Aligning data on the stack (C++)

Question

This question is specific to the MSVC compiler (specifically 2008), but I'm interested in non-compiler specific answers too.

I'm trying to figure out how to align a char buffer on the stack, based on the alignment of some arbitrary type. Ideally the code would read:

__declspec( align( __alignof(MyType) ) ) char buffer[16*sizeof(MyType)];

Unfortunately, this doesn't work

error C2059: syntax error : '__builtin_alignof'

The compiler just doesn't like the nested statements.

My only other idea is to do this:

char buffer[16*sizeof(MyType)+__alignof(MyType)-1];
char * alignedBuffer = (char*)((((unsigned long)buffer) + __alignof(MyType)-1)&~(__alignof(MyType)-1));

Does anyone know of a nicer way? It seems like the declspec thing should work, do I just have the syntax wrong or something?

Thanks for reading :)

Answer 1

You can use std::aligned_storage together with std::alignment_of as an alternative.

#include <type_traits>

template <class T, int N>
struct AlignedStorage
{
    typename std::aligned_storage<sizeof(T) * N, std::alignment_of<T>::value>::type data;
};

AlignedStorage<int, 16> myValue;

This is supported by MSVC 2008 and up. If you need portability to other non-C++11 compilers you can use std::tr1::aligned_storage and std::tr1::alignment_of and the <tr1/type_traits> header.

In the above code, AlignedStorage<T>::data will be a POD type (a char[] array in MSVC and GCC) of a suitable alignment for T and size T*N.

Answer 2

Update

Check Robert Knight's answer! Uses C++11 but is much cleaner than this...

---

Original Answer

How about this nasty hack:

namespace priv {

#define PRIVATE_STATICMEM(_A_) \
    template <size_t size> \
    struct StaticMem<size,_A_> { \
      __declspec(align(_A_)) char data[size]; \
      void *operator new(size_t parSize) { \
        return _aligned_malloc(parSize,_A_); \
      } \
      void operator delete(void *ptr) { \
        return _aligned_free(ptr); \
      } \
    };

    template <size_t size, size_t align> struct StaticMem {};
    template <size_t size> struct StaticMem<size,1> {char data[size];};

    PRIVATE_STATICMEM(2)
    PRIVATE_STATICMEM(4)
    PRIVATE_STATICMEM(8)
    PRIVATE_STATICMEM(16)
    PRIVATE_STATICMEM(32)
    PRIVATE_STATICMEM(64)
    PRIVATE_STATICMEM(128)
    PRIVATE_STATICMEM(256)
    PRIVATE_STATICMEM(512)
    PRIVATE_STATICMEM(1024)
    PRIVATE_STATICMEM(2048)
    PRIVATE_STATICMEM(4096)
    PRIVATE_STATICMEM(8192)

}

template <typename T, size_t size> struct StaticMem : public priv::StaticMem<sizeof(T)*size,__alignof(T)> {
    T *unhack() {return (T*)this;}
    T &unhack(size_t idx) {return *(T*)(data+idx*sizeof(T));}
    const T &unhack() const {return *(const T*)this;}
    const T &unhack(size_t idx) const {return *(const T*)(data+idx*sizeof(T));}
    StaticMem() {}
    StaticMem(const T &init) {unhack()=init;}
};

Looks scary, but you need all that only once (preferably in some well hidden header file :) ). Then you can use it in the following way:

StaticMem<T,N> array; //allocate an uninitialized array of size N for type T
array.data //this is a raw char array
array.unhack() //this is a reference to first T object in the array
array.unhack(5) //reference to 5th T object in the array

StaticMem<T,N> array; can appear in the code, but also as a member of some bigger class (that's how I use this hack) and should also behave correctly when allocated on the heap.

Bug fix:

Line 6 of the example: char data[_A_] corrected into char data[size]