global namespace always available
▪ But there is no way to “turn it off”
▪ Thus, global namespace is prone to name clashes
Program structure: creating a namespace
▪ To create a namespace:
namespace Space1
{
Some_Code }
▪ Places names defined in Some_Code into namespace Space1 ▪ Can then be made available by :
using namespace Name_Space_Name
▪ And any of the entities defined (e.g. NSentity) in it can be made available by: using Name_Space_Name::NSentity
Program structure: creating a namespace across header and implementation files
▪ As seen earlier, header files and implementation files hold different parts of the definition/declaration of the entities in a module.
▪ Thus, the same namespace needs to be declared in both ▪ In the header file (declarations):
namespace Space1
{
void greeting();
}
▪ In the implementation file (definitions):
namespace Space1
{
void greeting()
{
cout << "Hello from namespace Space1.\n"; }
}
Program structure: inline namespace qualification
▪ Can specify where name comes from
▪ Use "qualifier" and scope-resolution operator ▪ Used if only intend one use (or few)
▪ If overused, leads to less readable code
NS1::fun1();
▪ Specifies that fun1() comes from namespace NS1
▪ Especially useful for parameters:
int getInput(std::istream inputStream);
▪ Parameter found in istream’s std namespace
▪ Eliminates need for using directive or declaration
Program structure: namespaces example
Concordia University Department of Computer Science and Software Engineering
Program structure: namespaces example
Concordia University Department of Computer Science and Software Engineering
Program structure: unnamed namespace
▪ Compilation unit:
▪ A .cpp file, along with all files #included in the file
▪ Thus, in you #include a .cpp file, you merge them into a single compilation unit. Avoid!
▪ Every compilation unit has its own local unnamed namespace
▪ Declared in the same way as a named namespace, but with no name
▪ All names declared in an unnamed namespace are then local to the compilation unit
▪ Use unnamed namespace to keep things "local"
▪ Scope of unnamed namespace is compilation unit ▪ Not same as global namespace
▪ Global namespace:
▪ No namespace grouping at all; global scope
▪ Unnamed namespace:
▪ Has namespace grouping, just no name; local scope
Program structure: global vs. unnamed namespace example
#include
namespace {
const int i = 4;
}
int i = 2;
// this is local
// this is global
int main() {
cout << i << endl; // ERROR, i is ambiguous return 0;
}
References
▪ Walter Savitch, Absolute C++ (Chapter 1, 11), Addison-Wesley, 2006.
▪ Bjarne Stroustrup, The C++ Programming Language (Chapters 2,6,14,15), Addison-
Wesley, 2013.
▪ Y. Daniel Liang, Introduction to Programming with C++ (Chapter 2, 13). ▪ Joey Paquet COMP345 course Notes Concordia university.