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Constructors and Destructor

Posted By: Idelia Miller     Category: C++ Programming     Views: 48294

This is article explains about Constructor, Characteristics of Constructors, Parameterized Constructors, Multiple Constructors in a Class, Constructors With Default Arguments, Dynamic Initialization of Objects, Copy Constructor, Assignments and Initializations, Dynamic Constructors, Constructing Two Dimensional Arrays, Destructors.

Constructors and Destructors

  • Providing initial values with the definition
  • Make the user defined type behave like normal
  • Variable going out of scope to be deleted
  • A special member function constructor which enables an object to initialize when created
  • Destructor is a special member function which destroys the object when no longer required
  • Constructor bears the same name as the object
  • Destructor name is ~ prefixed to constructor 


  • It constructs the data values of a class
class integer 
{ int m,n; 
   public : <no data type> integer (void)
        { m = 0 ; n = 0; } };
integer int1;
  • There is no need to write any statement to invoke the constructor function
  • A constructor which accepts no arguments is called a default constructor // integer :: integer()!

Characteristics of Constructors

  • They should be declared in the public section
  • They are automatically invoked and have no return types, neither can they return values
  • They can not be inherited but called by DC Const
  • They can not be virtual nor can we refer to their addresses & they can have default arguments
  • An object with cons or dist can’t member of uni
  • They make implicit calls to new and delete
  • When cons is declared, init becomes mandatory

Parameterized Constructors

  • The constructors that can take arguments
  • integer( int x, int y)  { m = x; n = y; } is a PC
  • now integer int1; will not work
  • integer int1 = integer(8,7) is an explicit call
  • integer int1(8,7) is an implicit call
  • The parameters of a constructor can be any type except the class name to which it belongs
  • It can accept a reference to its own class as a parameter which is known as copy constructor

Multiple Constructors in a Class

integer() {m=0;n=0;}
integer(int x, int y) { m = x; n = y; }
integer(integer & i) { m = i.m; n = i.n }
integer i1;
integer i2;
integer i3(i2);
  • Above is constructor overloading
  • c++ compiler’s implicit constructor creates objects but once we define constructors, we must also define the do-nothing implicit constructor 

Constructors With Default Arguments

  • complex (float real, float imag = 0);
  • complex c(5.0) assigns value 5.0 to real and 0 to imag variable
  • complex c(2.0,3.0) assigns to both of them
  • default constructor and default argument constructor are different
  • A::A() and A::A(int i = 0)
  • When both of the above forms used in the class, they creates ambiguity for the compiler

Dynamic Initialization of Objects

public :
    fixed_deposit() {}
    fixed_deposit(long p, int y, float R = 0.12);
    fixed_deposit(long p, int y, int r);
{fixed_deposit FD1,FD2,FD3;
// defining and reading p, y, R and r here
FD1 = fixed_deposit(p,y,R); //parameter value 
FD2 = fixed_deposit(p,y);  // provided at run time
FD3 = fixed_deposit(p,y,r);
// displaying them here

Copy Constructor

  • integer i2 = i1 is same as integer i2(i1)
  • The process of initializing through a copy constructor is called copy initialization 
  • A copy constructor takes a reference to an object of the same class as itself
  • i2=i1 will copy member by member but does not call the copy constructor
  • We can not pass by value to a copy constructor
  • Compiler supplies its own CC when not defined

Assignments and Initializations

  • A value of an object is given to another in two distinct situations assignments and initializations
  • Initialization can occur in three ways
    • When one object explicitly initializes another such as declaration
    • When a copy of an object is made to be passed to a function
    • When a temp object is generated (most commonly as a return value
  • CC applies only to initializations

Dynamic Constructors

  • Allocating memory while init objects
class string 
 { char *name; 
     int length;
  public : 
     string() {length = 0; 
                    name=new char[length +1]; }
     string(char *s)
                 {length = strlen(s); 
                   name=new char[length +1];

void join(string & a, string & b)
   { length = a.length + b.length;
      delete name; name = new char[length+1];
      strcat(name,b.name); }
char *first=“This”; //In the main()
string name1(first),name2(“is”),name3(“testing”);
string s1,s2;

Constructing Two Dim Arrays

class matrix
{ int **p; int d1,d2;
  public :
     matrix(int x, int y)
      { d1 =x; d2 =y ; p = new int *[d1];
         for (int i = 0 ; i < d1; ++i)
                    p[i] = new int[d2];  }
     void get_element(int i,int j,int value)
       { p[i][j] = value}
     int & put_element(int i, int j)
       { return p[i][j] }                           


  • It’s primary purpose is to destroy memory used by an object
  • A default destructor is called when not defined
  • A default constructor allocates space which gets cleaned up by default destructors
  • If we are using dynamic objects, we have to do our own memory cleanup (garbage collection!)
  • It is implicitly called when objects go out of scope
  • It never takes an argument, nor it returns any val
  • Whenever new is used to allocate memory in the constructors, we should use delete to free it
  • A destructor for matrix class 
matrix ::~matrix()
 { for (int i = 0; i< d1; i++) delete p[i];
     delete p; }
  • It gives us control over the end of an object
  • Destruction of dynamic object only is controlled

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Idelia Miller
Idelia Miller author of Constructors and Destructor is from Frankfurt, Germany.
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