Today’s Topics

March 4, 2014 CS410 – Software Engineering Lecture #9: C++ Basics III

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Today’s Topics

• The operators new and delete• The scope resolution operator• Nested classes• Static and const members• The “this” pointer• Constructors and destructors


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The Operators new and delete• The unary operators new and delete are available

to manipulate free store.• They are more convenient than the C functions

malloc(), calloc(), and free().• Free store is a system-provided memory pool for

objects whose lifetime is directly managed by the programmer.

• This adds responsibility to the programmer and can easily lead to problems such as memory leaks.

• On the other hand, manipulating free store is an efficient and flexible way to handle data structures such as trees and lists.


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The Operators new and delete• The programmer creates an object using new, and

destroys the object using delete.• The operator new is typically used in the following

forms:•new type-name•new type-name initializer•new type-name [expression]

• In each case, there are at least two effects:•An appropriate amount of store is allocated

from free store to contain the named type.

•The base address of the object is returned as the value of the new expression.


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The Operators new and deleteExample:

…

int *p, *q;

p = new int(5);

q = new int[10];

…

In this code,• the pointer variable p is assigned the address of the store

obtained,• The location pointed at by p is initialized to the value 5,• the pointer variable q is assigned the base address of an int

array of size 10.


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The Operators new and deleteNotice the following things:• When memory is unavailable, the operator new can

either throw a bad_alloc exception or return the value 0.

• If no initializer is provided, the content of the allocated memory is undefined.

• Arrays cannot be initialized using the new operator.

• Objects created by the new operator always need to be destroyed by the delete operator as soon as they are not used by the program any more.


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The Operators new and delete• The operator delete destroys an object created by

new.• This returns its allocated storage to free store for

reuse.• The operator delete is used in the following forms:

•delete expression•delete [] expression

• The first form is used when the corresponding new expression has not allocated an array.

• The second form (empty brackets) is used when the original allocation was an array.

• The return type of delete is void (no return value).


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The Operators new and deleteExample: Dynamic allocation of an arrayint main(){ int *data; int size; cout << “\nEnter array size: “; cin >> size; assert(size > 0); data = new int[size]; assert(data != 0); for (int j = 0; j < size; j++) cout << (data[j] = j) << ‘\n’; delete [] data; return 0;}

Starting the program:

Enter array size: 40123


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The Scope Resolution Operator• The concept of classes adds new scope rules to

those of the kernel language.• You remember that one point of classes is to provide

an encapsulation technique.• It makes sense that all names declared within a

class be treated within their own scope as distinct from external names, function names, and other class names.

• This creates the need for the scope resolution operator.


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The Scope Resolution Operator• The scope resolution operator is the highest-

precedence operator in the C++ language.• It comes in two forms:

•::j (unary operator – refers to external scope)

•MyClass::j (binary operator – refers to class scope)

• Its unary form is used to access a name that has external scope and has been hidden by local or class scope.


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The Scope Resolution Operator

Example:

int count = 0;

void how_many(double w[], double x, int &count)

{

for (int i = 0; i < N; i++)

count += (w[i] == x); // local count

++ ::count; // global count tracks calls

}


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The Scope Resolution Operator

To better understand this program fragment, we change the parameter int &count to int &cnt:

int count = 0;

void how_many(double w[], double x, int &cnt)

{

for (int i = 0; i < N; i++)

cnt += (w[i] == x); // local count

++count; // global count tracks calls

}


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The Scope Resolution Operator• Binary scope resolution is used to clarify names that

are reused within classes.• For example, we need scope resolution to define

member functions:

class Student

{

public:

void PrintName();

private:

string studentName;

};

void Student::PrintName()

{

cout << studentName;

}


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Nested Classes

• Like blocks and namespaces, classes are scopes and can nest.

• Nesting allows local hiding of names and local allocation of resources.

• This is often desirable when a class is needed as part of the implementation of a larger construct.


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Nested ClassesExample:char c; // external scope ::cclass X // outer class declaration X::{public: char c; // X::c class Y // inner class declaration X::Y:: { public: void foo(char e) { X t; ::c = t.X::c = c = e; } private: char c; // X::Y::c };};

t.X::c is the same as t.c


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static and const Members• Using the modifier static in declaring a data

member means that the data member is independent of any given class variable.

• The data member is part of the class but separate from any single class object.

• You remember that nonstatic data members are created for each instance of the class.

• Using static data allows class data to be scoped to the class but still require only one object for its storage.

• Without static data members, data required by all instances of a class would have to be global.


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static and const Members

Since static members are independent of a particular instance, they can be accessed in the form

class-name :: identifier

Example:

class Point

{

public:

static int how_many;

};

Point::how_many = 0;

…

++Point::how_many;


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static and const Members

A static member function has the modifier static precede the return type inside the class declaration.Example:class Foo{ static int foo_function();};

int Foo::foo_function(){…}


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static and const Members• A data member declared with the const modifier

cannot be modified after initialization.• syntactically, a const member function has the

modifier follow the argument list inside the class declaration.

Example:

class Foo

{

int foo_function() const;

};

int Foo::foo_function() const {}


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static and const Members• The const and static member function

implementation can be understood terms of this pointer access.

• An ordinary member function is invoked as x.fcn(i, j, k).

• It has an explicit argument list i, j, k and an implicit argument list that includes the members of x (accessible through the this pointer).

• A static member function does not get the implicit arguments.

• A const member function cannot modify its implicit arguments.


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The this Pointer• The keyword this denotes a self-referential pointer to a class

object.• It cannot be used in static member functions.Example:class Point{public: void init(double u, double v) { x = u; y = v; } Point inverse() { x = -x; y = -y; return (*this); } Point* where_am_I() { return this; }Private: double x, y;};


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Constructors and Destructors

• A constructor is a member function whose name is the same as the class name.

• It constructs values of the class type.• This process involves initializing data members and,

frequently, allocating free store by using new.

• A destructor is a member function whose name is the class name preceded by the ~ character.

• It finalizes objects of the class type.• Typically, a destructor deallocates store assigned to

the object by using delete.


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Constructors• can take arguments,• can be overloaded.

A constructor is invoked whenever• its associated type is used in a definition,• call-by-value is used to pass a value to a function,• the return value of a function must create a value of

associated type.


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Destructors• cannot take arguments,• cannot be overloaded.

A destructor is invoked implicitly whenever an object goes out of scope.

Constructors and destructors do not have return types and cannot use return expression statements.


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Classes with ConstructorsExample: A data type ModInt for storing numbers that are computed with a modulus.

class ModInt{public:

ModInt(int i); // constructor declarationvoid assign(int i) { v = i % modulus; }void print() const {cout << v << ‘\n’; }const static int modulus;

private:int v;

};

ModInt::ModInt(int i) { v = i % modulus; } // constructor definitionconst int ModInt::modulus = 60;


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Classes with Constructors

void main()

{

ModInt a(5);

ModInt b(62);

a.print();

b.print();

}

What does the output look like?

5

2


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Classes with Constructors

What happens if we declare a variable c as follows:

ModInt c;

Since this class has only one constructor, and this constructor needs one int argument, this declaration causes a compile-time error.

The declaration above requires a default constructor.


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The Default Constructor

• A constructor requiring no arguments is called the default constructor.• It can be a constructor with an empty argument list or

one whose arguments all have default values.• It has the special purpose of initializing arrays of objects of its class.In the ModInt example, it would be useful to define a default value of v to be 0.To achieve this, we could add the following default constructor:

ModInt() { v = 0; }


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main ()

{

ModInt s1, s2;

ModInt d[5];

ModInt s1.print();

ModInt s2.print();

ModInt d[3].print();

}

Output:

0

0

0


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• If a class does not have a constructor, the system provides a default constructor.• If a class has constructors but no default constructor, array allocation causes a syntactic error.

In our ModInt example, the following constructor could serve as both a general initializer and a default constructor:

ModInt(int i = 0) { v = i % modulus; }


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Constructor Initializers

• A special syntax is used for initializing class members.• Constructor initializers for class members can be specified in a comma-separated list that follows the constructor parameter list.• The previous example can be recoded as:

ModInt(int i = 0): v(i % modulus) {}

• Notice that initialization replaces assignment.• The individual members must be initializable as member-name (expression list).


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Constructors as Conversions

• Constructors of a single parameter are used automatically for conversion unless declared with the keyword explicit.

• For example, T1::T1(T2) provides code that can be used to convert a T2 object to a T1 object.

• Let us take a look at the following class PrintChar, whose purpose is to print invisible characters with their ASCII designation (for example, the code 07 is alarm or bel).


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class PrintChar

{

public:

PrintChar(int i = 0) : c(i % 128) {}

void print() const { cout << rep[c]; }

private:

int c;

static const char* rep[128];

};

const char *PrintChar::rep[128] = {“nul”, “soh”, “stx”, …, “}”, “~”, “del”};


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int main()

{

PrintChar c;

for (int i = 0; i < 128; i++)

{

c = i; // or: c = static_cast<PrintChar>(i);

c.print();

cout << endl;

}

}

This program prints out the first 128 ASCII characters or their printable representations.

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