C++ Multidimensional array in existing memory

Question

(This is not a duplicate of this or this that refer to fixed sizes, the issue is not to understand how pointers are stored, but if the compiler can automate the manual funct

Answer 1

For anyone trying to achieve the same, I 've created a variadit template function that would create a n-dimension array in existing memory:

template  size_t CreateArrayAtMemory(void*, size_t bs)
{
    return bs*sizeof(T);
}

template 
size_t CreateArrayAtMemory(void* p, size_t bs, Args ... args)
{
    size_t R = 0;
    size_t PS = sizeof(void*);
    char* P = (char*)p;
    char* P0 = (char*)p;

    size_t BytesForAllPointers = bs*PS;
    R = BytesForAllPointers;

    char* pos = P0 + BytesForAllPointers;
    for (size_t i = 0; i < bs; i++)
    {
        char** pp = (char**)P;
        if (p)
            *pp = pos;
        size_t RLD = CreateArrayAtMemory(p ? pos : nullptr, args ...);
        P += PS;
        R += RLD;
        pos += RLD;
    }
    return R;
}

Usage:

Create a 2x3x4 char array:

int j = 0;
size_t n3 = CreateArrayAtMemory(nullptr,2,3,4);
std::vector a3(n3);
char*** f3 = (char***)a3.data();
CreateArrayAtMemory(f3,2,3,4);
for (int i1 = 0; i1 < 2; i1++)
{
    for (int i2 = 0; i2 < 3; i2++)
    {
        for (int i3 = 0; i3 < 4; i3++)
        {
            f3[i1][i2][i3] = j++;
        }
    }
}