Cross-platform ALIGN(x) macro?

Question

I would like to create a struct that has a certain alignment.

I would like to use the same struct definition for both GCC and VisualC++ compilers.

Answer 1

On Modern Compilers

The new versions of GCC (4.8.1) and VC++ (VS2013) supports the common syntax of having the attribute between struct and the identifier name. This is perhaps due to the new C++11 standard's introduction of the alignas keyword to do the same job the compilers' alignment attribute does; however, alignas can only make the alignment more stricter than the datatype's natural alignment, while the compiler directive can make it more lenient too.

Aside: On VS2013 alignof and stdalign.h are unsupported; although VS2015 supports alignof and alignas, it still does not have the header mandated by the C++ standard cstdalign (or its C counterpart); so availability should be checked manually based on the tool chain version. This is perhaps due to the poor support of the C language by Visual Studio. GCC, OTOH, has all of it; header, macros (__alignas_is_defined and __alignof_is_defined) and keywords (alignas and alignof).

With these information, we can define something that works and has a common syntax across compilers:

#ifdef _MSC_VER
#    if (_MSC_VER >= 1800)
#        define __alignas_is_defined 1
#    endif
#    if (_MSC_VER >= 1900)
#        define __alignof_is_defined 1
#    endif
#else
#    include    // __alignas/of_is_defined directly from the implementation
#endif

#ifdef __alignas_is_defined
#    define ALIGN(X) alignas(X)
#else
#    pragma message("C++11 alignas unsupported :( Falling back to compiler attributes")
#    ifdef __GNUG__
#        define ALIGN(X) __attribute__ ((aligned(X)))
#    elif defined(_MSC_VER)
#        define ALIGN(X) __declspec(align(X))
#    else
#        error Unknown compiler, unknown alignment attribute!
#    endif
#endif

#ifdef __alignof_is_defined
#    define ALIGNOF(X) alignof(x)
#else
#    pragma message("C++11 alignof unsupported :( Falling back to compiler attributes")
#    ifdef __GNUG__
#        define ALIGNOF(X) __alignof__ (X)
#    elif defined(_MSC_VER)
#        define ALIGNOF(X) __alignof(X)
#    else
#        error Unknown compiler, unknown alignment attribute!
#    endif
#endif

Example client code using the above

struct ALIGN(32) MyStruct
{
    ...
};

static_assert(ALIGNOF(MyStruct) == 32, "Error: MyStruct not on a 32-byte boundary!");