Answers to Memory Management in C and C++


Question #1
  • When will this line fail to compile:

    new myObj[100];
    
    a) Never
    b) When myObj is too large to fit into memory
    c) When myObj has no default constructor

    Question #2
  • Assuming that myObj is less than 1000 bytes, is there anything wrong with this code?
    char x[1000];
    myObj *obj = reinterpret_cast<myObj *>(x);
    
    new (obj) myObj;
    

    a) Nope, it works fine
    b) Yes, there could be byte alignment issues
    c) Yes, the syntax for calling new is incorrect

    Question #3

  • What is the functional difference between
    myObj *x = new myObj[100];
    delete x;
    
    and
    myObj *x = new myObj[100];
    delete [] x;
    

    a) There is none; they both work as expected
    b) They both do nothing.
    c) The first will not invoke all myObj destructors

    Question #4

  • What is wrong with the following code?

    int * x = (int *) malloc(100 * sizeof(int));
    x = realloc(x, sizeof(int) * 200);
    
    a) If realloc fails, then the original memory is lost
    b) Nothing, realloc is guaranteed to succeed (by returning the original pointer)
    c) Nothing, realloc frees the original memory passed to it

    Question #5
  • What is one possible symptom of having called free on the same block of memory twice?

    a) Nothing, calling free twice on one block of memory always works fine
    b) Malloc might always return the same block of memory
    c) You cannot free any memory in the future

  • Question #6
  • What's wrong with this line of code?
    int *x = NULL;
    delete x;
    

    a) It causes a segmentation fault when delete tries to access NULL
    b) Nothing
    c) It is undefined behavior

    Question #7

  • Assuming this code were being compiled using a standards-conforming C++, what is wrong with it?

    int * x = malloc(100 * sizeof(int));
    x = realloc(x, sizeof(int) * 200);
    
    a) malloc is undefined in C++, you can only allocate memory using new
    b) Invalid cast from a void* to an int*
    c) Nothing is wrong with this code

    Question #8
  • What is the default behavior of the normal new operator failing?

    a) This is left up to the implementation to decide
    b) It returns NULL
    c) It throws an exception

    Question #9

  • What is a difficulty that arises with handling out of memory errors?

    a) Many cleanup operations require extra memory
    b) Running out of memory rarely happens in systems that interact with users
    c) Once you've run out of memory, your program cannot continue

    Question #10

  • What is the result of this code?
    myObj *foo = operator new(sizeof(foo));
    

    a) That's not legal syntax!
    b) foo has memory allocated for it, but is not constructed
    c) foo has memory allocated for it, and is fully constructed