CMSC 202 Computer Science II, Fall 2015, Section 06

Lecture 13: Dynamic Memory Allocation

Tue 10/13, 2015

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Assigned Reading: 

Slides As Shown:

• Announcements, etc: [slide view] [print view]

• Destructors: [slide view] [print view]

Topics Covered:

• Danger of destructors: objects are copied in many situations. We need to make sure that the copies are deep copies and not shallow copies. Otherwise, when a destructor is called it frees the memory that other objects are still pointing to.

  Examples of bad behavior:

  • Using delete instead of delete [] : bad1.cpp.
  • After assignment: bad2.cpp.
  • After initializing with DMC X = DMC(Y) : bad3.cpp.
    Note: default copy constructor called instead of default constructor.
  • Passing a DMC by value: bad4.cpp.
  • Returning a DMC value: bad5.cpp.
  • Explicitly calling a destructor: bad6.cpp.
  • Explicitly calling a destructor thru a pointer: bad7.cpp.

• For classes with dynamically allocated members, we need to implement a copy constructor and an overloaded assignment operator to perform the deep copying. (See slides.)

• Finished discussion of copy constructors and the overloaded assignment operator. See slides linked above.

  • New version of DMC with copy constructor and assignment operator dmc2.h, dmc2.cpp.

  • Check that assignment and copy constructors work: good1.cpp.

  • DMC object as a data member of AClass: good2.cpp.

  • Passing a DMC object by value and returning by value: good3.cpp

• Did not show these examples in class.
  Comparing memory initializers versus assignments in a constructor: we have AClass that has a DMC object as a data member. The alternate constructor for AClass can assign values to its data members using an assignment, or it can use memberinitializers.
  • This version uses assignment: meminit1.cpp
  • This version uses member initializers: meminit2.cpp