How to write the first test for an “add” functionality

I had problems finding a good question title, so feel free to improve on that.

I want to implement an Entity Component System in C++, and wish to use TDD for the first time. Now I have doubts about how useful TDD is when the target “architecture” is already specified, but want to try it anyway. My problem is more of a TDD problem though, and I could have run into it in another context as well:

The first test was supposed to check that I can add a Component to an Entity as an Entity should “possess” a set of Components (maybe this is a bad test and the problem lies here already?). So far no code exists and I’m facing a dilemma which might not be one. I could write a test that does this:

Entity e;
Component c;

But how would I assert that the add has worked correctly? I know I am going to need some sort of has_component method sooner or later, but I shouldn’t implement additional functionality now, or should I?
I could also make Entity have some list type in its public interface and use that in the test, and refactor that “away” later.

What is a correct approach here? It might be my mindset for TDD is totally off, so I’m grateful for any input!


Your first test is a good start. Maybe think about error checking: is there a return code from add, to tell that it did work or not ? Or is it supposed to throw an exception if something goes wrong ?

Preferred approach

Knowing that you’ll need a method has_component() for the Entity class, just add it to the class definition, with a stub implementation that always return false (and a big comment // TO DO !!).

Your next test would then be:

assert(! e.has_component(c) ); 
assert(e.has_component(c)); // or return FAIL to your test framework.  

The principle is that your test will fail until both functions are correct.

What are the alternatives

  • use a simpler member, such as number_of_components() to check that you’re on the right track (i.e. number increases when adding a new component; number remains same when adding same component twice).
  • let the test code inspect the internals of your Entity. I don’t like this approach, because it breaks the rules of encapsulation.
  • make a a test double that provides emulated/simulated answer for has_component(). Would it be a very complex feature, this would be the way to go. But for simple features like here, this might be overkill.
  • if you’d have a dispatching function that forwards calls from Entity to Components you could use it to check if c reacts.


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