Template Specialization                

In this lab, we will look at some examples of Template Specialization. The files are in ~cs253/Lab/Template. Create a file called recit11.txt, 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 integers, uses it to count four integers, and then produce this bar graph:                 

     4 *
     6 ***
    12 *

This template 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 required.                 

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 3.cc, and modify that from now on.
• Instead of .dump(), overload << so that you can insert a BarGraph object into an ostream.
• Create a specialization for BarGraph<char>:
  • It must use a fixed-size C array, not a map.
  • 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"; for the operator you just implemented.
• Add the code in 3.cc to recit11.txt, so the TA can see what you did.