[tayo42]

Unit tests catch that kind of stuff

[shagie]

The code works perfectly - there is no issue that a unit test could catch... unless you are spying on internally created objects to a method and verifying that certain functions are called some number of times for given data.

[tayo42]

Sure and you can do that

[shagie]

Trying to write the easiest code that I could test... I don't think I can without writing an excessively brittle test that would break at the slightest implementation change.

So you've got this Java:

    public List<Integer> someCall() {
        return IntStream.range(1,10).boxed().toList();
    }

    public List<Integer> filterEvens(List<Integer> ints) {
        return ints.stream()
                .filter(i -> i % 2 == 0)
                .toList();
    }

    int aMethod() {
        List<Integer> data = someCall();
        return filterEvens(data.stream().filter(i -> i % 2 == 0).toList()).size();
    }

And I can mock the class and return a spied'ed List. But now I've got to have that spied List return a spied stream that checks to see if .filter(i -> i % 2 == 0) was called. But then someone comes and writes it later as .filter(i -> i % 2 != 1) and the test breaks. Or someone adds another call to sort them first, and the test breaks.

To that end, I'd be very curious to see the test code that verifies that when aMethod() is called that the List returned by SomeCall is not filtered twice.

What's more, it's not a useful test - "not filtered twice" isn't something that is observable. It's an implementation detail that could change with a refactoring.

Writing a test that verifies that filterEvens returns a list that only contains even numbers? That's a useful test.

Writing a test that verifies that aMethod returns back the size of the even numbers that someCall produced? That's a useful test.

Writing a test that tries to enforce a particular implementation between the {} of aMethod? That's not useful and incredibly brittle (assuming that it can be written).

[nl]

You are correct and the objection is just completely invalid. There's no way anyone would or should write tests like this at the client level.

I think they are just arguing for the sake of arguing.

[tayo42]

You mention the tools you can use to make it happen.

I think we're at the point where you need concrete examples to talk about whether it's worth it or not. If you have functions that can't be called twice, then you have no other option to test details in the implementation like that.

Yeah there's a tradeoff between torturing your code to make everything about it testable and enforce certain behavior or keeping it simpler.

I have worked in multiple code bases where every function call had asserts on how many times it was called and what the args were.

[shagie]

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;
    }

[tayo42]

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.

[anthonypasq96]

youre verifying std lib function call counts in unit tests? lmao.

[tayo42]

You can do that with mocks if it's important that something is only called once, or likely there's some unintended side effect of calling it twice and tests woukd catch the bug

[anthonypasq96]

i know you could do it, im asking why on earth you would feel its vital to verify stream.filter() was called twice in a function

[noitpmeder]

You're not verifying the observable behavior of your application? lmao

[shagie]

How would you suggest tests around:

    void func() {
        printEvens(someCall().stream().filter(n -> n % 2 == 0).toList());
    }

    void printEvens(List<Integer> nums) {
        nums.stream().filter(n -> n % 2 == 0).forEach(n -> System.out.println(n));
    }

The first filter is redundant in this example. Duplicate code checkers are checking for exactly matching lines.

I am unaware of any linter or static analyzer that would flag this.

What's more, unit tests to test the code for printEvens (there exists one) pass because they're working properly... and the unit test that calls the calling function passes because it is working properly too.

Alternatively, write the failing test for this code.

[tayo42]

Idk how exactly to do it in cpp becasue I'm not familiar with the tooling

You could write a test that makes sure the output of someCall is passed directly to printeven without being modified.

The example as you wrote is hard to test in general. It's probably not something you would write if your serious about testing.

[shagie]

In C++, the code would look like:

    #include <vector>
    #include <iostream>
    #include <algorithm>

    std::vector<int> someCall()
    {
        return {1, 2, 3, 4, 5, 6, 7, 8, 9, 10};
    }

    void printEvens(const std::vector<int>& nums)
    {
        std::ranges::for_each(nums, [](int n)
        {
            if (n % 2 == 0)
            {
                std::cout << n << '\n';
            }
        });
    }

    int main()
    {
        std::vector<int> data = someCall();
        std::vector<int> tmp;

        std::ranges::copy_if(data,
                             std::back_inserter(tmp),
                             [](int n) { return n % 2 == 0; }
        );
    
        printEvens(tmp);
        return 0;
    }

---

Nothing in there is wrong. There is no test that would fail short of going through the hassle of creating a new type that does some sort of introspection of its call stack to verify which function its being called in.

Likewise, identify if a linter or other static analysis tool could catch this issue.

Yes, this is a contrived example and it likely isn't idiomatic C++ (C++ isn't my 'native' language). The actual code in Java was more complex and had a lot more going on in other parts of the files. However, it should serve to show that there isn't a test for printEvens or someCall that would fail because it was filtered twice. Additionally, it should show that a linter or other static analysis wouldn't catch the problem (I would be rather impressed with one that did).

From ChatGPT a code review of the code: https://chatgpt.com/share/69780ce6-03e0-8011-a488-e9f3f8173f...

[wtetzner]

> You could write a test that makes sure the output of someCall is passed directly to printeven without being modified.

But why would anyone ever do that? There's nothing incorrect about the code, it's just less efficient than it should be. There's no reason to limit calls to printEven to accept only output from someCall.

[quietbritishjim]

A redundant filter() isn't observable (except in execution time).

You could pick it up if you were to explicitly track whether it's being called redundantly but it'd be very hard and by the time you'd thought of doing that you'd certainly have already manually checked the code for it.

[anthonypasq96]

what happened to not testing implementation details?