Dealing with extra parentheses in macros in MSVC

Question

There are some cases where you need to (a) have a macro which (b) parenthesizes one of its arguments in (c) a case where the language does not allow parentheses.

Simple example:

#define MAKE_STRUCT(name, base) struct name : base { }

struct X { };
MAKE_STRUCT(Y, X);   // ok
MAKE_STRUCT(Z, (X)); // error

This is an error because we're expecting a type name and we get (. ecatmur provides a clever workaround for this by taking advantage of the fact that there are other parts of the language that do allow for an extra set of parentheses:

template<typename T> struct argument_type;
template<typename T, typename U> struct argument_type<T(U)> { typedef U type; };

#define MAKE_STRUCT(name, base) struct name : argument_type<void(base)>::type { }

struct X { };
MAKE_STRUCT(Y, X);   // ok
MAKE_STRUCT(Z, (X)); // ok

This compiles on gcc and clang, but it does not compile on MSVC. Is there either an MSVC-specific trick to allow for parentheses or a different trick that works on all three compilers?

This possibly-parenthesized type is passed through several macros - so I'm not sure that the other answers could apply (the context here is trying to pass multiple template parameters to a fixture using Catch2's TEST_CASE_METHOD()).

score 4 · Accepted Answer · answered Apr 20 '18 at 00:46

4

So I came to the following pure-preprocessor solution that will strip parentheses only when they are present. Seems to work in vc++, g++ and clang++ and probably can be modified to deal with several levels of parentheses.

#define MAKE_STRUCT(name, base) struct name: EXPAND1(DEF base) {}
#define EXPAND1(param) EXPAND2(param)
#define EXPAND2(param) NO## param
#define DEF(param) DEF param
#define NODEF

struct X { };
MAKE_STRUCT(Y, X);   // ok
MAKE_STRUCT(Z, (X)); // ok

online compiler

answered Apr 20 '18 at 00:46

user7860670

35,849
4
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In my case the `X` really has a comma in it, but that's a trivial change to make `DEF` and `EXPAND2` take `...` instead. Wonderful, thank you. The preprocessor is still basically magic to me. – Barry Apr 20 '18 at 03:55

score -1 · Answer 2 · answered Apr 19 '18 at 21:36

-1

Not the great solution, but if structs in question can be tinkered with, and only ONE level of parenthesis is to be supported, this seems to work:

struct tag {};

#define MAKE_STRUCT(name, base) struct name : \
  decltype(base(tag{})) { };

struct X {
     X(tag );
     X() = default;
};

MAKE_STRUCT(Y, X);   // ok
MAKE_STRUCT(Z, (X)); // ok

answered Apr 19 '18 at 21:36

SergeyA

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Unfortunately, no. But I'm fascinated that this works whereas, like, `base()` doesn't? – Barry Apr 20 '18 at 03:53

Dealing with extra parentheses in macros in MSVC

2 Answers2