Redefining new & delete

Description

In this lab, we’ll show how you can redefine the new/delete operators to achieve a variety of useful or bizarre results.

The files for this lab are available in ~cs253/pub/Labs/NewLab.

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 returns void. Since these functions deal with raw memory, they work with void *.

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.

• Why are operator new and operator delete declared static?

Forbid new/delete for a specific class.

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

Same thing, but with a Mixin

forbid-mixin.cc does the same thing as forbid.cc, but does it better.

For another popular example of a mixin, see the Boost class noncopyable

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?

Scrubbing the memory

If you’re concerned about the contents of deleted memory getting reused, then you might consider the technique used in scrub.cc. What criteria would you use to decide on a value for memory? I think that it should be:

• a lousy integer
• a lousy floating-point value
• a lousy pointer
• a printable value

For example, a repeated string of “Jack” or “WTF!” would fit all of the above requirements. Notably, an odd value makes for a poor pointer, and would often cause a segmentation violation upon use.

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?

Templated version

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.

Problems with this approach

• 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?

Exercises

• Measure the speed of normal.cc vs. pool.cc. Of course, they’re too fast to measure, so you have to modify them to run longer. Which is faster? Why?
• Measure the space requirements of normal.cc vs. pool.cc. They will have to be similarly modified.

For extra fame & glory

• Create a verion of errchk.cc that uses backtrace to print a stack backtrace in case of error.
• 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.