Wednesday, March 5, 2014

Extracting reusable C++ templated functions from StringCalculator code

I've recently coded a solution to the StringCalculator kata in C++ using GoogleMock, (you can check its code in GitHub).

In one of the refactoring steps, I extracted the code that filters out not numeric tokens to a separated helper method: filterOutNotNumericTokens:

Once all the kata requirements were satisfied, I refactored this method a little bit more using C++11 copy_if algorithm:

We can use the same technique to refactor the method that ignores numbers greater than 1000, ignoreTooBig, from this:

to this:
Notice that we had to make notTooBig a free function in order to pass it to copy_if.

To keep it as a member function we'd need to write this less readable version:

which uses the bind1st and mem_fun functions from functional.

In this case, I think it's probably better to use a lambda:

In the two previous examples we eliminated duplication using the copy_if algorithm.

We can improve the readability of this code introducing a helper templated functions, filter:

Notice that filter works only for vector, copy_if is more general. But this leads to a bit more readable code for filterOutNotNumericTokens and ignoreTooBig:
We can also use filter to refactor the getNegatives method:
The same procedure can be applied to have a map templated function that uses the transform algorithm which can then be used to refactor the convertToInts method:

This map and filter functions that we'd refactored out from the StringCalculator code are pure functions that are highly reusable and will help us to avoid duplication in another projects.

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