Feature request: more print_log_value specializations

[mikeiovine]

In my project, I have to test for equality of std::pairs. In other testing frameworks, this is very easy. For instance, in GoogleTest, this works: ASSERT_EQ(std::make_pair(...), std::make_pair(...)).

This task turns out to be very awkward in Boost. The docs recommend defining operator<< or boost_test_print_type. Unfortunately, these need to be defined in the std namespace, or else the function lookup will fail. Extending the namespace std with a new function definition is technically undefined behavior.

For testing two std::pairs, it's easy (albeit slightly annoying) to just test them element-wise, e.g. with BOOST_CHECK_EQUAL(pair1.first, pair2.first). However, this option is not available for comparing collections of pairs with BOOST_TEST(collection1 == collection2, boost::test_tools::per_element())

The following alternative solution works:

namespace boost {
namespace test_tools {
namespace tt_detail {

template <class T, class U>
struct print_log_value<std::pair<T, U>> {
  void operator()(std::ostream &os, std::pair<T, U> p) {
    os << "{" << p.first << ", " << p.second << "}";
  }
};

}  // namespace tt_detail
}  // namespace test_tools
}  // namespace boost

Injecting a specialization into a detail namespace feels equally hacky.

I think adding this specialization for std::pair would be very convenient.

It would also be helpful to add a specialization for std::optional and std::tuple, though I'm not sure how these would work with respect to backwards compatibility since these are not available in C++03.