Why did you write it with two structs though? You could do

    #define defer(body) DeferHolder COMMON_CAT(_defer_instance, __LINE__) {([&]()->void body)};

and call it as

    defer({
        function body here;
    });

Which looks much nicer. The preprocessor treats balanced curlies as one single token regardless of how many lines it spans, precisely to enable this usage.