Demystifying the Obscure Error: When Parametrized GTest with std::pairs Fails

Are you tired of banging your head against the wall, trying to figure out why your parametrized GTest with std::pairs is throwing an obscure error? You’re not alone! This article will guide you through the common mistakes, detailed explanations, and step-by-step solutions to get your tests running smoothly.

What’s the Problem?

When using parametrized tests with std::pairs in Google Test (GTest), you might encounter an error that looks something like this:

error: static assertion failed: Types are not equal
  static_assert(std::is_same<std::decay_t<typename std::tuple_element<I, T>::type>::value_type>, 
               typename std::tuple_element<I, T>::type>::value_type>{},
               "Types are not equal");

This error message is cryptic, to say the least. But don’t worry, we’ll break it down and provide a clear explanation of what’s going on.

Understanding the Issue

The problem arises when GTest tries to instantiate the test fixture with the parametrized type. In the case of std::pairs, the type deduction mechanism gets confused, leading to the obscure error message.

What’s Happening Behind the Scenes?

When you use a parametrized test with std::pairs, GTest creates an internal tuple to store the test parameters. This tuple is then used to instantiate the test fixture. Here’s a simplified example:

TEST_P( MyTest, MyTestCase ) {
  for ( auto param : GetParam() ) {
    // Your test code here
  }
}

INSTANTIATE_TEST_SUITE_P( MyTestSuite, MyTest,
                          ::testing::Values( 
                            std::make_pair(1, 2),
                            std::make_pair(3, 4),
                            std::make_pair(5, 6)
                          )
                        );

In this example, `GetParam()` returns a reference to an internal tuple, which contains the test parameters. When GTest tries to instantiate the test fixture, it uses this internal tuple to deduce the types. However, this is where things get tricky with std::pairs.

The std::pair Conundrum

The issue arises because std::pairs have two types: `first_type` and `second_type`. When GTest tries to deduce the types from the internal tuple, it gets confused about which type to use. This confusion leads to the obscure error message.

Solutions and Workarounds

Now that we understand the problem, let’s dive into the solutions and workarounds to get your parametrized tests with std::pairs working again.

Solution 1: Use `std::tuple` Instead of `std::pair`

A simple solution is to use `std::tuple` instead of `std::pair`. This eliminates the type deduction confusion and allows GTest to instantiate the test fixture correctly.

INSTANTIATE_TEST_SUITE_P( MyTestSuite, MyTest,
                          ::testing::Values( 
                            std::make_tuple(1, 2),
                            std::make_tuple(3, 4),
                            std::make_tuple(5, 6)
                          )
                        );

This solution works, but it might not be suitable if you need to use `std::pair` specifically.

Solution 2: Use a Custom Type

Another approach is to create a custom type that wraps the `std::pair`. This allows you to explicitly specify the types, eliminating the type deduction confusion.

struct MyPair {
  int first;
  int second;
};

INSTANTIATE_TEST_SUITE_P( MyTestSuite, MyTest,
                          ::testing::Values( 
                            MyPair{1, 2},
                            MyPair{3, 4},
                            MyPair{5, 6}
                          )
                        );

This solution provides more control over the types, but it requires additional code and might not be suitable for all scenarios.

Solution 3: Use a Type Wrapper

A more elegant solution is to use a type wrapper that provides an explicit type definition for the `std::pair`. This allows you to use `std::pair` while avoiding the type deduction issue.

template <typename T1, typename T2>
struct PairWrapper {
  std::pair<T1, T2> pair;
};

INSTANTIATE_TEST_SUITE_P( MyTestSuite, MyTest,
                          ::testing::Values( 
                            PairWrapper<int, int>{{1, 2}},
                            PairWrapper<int, int>{{3, 4}},
                            PairWrapper<int, int>{{5, 6}}
                          )
                        );

This solution provides a flexible and reusable way to work with `std::pair` in parametrized tests.

Best Practices and Conclusion

To avoid obscure errors when using parametrized GTest with std::pairs, follow these best practices:

• Use `std::tuple` instead of `std::pair` when possible.
• Create a custom type or type wrapper to explicitly define the types.
• Avoid using complex types or nested types in parametrized tests.
• Test your parametrized tests thoroughly to ensure they’re working as expected.

In conclusion, the obscure error when using parametrized GTest with std::pairs is a common issue that can be resolved with the right approach. By understanding the underlying issue and applying the solutions and workarounds provided in this article, you’ll be able to write robust and efficient parametrized tests with confidence.

Solution	Description	Pros	Cons
Use `std::tuple`	Replace `std::pair` with `std::tuple`	Easy to implement, eliminates type deduction issue	May not be suitable for all scenarios
Create a custom type	Define a custom type that wraps the `std::pair`	Provides explicit type definition, reusable	Requires additional code, might not be suitable for complex scenarios
Use a type wrapper	Use a type wrapper that provides an explicit type definition for the `std::pair`	Flexible, reusable, and provides explicit type definition	Requires additional code, might not be suitable for extremely complex scenarios

By following these guidelines and applying the solutions provided, you’ll be well-equipped to tackle the obscure error and create robust parametrized tests with std::pairs.

Frequently Asked Question

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