How to add support for comparing std::vector to std::initializer_list

[TorstenRobitzki]

Hello,
I would like to have support for expressions, like this in Catch:

CHECK( std::vector< char >() == { 0 } );

Could some of you give me some advice, how to add comparison of containers with std::initializer_list? (beside using CHECK_THAT and a custom matcher).

As comparison of std::vector< T > with other std::vector< char >() works, I was looking for tests to find out how this works. Are there any unit tests in Catch?

cheers,
Torsten

[refi64]

Can't you just do:

CHECK( std::vector< char >() == std::vector< char >{ 0 } );

[TorstenRobitzki]

Sure, but that would add overhead (in terms of code to read) that I would like to eliminate.

CHECK( a == { 0x01, 0x02 } );

just reads nicer.

[refi64]

Unfortunately, there's no other way. C++11 just doesn't allow an initializer list on the right-hand-side of ==, no matter whether or not there's an overload. So it's impossible for Catch to do anything...

...without metaprogramming. You could create a function and then call it like:

CHECK( a == mkv(0x1, 0x2) );

Working on an implementation right now...

[refi64]

template <typename... Args>
std::vector<typename std::common_type<Args...>::type> mkv(Args&&... args) {
    return {args...};
}

You can technically name it whatever you want.

[TorstenRobitzki]

Nice, thank you. What would be the usual ways to add support for custom type comparison operators? Does the std::vector support only based on Catch::toString()? I ask, because I could change std::vector in my tests to something else.

In case I test with larger amounts of data, it could be handy if the comparison makes a diff between the two data sets and reports the difference instead of the content of both sets.

[horenmar]

This is rather old issue, but here we go:

• To add support for comparing with new types, you just need to add proper bool operator== overload where the compiler finds it.
• To change how the results are displayed, you need to either give your type overload for operator<<, or specialize Catch::StringMaker for your type.

[philsquared]

I missed this one the first time around, sorry.
Reading through now it seems this would be a good fit for matchers (which I've just revamped and added vector matchers for).
At the moment you can do Equals matchers between vector and another vector - or Contains matchers between two vectors or a vector and an element.
Adding versions that compare against initialiser_lists (if detected to be available) should be easy to do (in fact I just tested this theory in a local proof-of-concept.
I'll have look at adding these. That will mean you'll be able to do:

REQUIRE_THAT( myVec, EqualsVector( { 1, 2, 3 } ) );

But also:

REQUIRE_THAT( myVec, ContainsVector( { 1, 2 } ) );

How does that sound?

[TorstenRobitzki]

That sounds perfect! What do you think about having a diff in output of the error case, in case that the containers becomes larger?

[DanielSWolf]

Has there been any progress? I have a lot of Google Test unit tests that I'd like to migrate to Catch2. But I got blocked on the very first test suite, which heavily relies on std::initializer_lists for expected values.

Google Test's EXPECT_THAT(value, ElementsAreArray(...)) supports initializer lists.

[horenmar]

@DanielSWolf

	std::vector<int> values{ 1, 2, 3 };
	REQUIRE_THAT(values, Catch::Equals<int>({1, 2, 3}));

[DanielSWolf]

Thank you! I didn't realize I needed to specify the generic type.