A Closer Look at Classes in .NET

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A Closer Look at Classes
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This program displays the following output:
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ob1 before assignment: a is 10 b is 20 ob2 before assignment: a is 0 b is 0 ob1 after assignment: a is 10 b is 20 ob2 after assignment: a is 10 b is 20
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By default, all data from one object is assigned to the other using a bit-by-bit copy (That is, an exact duplicate is created) However, as you will see later, it is possible to overload the assignment operator so that customized assignment operations can be defined Remember: Assignment of one object to another simply makes the data in those objects identical The two objects are still completely separate Thus, a subsequent modification of one object s data has no effect on that of the other
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Passing Objects to Functions
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An object can be passed to a function in the same way as any other data type Objects are passed to functions by using the normal C++ call-by-value parameter-passing convention This means that a copy of the object, not the actual object itself, is passed to the function Therefore, changes made to the object inside the function do not affect the object used as the argument to the function The following program illustrates this point:
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#include <iostream> using namespace std; class OBJ { int i; public: void set_i(int x) { i = x; } void out_i() { cout << i << " "; } }; void f(OBJ x) { xout_i(); // outputs 10 xset_i(100); // this affects only local copy xout_i(); // outputs 100 }
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int main() { OBJ o;
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oset_i(10); f(o); oout_i(); // still outputs 10, value of i unchanged return 0; }
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The output from the program is shown here
10 100 10
As the output shows, the modification of x within f( ) has no effect on object o inside main( )
Constructors, Destructors, and Passing Objects
Although passing simple objects as arguments to functions is a straightforward procedure, some rather unexpected events occur that relate to constructors and destructors To understand why, consider this short program:
// Constructors, destructors, and passing objects #include <iostream> using namespace std; class myclass { int val; public: myclass(int i) { val = i; cout << "Constructing\n"; } ~myclass() { cout << "Destructing\n"; } int getval() { return val; } }; void display(myclass ob) { cout << obgetval() << '\n'; } int main() { myclass a(10); display(a); return 0; }
A Closer Look at Classes
This program produces the following, unexpected output:
Constructing 10 Destructing Destructing
As you can see, there is one call to the constructor function (which occurs when a is created), but there are two calls to the destructor Let s see why this is the case When an object is passed to a function, a copy of that object is made (and this copy becomes the parameter in the function) This means that a new object comes into existence When the function terminates, the copy of the argument (ie, the parameter) is destroyed This raises two fundamental questions: First, is the object s constructor called when the copy is made Second, is the object s destructor called when the copy is destroyed The answers may, at first, surprise you When a copy of an argument is made during a function call, the normal constructor is not called Instead, the object s copy constructor is called A copy constructor defines how a copy of an object is made (Later in this chapter you will see how to create a copy constructor) However, if a class does not explicitly define a copy constructor, then C++ provides one by default The default copy constructor creates a bitwise (that is, identical) copy of the object The reason a bitwise copy is made is easy to understand if you think about it Since a normal constructor is used to initialize some aspect of an object, it must not be called to make a copy of an already existing object Such a call would alter the contents of the object When passing an object to a function, you want to use the current state of the object, not its initial state However, when the function terminates and the copy of the object used as an argument is destroyed, the destructor is called This is necessary because the object has gone out of scope This is why the preceding program had two calls to the destructor The first was when the parameter to display( ) went out of scope The second is when a inside main( ) was destroyed when the program ended To summarize: When a copy of an object is created to be used as an argument to a function, the normal constructor is not called Instead, the default copy constructor makes a bit-by-bit identical copy However, when the copy is destroyed (usually by going out of scope when the function returns), the destructor is called
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