How can I match one element from a C++ tuple using gmock?

How can I match one element from a C++ tuple using gmock ?

For example let's try to extract the std::string from a std::tuple<std::string, int>.

I know I could write a custom matcher like this:

MATCHER_P(match0thOfTuple, expected, "") { return (std::get<0>(arg) == expected); }

But since I found the Pair(m1, m2) matcher for the std::pair, I expected also to find something similar for the std::tuple.

Gmock has Args<N1, N2, ..., Nk>(m) for selecting a subset of tuple arguments. When using it with just 1 argument, it still expects a tuple matcher. The following attempt does not seem to compile:

struct {
  MOCK_METHOD1(mockedFunction, void(std::tuple<std::string, int>&));
} mock;
EXPECT_CALL(mock, mockedFunction(testing::Args<0>(testing::Eq(expectedStringValue))));

And makes my clang give a compilation error like this:

.../gtest/googlemock/include/gmock/gmock-matchers.h:204:60: error: invalid operands to binary expression ('const std::__1::tuple<std::__1::basic_string<char> >' and 'const std::__1::basic_string<char>')
  bool operator()(const A& a, const B& b) const { return a == b; }
...

Is there a gmock solution for the std::tuple similar to the one for the std::pair, which uses the gmock Pair matcher?

Solution

testing::Args is for packing function arguments to tuple - quite opposite to what you want to achieve.

My advice - in your case - unpack in Mock class, see:

struct mock 
{
  void mockedFunction(std::tuple<std::string, int>& tt)
  {
      mockedFunctionUnpacked(std::get<0>(tt), std::get<1>(tt));
  }
  MOCK_METHOD2(mockedFunctionUnpacked, void(std::string&, int&));
};

Then:

EXPECT_CALL(mock, mockedFunctionUnpacked(expectedStringValue, ::testing::_));

Unfortunately none of current gmock matchers works for std::tuple arguments.

If you like to get learn about C++ template - you might try this (not complete - just an idea how it can be achieved to make general function for tuple matching):

// Needed to use ::testing::Property - no other way to access one 
// tuple element as "member function"
template <typename Tuple>
struct TupleView
{
public:
    TupleView(Tuple const& tuple) : tuple(tuple) {}
    template <std::size_t I>
    const typename std::tuple_element<I, Tuple>::type& get() const
    {
        return std::get<I>(tuple);
    }
private:
    Tuple const& tuple;
};

// matcher for TupleView as defined above
template <typename Tuple, typename ...M, std::size_t ...I>
auto matchTupleView(M ...m, std::index_sequence<I...>)
{
    namespace tst = ::testing;
    using TV = TupleView<Tuple>;
    return tst::AllOf(tst::Property(&TV::template get<I>, m)...);
}

// Matcher to Tuple - big disadvantage - requires to provide tuple type:
template <typename Tuple, typename ...M>
auto matchTupleElements(M ...m)
{
    auto mtv = matchTupleView<Tuple, M...>(m..., std::make_index_sequence<sizeof...(M)>{});
    return ::testing::MatcherCast<TupleView<Tuple>>(mtv);
}

Then use like this:

EXPECT_CALL(mock, mockedFunction(matchTupleElements<std::tuple<std::string, int>>(expectedStringValue, ::testing::_)));