I'm trying to make a memory pool class that allows me to return pointers from within a char* array, and cast them as various pointer. Very very simplified implementation:

class Mempool
{
     char* mData=new char[1000];
     int mCursor=0;

     template <typename vtype>
     vtype* New()
     {
        vtype* aResult=(vtype*)mData[mCursor];
        mCursor =sizeof(vtype);
        return aResult;
     }
};

(Remember, it's not intended to be dynamic, this is just random access memory that I only need for a short time and then it can all be tossed)

Here's my issue: I know that on some systems (iOS, at least) a float* has to be aligned. I assume the alignment is obtained via alignof(float)... but I can't ever be sure of the alignment of the initial char*'s allocation could be anywhere, because alignof(char) is 1.

What can I do to make sure that in my New() function, my mCursor is advanced to a proper alignment for the requested object?

CodePudding user response：

alignof(std::max_align_t) will tell you the alignment required on your platform for standard functions like malloc(). You should use that if you don't know what alignment requirements the users of your allocator will have.

Simply round sizeof(vtype) up to the nearest multiple of alignof(std::max_align_t) when you increment mCursor.

Or for perhaps a little more efficiency, increment mCursor as you do now, then round it up as needed when the user requests an allocation. When allocating an object of size 2 for example, you know that 2-byte alignment is sufficient, you don't need the full alignof(std::max_align_t). You can use alignof(vtype) for this, to avoid wasted space if users allocate small objects next to each other.

As for this:

I can't ever be sure of the alignment of the initial char*'s allocation could be anywhere, because alignof(char) is 1

You can be sure: new char[N] will always be aligned to at least alignof(std::max_align_t). But my second approach (pad when New is called) does not rely on this assumption.