A macro for migrating to `static operator()`

P1169 “static operator()” (Barry Revzin and Casey Carter, August 2021) proposes that we should be able to define operator() as a static member function. This is a very good idea. As of this writing, it also looks likely to get into C++2b. So instead of e.g.

struct ByAuthor {
    bool operator()(const Book& a, const Book& b) const {
        return a.author() < b.author();
    }
};

we’ll soon be able to write simply

struct ByAuthor {
    static bool operator()(const Book& a, const Book& b) {
        return a.author() < b.author();
    }
};

This may avoid the overhead of passing an unused this pointer (when the call isn’t simply inlined away), and it certainly can’t hurt.

Jonathan Müller recently posed a question about migration strategy. If your codebase currently uses non-static operator() const, but wants to use static operator() in C++2b, then how should you write your code so that it will be valid in both C++20 and C++2b?

I responded:

#if __cplusplus > 202002L
 #define CALL_OPERATOR(...) static operator()(__VA_ARGS__)
#else
 #define CALL_OPERATOR(...) operator()(__VA_ARGS__) const
#endif

struct ByAuthor {
    bool CALL_OPERATOR(const Book& a, const Book& b) {
        return a.author() < b.author();
    }
};

This works because C++’s declaration syntax treats bool static f() and static bool f() synonymously.

P1169 does have some downsides: for example, it proposes that captureless lambdas’ call operators must be non-static by default, unless you manually cruft them up with yet another trailing keyword: [](int x) static { return x + 1; }. I’d rather that compilers and/or platform-ABI owners be free to do this optimization themselves, without any additional keywords. But P1169 is definitely a step forward for the non-lambda case, and maybe lambdas will catch up in a few years.

CppCon Pub Quiz practice: Which six C++20 keywords can already appear in trailing position, directly after a lambda’s parameter-list?

[UPDATES, 2021-10-08: Changed “three” to “four,” then again to “six.” Thanks to Reddit commenters “SentientChowMein” and “i_lack_chromosomes” for pointing out that while there are currently only three decl-specifiers that can appear in trailing position, there are several more keywords that can appear there.]

A macro for migrating to static operator()

A macro for migrating to `static operator()`