CS253: Software Development with C++
Spring 2019
New
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/Lab/New.
Consider glob.cc. It redefines the global new
and delete operators (yes, 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 *?
new/deleteConsider 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.
main, which function is called and why?
operator new and operator delete declared static?
new/delete for a specific classTry to compile forbid1.cc. Does it compile? Why not?
=deleteTry to compile forbid2.cc. Does it compile? Why not?
forbid-mixin.cc does the same thing as forbid1.cc & forbid2.cc, but encapsulates it in a “mixin”.
Why does the mixin class declare its ctor & dtor protected?
For another popular example of a mixin, see the Boost class noncopyable
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.)
There are several reasons for scrubbing memory.
new time, to ensure that you’re not erroneously counting
on the memory being any particular value.
delete time, if you fear that the code may erroneously keep
pointers to the deleted memory.
Consider the technique used in scrub.cc. What criteria would you use to decide on a value for memory? It should be:
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
may cause a segmentation violation upon use, since some architectures
require a four-byte int to be at an address that’s a multiple of
four.
Run normal.cc.
class Foo?
p and q? Why?
Now, run pool.cc.
class Bar?
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.
new and delete for class Foo,
and then derive class Bar from class Foo?
Bar inherit the redefined new and delete?
sizeof(Bar) > sizeof(Foo)?
NoNewDelete in
forbid-mixin.cc does—it makes its subclass
require the use of new.
