Dynamic Memory Continued

• You author addresses pointer math.
  • I do not wish to cover this.
  • So I will not.
• In C++ you can override new and delete if you wish.
  • This is, in my opinion, outside the scope of this class.
  • This is an operator overload item
  • And is covered in chapter 15.
• He also discusses Garbage Collection
  • Java, python, javascript, ... all do this.
  • This eliminates the need for direct calls to delete.
  • But is not implemented in C++
  • The environment keeps track of all dynamically allocated objects
  • When the last reference to the object is removed, the object is automatically deleted.
  • This is an OS topic.
• I am going to skip Smart Pointers (182-192) for now.
  • You need at least one more pass with raw pointers.
  • We will be back if we get time.
• Common Memory Pitfalls
  • Page 192
    • Read this and go back and read it again.
  • His example on the bottom of 192 is the definition of a buffer overflow.
    • A buffer overflow occurs when data written to a buffer also corrupts data values in memory addresses adjacent to the destination buffer due to insufficient bounds checking. This can occur when copying data from one buffer to another without first checking that the data fits within the destination buffer. (Stolen shamelessly from wikipedia)

    • char buffer[1024] = {0};
      pos = 0;
      size_t i;
      bool done = false;
      while (not done) {
         char * nextChunk = GetMoreData();
         if (nextChunk != nullptr) {
             for(i = 0; i < nextChunk.size(); i++) {
                buffer[pos] = nextChunk[i];
                pos++;
             }
         } else {
             done = true;
         }
      } 

    • What is wrong with this code?
    • How can this break things?
  • Out of Bounds Memory
    • This is an array problem that we have hopefully handled.
    • Read it anyway.
  • Memory leaks
    • What is a memory leak?
    • How can they occur?

    • struct MyStruct{
      ...
      };
      
      void Fix(MyStruct * & ptr) {
       ...
       ptr = new MyStruct
       ...
      }
      ...
      
      MyStruct * x;
      
      x = new MyStruct;
      Fix(x);
      
      delete x;
      
               

    • Look at leak.cpp
    • Valgrind is our friend
  • Double Delete
    • Look at crash.cpp
• What does the overloaded assignment operator do?
• What does the copy constructor do?
• What does the destructor do?