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<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="utf-8" />
<title>ClassRelationships</title>
<meta name="description" content="Software Engineering course notes. Code Samples. Software Links" />
<meta name="keywords" content="Lecture, Notes, Code, Syracuse,University" />
<meta name="Author" content="Jim Fawcett" />
<meta name="Author" content="James Fawcett" />
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<body onload="initializeMenu()" oncontextmenu="return false;">
<nav>
<div id="navbar"></div>
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<a id="Next" href="Inheritance.htm">N</a>
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<header>
<hgroup id="pagetitle">
<h1 id="title">Class Relationships</h1>
</hgroup>
</header>
<!-- page content -->
<main id="main">
<h3 style="margin-top: 0px;">Synopsis:</h3>
<div class="indent">
<p>
The C++ language was built from the ground up to support user defined value types. It does that with copy constructors and copy
assignment operators that allow the designer to implement deep copies. That makes the state of the destination object have the
same value as the state of the source, but those states are independent. If the source changes its state later, that does not
affect the state of the destination.
</p>
<p>
C++ provides destructors that are called when the thread of execution leaves the scope where the instance was declared. This means
the developers have tight control over how and when an instance's resources are returned to the process, for use by other instances.
More importantly, the users of C++ classes do not need to be aware of these resource allocations and deallocations. They simply
use instances of the classes and the instances take care of all resource management.
</p>
<p>
The language also provides move construction and move assignment operations. Move construction builds an instance, taking ownership
of another instance's state. This transfer is very efficient, usually effected with a pointer swap. Similarly, move assignment
discards the destination's state and it takes ownership of the source's state.
</p>
<p>
Move operations usually only occur when the source is a temporary object, but the compiler can be coerced to execute a move on
a non-temporary object by using the std::move(...) operation. Note that you need to be careful with this, as the source no longer
owns it's former state and may be unable to continue. Essentially, the use of the std::move(...) is a promise not to use the
source object any longer, or to reinitialize it in some effective way.
</p>
</div>
<div class="indent" style="margin-bottom:10px;">
There are four <a href="../Presentations/ClassRelationships.pdf">class relationships</a>,
<a href="Repository/Presentations/ClassRelationships.doc">doc</a>,
we use to construct Object Oriented Designs: Inheritance, Composition, Aggregation,
and Using. Inheritance is an "is-a" relationship. Composition is a strong ownership or "part-of" relationship. Aggregation
is also a "part-of" relationship, but is weaker than composition because it does not guarantee that the aggregated part is
owned, only that it can be owned by the aggregator. Finally, using is a dependency on an object that is not part of the user,
was not created by the user, and should not be destroyed by the user.
</div>
<h3>Compound Object Example:</h3>
<div class="indent" style="float:right; padding-left:0px; padding-bottom:10px;">
<div style="float:left; margin:10px; border:1px solid gray; padding:5px; text-align:center; box-shadow:5px 5px 2px #888;">
<img src="Pictures/ObjectRelationships.JPG" ; width="700" />
<div style="text-align:center">
Compound Object Classes
</div>
</div>
</div>
<div>
<h4 style="padding-bottom:0px; margin-bottom:0px;">Construction Behaviors:</h4>
<ol style="margin-top:5px;">
<li>An instance of C is always contained within the footprint of each instance of B.</li>
<li>Clients of B do not see C's public interface. That is only accessible to B.</li>
<li>Similarly, an instance of B is always contained within each instance of D.</li>
<li>Since D derives from B via public inheritance, its public methods become public methods of D.</li>
<li>Note that this does not hold for constructors, destructor, or assignment operators.</li>
<li>
The C++ language guarantees that a constructor call is made for the creation of every object
even if the class designer did not provide constructors. It does that by generating default and copy constructors
that do member-wise construction of the class's bases and data elements if the class definition does not explicitly
provide them.
</li>
<li>
Similarly, the language guarantees that a destructor call is made for every object as it leaves the scope
in which it was declared, even if the class designer did not provide a destructor. In that case the compiler
generates a destructor that does member-wise destruction of each of the class's bases and member data.
</li>
<li>
Note:
<span class="itemcallout top">
This means that you must always decide whether to provide default and copy constructors and destructors for
every class you design.
</span>
</li>
<li>
<span>
If a class's bases and member data have correct copy, assignment, and destruction semantics, then you
don't need to, and should not, provide them. That is always true if your class holds only primitive types
and containers from the Standard Template Library, or holds instances of classes for which that is true.
</span>
</li>
<li>
If your class holds pointers to data on the heap, then you must provide copy, assignment, and destruction
operations, or qualify those operations with delete.
</li>
<li>
If you class holds any types that do not support copying or assignment, like mutexes and critical sections,
then you must qualify copy, assignment, and destruction operations with delete.
</li>
</ol>
</div>
<div style="clear:both;">
</div>
<div id="spacer"></div>
<div>
<h4>Example Code:</h4>
<div class="indent">
<a href="CodeSnap-CompoundObjects.cpp.html">Compound Objects CodeSnap</a>,
<div class="itemcallout">
This code has classes C, B, D, and U, shown in the diagram above. All of these classes,
in the demo code, have correct
copy,assignment, destruction, and move semantics. So we normally would not provide
those operations, deferring to the compiler generated ones.
<div style="height:5px;"></div>
However, they are provided
here so that they can write to the console to indicate to you how and when they are called.
</div>
</div>
</div>
<!--<p>
The classes C, B, D, and U, in the code below, demonstrate these operations with both class declarations and method implementations.
</p>
<div style="margin:20px 20px; padding-top:0px;">
<codewrapper>
<codecaption style="text-align:left; padding-left:45px;">
Example CompObj.cpp - only cpp file for this demonstration
</codecaption>
<code>
///////////////////////////////////////////////////////////////////////
// CompObj.cpp - Compound Object Operations //
// //
// Jim Fawcett, CSE687 - Object Oriented Design, Midterm Spring 2016 //
///////////////////////////////////////////////////////////////////////
/*
* The main objectives of this demonstrations are to:
* - illustrate the importance of constructor initialization sequences
* - expose all of the operations that compound objects are expected
* to provide.
*/
#include <iostream>
#include <string>
#include "../Utilities/Utilities.h"
void putMsg(const std::string& msg)
{
std::cout << "\n " << msg.c_str();
}
struct Cosmetic {
~Cosmetic() { std::cout << "\n\n"; }
} GlobalCosm;
///////////////////////////////////////////////////////////////////////
// C class - instances are composed by the base class B
class C
{
public:
C() { putMsg("C default construction"); }
C(const C& c) : name_(c.name_) // note initialization
{
putMsg("C copy construction");
}
C& operator=(const C& c)
{
putMsg("C assignment");
if (this == &c)
return *this;
name_ = c.name_;
return *this;
}
C(C&& c) : name_(std::move(c.name_))
{
putMsg("C move construction");
}
C& operator=(C&& c)
{
putMsg("C move assignment");
if (this == &c)
return *this;
name_ = std::move(c.name_);
return *this;
}
~C()
{
putMsg("C destruction");
}
private:
std::string name_;
};
///////////////////////////////////////////////////////////////////////
// B class - serves as the base of inheritance for D
/*
* B's constructors, as their first act, invoke a C constructor. They
* don't execute their bodies until C's construction completes.
*/
class B
{
public:
B()
{
putMsg("B default construction");
}
B(const B& b) : c(b.c)
{
putMsg("B copy construction");
}
B& operator=(const B& b)
{
putMsg("B copy assignment");
if (this == &b)
return *this;
c = b.c;
return *this;
}
B(B&& b) : c(std::move(b.c))
{
putMsg("B move construction");
}
B& operator=(B&& b)
{
putMsg("B move assignment");
if (this == &b)
return *this;
c = std::move(b.c);
return *this;
}
virtual void g()
{
putMsg("Calling g()");
}
virtual ~B()
{
putMsg("B destruction");
}
private:
C c;
};
///////////////////////////////////////////////////////////////////////
// U class - instances are used by D
class U
{
public:
U()
{
putMsg("U default construction");
}
U(const U& u)
{
putMsg("U copy construction");
}
U& operator=(const U& u)
{
putMsg("U assignment");
return *this;
}
U(U&& u)
{
putMsg("U move construction");
}
U& operator=(U&& u)
{
putMsg("U move assignment");
return *this;
}
~U()
{
putMsg("U destruction");
}
};
///////////////////////////////////////////////////////////////////////
// D class
// - D derives from B.
// - There are no virtual functions because our goal is to
// illustrate operations that occur when a compound object
// is created, assigned, and destroyed.
/*
* D's constructors, as their first act, invoke a B constructor. They
* don't execute their bodies until B's construction completes.
*/
class D : public B
{
public:
D()
{
putMsg("D default construction");
}
D(const D& d) : B(d)
{
putMsg("D copy construction");
}
D& operator=(const D& d)
{
putMsg("D assignment");
if (this == &d)
return *this;
B::operator=(d);
return *this;
}
D(D&& d) : B(std::move(d))
{
putMsg("D move construction");
}
D& operator=(D&& d)
{
putMsg("D move assignment");
if (this == &d)
return *this;
B::operator=(std::move(d));
return *this;
}
virtual void g()
{
putMsg("Calling D::g()");
}
void f(U& u)
{
putMsg("D using U");
}
~D()
{
putMsg("D destruction");
}
};
///////////////////////////////////////////////////////////////////////
// global function testFunction
// - Illustrates move construction from temporary objects.
D testFunction()
{
Utilities::title("Running testFunction");
D d;
return d; // d will be moved, not copied
}
///////////////////////////////////////////////////////////////////////
// Test stub
// - You should experiment with this and with the parts provided
// for the classes.
// - What do you think will happen if you comment out the move
// operations?
int main()
{
Utilities::title("Demonstrating Operation of Compound Object", '=');
U u;
D d;
d.f(u);
d = testFunction();
Utilities::title("starting copy construction");
D d2 = d;
Utilities::title("starting move construction");
D d3 = std::move(d);
// d is now invalid
Utilities::title("Demonstrating polymorphism");
B* ptr = new B();
ptr->g();
ptr = &d;
ptr->g();
Utilities::title("leaving main's scope");
}
</code>
</codewrapper>
</div>
<div class="spacer"></div>
<div style="float:left; margin:30px; border:1px solid gray; padding:5px; text-align:center; box-shadow:5px 5px 2px #888;">
<img src="../Presentations/CompoundObject.out.jpg" ; width="550" />
<div style="text-align:center">
Compound Object Demo Output
</div>
</div>
<p style="padding-left:50px;">
The <a href="../code/CompoundObjects">Compound Objects Code</a><br />
folder provides this code in a Visual Studio project so you can try it out.
</p>
<div class="spacer"></div>
</div>
<div id="spacer"></div>-->
</main>
<div style="margin-top:25px; border:1px solid black; padding:10px;">
<img class="photo" src="pictures/Campusstrip.jpg" alt="CST strip" width="100%" />
</div>
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