Too many arguments provided to function-like macro invocation [duplicate]

Question

Say we have an implementation of std::aligned_storage. I've defined two macros for the alignof and alignas operators.

#include <iostream>
#include <cstddef>

#define ALIGNOF(x) alignof(x)
#define ALIGNAS(x) alignas(x)

template<std::size_t N, std::size_t Al = ALIGNOF(std::max_align_t)>
struct aligned_storage
{
    struct type {
        ALIGNAS(Al) unsigned char data[N];
    };
};

int main()
{
    // first case
    std::cout << ALIGNOF(aligned_storage<16>::type); // Works fine

    // second case
    std::cout << ALIGNOF(aligned_storage<16, 16>::type); // compiler error
}

In the second case I get the error in the title of the question (compiling with Clang, similar error with GCC). The error is not present if I replace the macros with alignof and alignas respectively. Why is this?

Before you start asking me why I'm doing this - the original macros have C++98 compatible code such as __alignof and __attribute__((__aligned__(x))) and those are compiler specific, so macros are my only choice...

EDIT: So according to the question marked as duplicate, an extra set of parenthesis would fix the issue.

std::cout << ALIGNOF((aligned_storage<16, 16>::type)); // compiler error

It doesn't. So, how would I go about doing this? (Satisfiable question?)

@melpomene, That doesn't really give any way of working around the problem — Qaz
– Qaz, Commented Jun 25, 2016 at 15:44
Why? Because comma. - Macro expansion sees the comma before making syntactic sense of any other content — Hagen von Eitzen
– Hagen von Eitzen, Commented Jun 25, 2016 at 15:44
btw - simply use #define MACRO(...) something(__VA_ARGS__) to work around it. — user719662
– user719662, Commented Jun 25, 2016 at 15:47
The extra parens thing only works for expressions. You're dealing with types/parameter lists, so a different workaround is required. — melpomene
– melpomene, Commented Jun 25, 2016 at 15:49

melpomene · Accepted Answer · 2016-06-25 15:48:33Z

34

C/C++ preprocessor is not aware of any C/C++ language constructs, it is just text preprocessor with its own syntax and rules. According to that syntax the following code ALIGNOF(aligned_storage<16, 16>::type) is invocation of macro ALIGNOF with 2 arguments (aligned_storage<16 and 16>::type) because there is comma inside parentheses.

I would suggest you to typedef aligned_storage<16, 16> and use that type inside this macro invocation.

edited Jun 25, 2016 at 15:48

melpomene

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answered Jun 25, 2016 at 15:45

mvidelgauz

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1 Comment

mvidelgauz Over a year ago

@melpomene Thank you for editing, I didn't know ** ** doesn't work inside ` ` :) and my reaction was slower than yours :)

Qaz · Accepted Answer · 2016-06-25 15:58:40Z

As has been explained, macro arguments are separated by commas that are not within extra parentheses. There are a few easy-ish ways to get around this, some more general than others:

Use a variadic macro (or a corresponding compiler extension for C++98) (live example):
```
#define ALIGNOF(...) alignof(__VA_ARGS__)
ALIGNOF(aligned_storage<16, 16>::type)
```

Get the caller to pass the number of arguments and wrap the arguments in extra parentheses (live example):

#define ALIGNOF(n, tuple) alignof(BOOST_PP_TUPLE_ENUM(n, tuple))
ALIGNOF(2 (aligned_storage<16, 16>::type))

Get the caller to pass in a "sequence" (live example):

#define ALIGNOF(seq) alignof(BOOST_PP_SEQ_ENUM(seq))
ALIGNOF((aligned_storage<16)(16>::type))

If the argument is a type, use a typedef (live example):

typedef aligned_storage<16, 16>::type storage_t;
ALIGNOF(storage_t)

Note that templated aliases can be created prior to C++11 by templating a struct and exposing a type member:

template<int N> 
struct alias {
    typedef typename aligned_storage<N, N>::type type;
};

If the argument can be used parenthesized, get the caller to wrap the argument in parentheses and use it directly. That is not the case with alignof.

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