Lambda with structured bindings inside macro call

Question

This may be related to How to correctly forward structured binding arguments in a macro , but I feel the real problem is not the same, so may have a different solution.

I want to pass a lambda as a name generator inside gtest's INSTANTIATE_TEST_SUITE_P. This works well until I try to use structured bindings, whose comma makes the preprocessor think it gets split into separate arguments.

using param = std::tuple<int, int>;

// This works
INSTANTIATE_TEST_SUITE_P(
    instance, suite, cases,
    [](const testing::TestParamInfo<param>& info) {
        return std::string(std::get<0>(info.param)
            + "_to_" + std::string(std::get<1>(info.param));
    }
);

// This also works
static constexpr auto name = [](const testing::TestParamInfo<param>& info) {
    const auto [a, b] = info.param;
    return std::string(a) + "_to_" + std::to_string(b);
};

INSTANTIATE_TEST_SUITE_P(
    instance, suite, cases, name
);

// But this does not compile
INSTANTIATE_TEST_SUITE_P(
    instance, suite, cases,
    [](const testing::TestParamInfo<param>& info) {
        const auto [a, b] = info.param;
        return std::string(a) + "_to_" + std::to_string(b);
    }
);

I know that in some cases like template arguments, the comma can be protected from preprocessor by an extra set of parentheses. Can the same be done here? I tried placing parentheses around the structured binding, but it did not compile either.

Have you tried putting the whole lambda in a set of parentheses? Like ([](const testing::TestParamInfo<param>& info) { ... })? — Some programmer dude
– Some programmer dude, Commented Feb 14 at 9:35
if you dont remove includes from the code then anybody passing by can copy the code to compile it and see the error within a blink. If you remove them one would have to redo work you already did, thats not nice — 463035818_is_not_an_ai
– 463035818_is_not_an_ai, Commented Feb 14 at 9:55

Dominik Kaszewski · Accepted Answer · 2025-02-14 09:52:35Z

3

Answered in comments: putting entire lambda in parentheses solves the issue.

INSTANTIATE_TEST_SUITE_P(
    instance, suite, cases,
    ([](const testing::TestParamInfo<param>& info) {
        const auto [a, b] = info.param;
        return std::string(a) + "_to_" + std::to_string(b);
    })
);

answered Feb 14 at 9:52

Dominik Kaszewski

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