In functions that you write, that might be possible.

How would you assert that a given std::vector only was filtered by std::ranges::copy_if once? And how would you test that the code that was in the predicate for it wasn't duplicated?

How would you write a failing test for this function keeping the constraint that you are working with std::vector?

    std::vector<int> doThing(const std::vector<int>& nums) {
        std::vector<int> tmp1;
        std::vector<int> tmp2;
        std::ranges::copy_if(data,
                             std::back_inserter(tmp1),
                             [](int n) { return n % 2 == 0; }
        );
        std::ranges::copy_if(tmp1,
                             std::back_inserter(tmp2),
                             [](int n) { return n % 2 == 0; }
        );
        return tmp2;
    }

I know how I would do it in python. This is built into the stdlibs testing library, with mocks.

Maybe dependency injection and function pointers for the copy if function. Then you can check the call counts in your tests. But idk the cpp eco system and what's available there to do it.

The python code would be

    def some_call():
        return [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]
    
    def print_evens(nums):
        for n in filter(lambda n: n % 2 == 0, nums):
            print(n)
    
    def func():
        filtered = list(filter(lambda n: n % 2 == 0, some_call()))
        print_evens(filtered)
    
    if __name__ == "__main__":
        func()

How would you write a failing test that prevents the list from some_call() from having the same filter applied to it twice?

You would use something like this

https://docs.python.org/3/library/unittest.mock.html#unittes...

Then you would make the mock filter with patch and test the `func` function

psuedo python code would be

   @patch(builtins.filter)
   def test_func_filter_calls(mock_filter):
     mock_filter.return_value = [2,4,6]
     func()
     mock_filter.assert_called_once_with([1, 2, 3, 4, 5, 6, 7, 8, 9, 10])

That wouldn't fail though. It was called only once with [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]. The second time it was called with [2, 4, 6, 8, 10].

Likewise, if some_call returned [2, 4, 6, 8, 10] instead, it should only be called with [2, 4, 6, 8, 10] once then.

However, the purpose of this test then becomes questionable. Why are you testing implementation details rather than observable? Is there anything that you could observe that depended on the filter being called once or twice with the same filter function?

Did you try it? If it doesn't work there's also called once if you scroll up on the doc

And as far as whether it's a good idea or not, I generally wouldn't, but was saying when it is important then you do have these tools available,llms aren't the first thing to check for these mistakes. It's up to the engineer to choose between trade offs for your scenario.

Yes. The test passes. https://imgur.com/a/4qlTKlc

To try to monkey patch this in, you would need to also assert that it wasn't called with [2, 4, 6, 8, 10].

At which point, I would again ask "why are you testing that it _wasn't_ called with a given set of values?"

The comment at the root of this is "Unit tests catch that kind of stuff".

... But unit tests aren't for testing internals of implementation but rather observable aspects of a function.

Consider if the code was written so that it was

    def print_evens(nums):
        for n in nums:
            if n % 2 == 0:
                print(n)

instead (with the filter being used in func())

This isn't something that unit tests can (or should) identify. It would come out in a code review that there is redundant functionality in func and print_evens.

Using ChatGPT or another tool to assist in doing code reviews can be helpful (my original premise).

https://chatgpt.com/share/697a64a6-33c0-8011-a0f8-ca4fec74ab...

ChatGPT properly identifies the duplicated functionality (even though the code is using different idioms for doing the filtering for even numbers).

The test pass because your patching print_even so the 2nd filter is never called.

Which I guess, idk maybe think through the testing more and your code more before jumping to conclusions about how things are?

Testing is one tool you have, and it can test the internal like this. Obviously there's a use for it if its in the Python stdlib

This is in mockito

https://stackoverflow.com/questions/39452438/mockito-how-to-...

this is in google testing library for cpp

https://google.github.io/googletest/gmock_for_dummies.html

> Specify your expectations on them (How many times will a method be called? With what arguments? What should it do? etc.).

If you never heard of this, I guess you learned something new? Im not a tutor though. I would read the docs more and experiment. Maybe chatgpt can help you with how tests can be written.

With Mockito, I can mock the returned result of someCall().

However, it also means mocking list.stream() and mocking the Stream for stream.filter() and mock the call stream.toList() to return a new mocked object that has those mocks on it again.

I could catch the object passed in to printEven(...) but that has no history on it to see if filter was called on it before.

Trying to do the filter(...) call would be especially hard since you'd be parameterize it with a code block.

And all this returns to "is this a useful test?"

Testing should only be done on the observable parts of the function. Does printEven only print even numbers?

The tests that you are proposing are testing the implementation of those calls to work in a specific way. "It must call filter" - but if it's changed to a different filter or if it's changed to not use a filter but has the same functionality the code breaks.

Inefficient? Yes. Bad? Yes. Wrong - no. And not being wrong it isn't something that a unit test could validate without going unnecessarily into the implementation of the internals for the method. Internals changing while the contract remains the same is perfectly acceptable and shouldn't be breaking a unit test.