new & delete Lab                

Description                

The files for this lab are available in ~cs253/Lab/New. In this lab, we’ll show how you can redefine the new/delete operators to achieve a variety of useful or bizarre results. Record your answers to the questions asked in a file called results.txt, and check them in (see below). A video introduction is available.                 

new and delete are operators. We use them like this:                 

constexpr int SIZE = 20;
int *p = new int[SIZE];
for (int i=0; i<SIZE; i++)
    p[i] = i*i;             // table of squares
for (int i=0; i<SIZE; i++)
    cout << p[i] << ' ';
delete[] p;

0 1 4 9 16 25 36 49 64 81 100 121 144 169 196 225 256 289 324 361 

I don’t see any parentheses, so new isn’t a function, it’s an operator. Of course, when we override an operator, to provide our own implementation, we write a function. Therefore, we write functions named operator new and operator delete.                 

1. Redefining global new/delete                

Consider glob.cc. It redefines the global new and delete operators to add trace output. This could be useful in figuring out just when new and delete are being invoked.                 

Note the function signatures—at this level, new takes a size_t and returns a void *, whereas delete takes a void * and a size_t and returns void. Why does new return void *, as opposed to int *, or double *, or whatever type we’re allocating?                 

2. Redefining class-specific new/delete                

Consider class.cc. It redefines the class-specific new and delete operators to add trace output. This will not affect new and delete operations on other types, such as int or double.

• Run the code, consider the output. Now explain briefly, for each line of main(), which function is called and why?
• Why are operator new and operator delete declared static?

3. Forbid new/delete for a specific class                

Try to compile forbid1.cc. Does it compile? Why not?                 

4. Same thing, but using =delete                

Try to compile forbid2.cc. Does it compile? Why not?                 

5. Same thing, but with a Mixin                

🍨 forbid-mixin.cc does the same thing as forbid1.cc & forbid2.cc, but encapsulates it in a “mixin”.                 

A “mixin” is a base class, designed for inheritance, but don’t think of it as an “is-a” relationship. Instead, think of the base class as contributing methods to the derived class. It’s similar to how they mix chocolate sprinkles or crushed cookies into ice cream at places like Cold Stone Creamery.                 

Why does the mixin class declare its ctor & dtor protected?                 

6. Error checking                

Compile and execute errchk.cc. Observe the error message, and decide what caused it. How would you improve it to detect multiple deletions of the same memory? (You don’t need to write the code, just explain it.)                 

7. Scrubbing the memory                

There are several reasons for scrubbing memory.

• At new time, to ensure that you’re not erroneously counting on the memory being any particular value.
• At delete time, if you fear that the code may erroneously keep pointers to the deleted memory.

Consider the technique used in scrub.cc.                 

What is the best byte value to use for writing to memory? For example, zero is a poor choice, because it makes nice zero integers and zero floating-point values. You want something more disruptive, more obvious, something that makes a poor int, double, pointer, or string. Explain your reasoning.                 

8. Memory pooling                

Run normal.cc.

• What is the size of class Foo?
• What is the difference between the values of p and q? Why?

Now, run pool.cc.

• What is the size of class Bar?
• What is the difference between the values of r and s? Why?

Of course, it’s a pain to have to redefine new and delete for every class. After we study templates, you will be able to create a templated mixin to do the same for all classes.                 

9. Problems with memory pooling                

• What if you redefine new and delete for class Foo, and then derive class Bar from class Foo?
• Will Bar inherit the redefined new and delete?
• Will that work, if sizeof(Bar) > sizeof(Foo)?
• How would you fix that?

10. Speed                

• Measure the speed of normal.cc vs. pool.cc.
  • They’re too fast to measure, so you have to modify them to run longer, for at least several seconds. Instead of only allocating one or two objects, allocate ten million of them, one at a time (but don’t free the memory between allocations). That should be big enough to measure.
  • What is the ratio of time used between the two? Why?
• Don’t bother deleting the allocated objects. This is a lab; we can break the rules.
• To find out how long the program runs, use the time program like this, to display the elapsed time in seconds: /bin/time -f %e ./a.out
  For example, where the initial % is my shell prompt:

% /bin/time -f %e sleep 0.123456789
0.12

11. Space                

• Measure the space requirements of normal.cc vs. pool.cc.
  • Leave the modifications from the Speed exercise in place.
  • What is the ratio of memory used between the two? Why?
• Surprisingly, the time command can also measure how much space (in kilobyte chunks) a command uses, like this:

% /bin/time -f %M date
Fri Nov 22 02:34:40 MST 2024
2032

12. For extra fame & glory (but no points)                

• Create a mixin class that does the opposite of what NoNewDelete in forbid-mixin.cc does—it makes its subclass require the use of new, and so forbids declaring one on the stack.

How to submit your work:                

In Canvas, check in the file results.txt to the assignment “Lab09”. It’s due 11:59ᴘᴍ MT Saturday, with a five-day late period.                 

How to receive negative points:                

Turn in someone else’s work.