CS253 | Lab / Template

a Template Lab

In this lab, we will look at some examples of Template Specialization. The files are in ~cs253/Lab/Template. Create a file called results.cc, and turn it in for credit.

A Bar Graph Template

In 1.cc, we have a program that defines the BarGraph template. Here’s how to use it:

    BarGraph<int> alpha;
    alpha += 12;
    alpha += 6;
    alpha += 4;
    alpha += 6;
    alpha += 6;
    alpha.dump();

This create a BarGraph of ints, uses it to count four ints, and then produce this bar graph:

     4 *
     6 ***
    12 *

This template class has several features worth discussion:

It uses a std::map to count the elements as they come in.
- The key of the map is the templated type T, an int, in this case.
- The value of the map is an unsigned int. It’s unsigned because it’s a counter, and counters can’t be negative. We may as well use an unsigned int, which has no negative values, but can hold twice as many positive values.
- The map is sorted by the key (an int, in this case). Therefore, the output is in the order 4, 6, 12.
It overloads += to “add” items to the bar graph. The return type of this operator+= is void. += usually returns a reference to *this, because all standard assignment operators do that, but it’s only a convention, not a rule.
BarGraph uses a std::string temporary to print a number of stars. That is, string(23, '*') calls the string ctor to creates a temporary unnamed string that contains 23 asterisks.
It uses an iterator to traverse (walk through) the sorted map.

Advantages of templates

The benefit of a template is its generality. BarGraph should work for any type, as long as it is (think “Duck Typing”): 🦆

copyable (copy ctor & operator=)
comparable via <
printable via <<

including int, float, string, etc.

Disadvantages of templates

Sure, BarGraph works for lots of types, but it’s not the most efficient solution for all types. It uses a map, which is a general solution, but that generality is not always needed. It’s like how a chainsaw can cut through just about anything, but it’s not really practical for eating dinner. Sometimes, a more specialized tool is appropriate.

Consider 2.cc, which introduces template specialization. It features a hand-written version BarGraph<bool>. This is a version of BarGraph tuned specially for counting booleans. Of course, the general version of BarGraph still exists, so it works for all other types, too.

The specialization BarGraph<bool> is:

Smaller: only has two unsigned variables.
Faster: does a boolean test instead of a map lookup, and doesn’t have to iterate through the possible values, since there are only false and true.
Better: outputs false and true instead of 0 and 1.

Of course, it’s possible that g++’s implemention of map has a specialization for map<bool,whatever> that is smaller & faster in just that way. Who knows?

Exercises

Copy 2.cc to results.cc, and use that from now on.
Instead of .dump(), overload << so that you can insert a BarGraph object into an ostream.
- Do this for the templated BarGraph class, and all of its template specializations. A template specialization should present the same interface as the unspecialized class.
Create a specialization for BarGraph<char>:
- << must work for this specialization, also.
- It must use a fixed-size C array, not a map.
  - You may assume that the input will be only ASCII characters. Unicode characters will fail horribly, but we’ll live with that for this academic example.
- Add an operator so that you can do delta += "foo" for a BarGraph<char>. This should be equivalent to:
  delta += 'f';
  delta += 'o';
  delta += 'o';
- Add a test case delta += "Colorado"; to main() for the operator you just implemented.

How to submit your work:

In Canvas, check in the file results.cc to the assignment “Lab11”. It’s due 10:00:00ᴘᴍ MT Saturday, with a 24-hour late period for a 25% penalty.

How to receive negative points:

Turn in someone else’s work.

CS253: Software Development with C++