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Dynamic Arrays in C++: the new operator

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• The new operator
  • C++ has the new operator that is identical to Java's own new operator

    (So I don't need to spend much time explaining what it does.

    I have explained new before, if you need a refresher, see: click here)

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  • Syntax of C++'s new operator:

    new TYPE // Allocate space to store ONE variable of // the type TYPE new TYPE [ N ] // Allocate space to store N variables of // the type TYPE

  • Just like in Java, the new in C++ returns the address of the reserved memory block

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• Storing the address returned by new
  • The new operator returns an address

  • A reference variable can store an address

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  • Clearly, we must use a reference variable to store the address returned by new

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  • Example:

    int *p; // p can store an address int i; p = new int[10]; // We are using array notation with a pointer variable ! p[0] = 12; p[1] = 23; p[2] = 34; p[3] = 45; p[4] = 56; p[5] = 67; p[6] = 78; p[7] = 89; p[8] = 90; p[9] = 111; for ( i = 0; i < 10; i++ ) { cout << p[i] << " "; }

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  • Graphically, this data structure is created by the above code:

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  • NOTE:

    The above data structure should look familiar to you... It is the same way that an array is stored in Java !!! Example: int[] a; a = new int[10]; results in:

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• The delete operator
  • Unlike Java, the C++ programming language does not provide a garbage collection system to reclaim garbage

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  • Recall that garbage is an inaccessible variable that was previous created by the new operator.

    Example:

    ListElem p; p = new ListElem; Result: p = new ListElem; // Overwrites p with a new address Result:

    The first ListElem can no longer be accessed (because the location is "forgotten")

    Such memory variable is known as garbage

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  • When a dynamically created/allocated variable is no longer used (needed), you must de-allocate the reserved memory using:

    delete PointerVariable;

    Example:

    ListElem p; p = new ListElem; // Allocate space for one ListElem variable ... delete p; // **** Deallocate space allocated !!!

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• Dynamic arrays in C++
  • Arrays in Java is dynamic

    This means that the size of an array in Java can change during the program execution

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  • In contrast, Arrays in C++ is static

    This means that the size of an array in C++ cannot change during the program execution

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  • BUT...., C++ does support dynamic arrays using pointer variables

  • Example: double the size of an array

    int i, N; int *p, *q; /* ------------------------------------ Make an array ------------------------------------ */ N = 10; p = new int[N]; // array of 10 elements for ( i = 0; i < N; i++ ) p[i] = i; /* ------------------------------------ Double the side of the array ------------------------------------ */ q = new int[2*N]; // array of 20 elements for ( i = 0; i < N; i++ ) q[i] = p[i]; // Copy old array to new array delete(p); // **** Remember to free unused space !!! p = q; // Switch p over to the new array N = 2*N; // Remember new array size

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  • Example Program: (Demo above code)                                                
    • Prog file: click here

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• REPEAT: always de-allocate unused memory in C++
  • The new operator in C++ is just like the one in Java which is used to reserve unused memory

  • But unlike Java, the C++ programming language does not have a garbage collector

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  • Very very very Important fact:

    You must return the reserved memory obtained by new when the variable (memory) become unnecessary