Cast pointer to fixed-size array in return statement

Question

The simplest way to ask this question is with some code:

struct Point
{
    int x;
    int y;
    int z;

    int* as_pointer() { return &x; }        // works
    int (&as_array_ref())[3] { return &x; } // does not work   
};

as_pointer compiles, as_array_ref does not. A cast seems to be in order but I can't figure out the appropriate syntax. Any ideas?

Answer 1

I find that array types are easier to deal with with a typedef:

typedef int ints[3];

Then your as_array_ref must be written so that &as_array_ref() == &x.

The following syntaxes are possible:

1. plain C-style cast from int* to ints*:

   ints& as_array_ref() { return *( (ints*)(&x) ); }

2. C++ style reinterpret_cast (suggested by @Mike Seymour - see also his answer). It is often considered a better practice in C++:

   ints& as_array_ref() { return *reinterpret_cast<ints*>(&x); }

3. Cast from int& to ints& which is slightly shorter but (for me) less intuitive:

   ints& as_array_ref() { return reinterpret_cast<ints&>(x); }

Answer 2

The cast you need to reinterpret a reference to a variable as a reference to an array is:

reinterpret_cast<int(&)[3]>(x);

Be aware that using this gives undefined behaviour; it will probably work on any sensible implementation, but there's no guarantee that there won't be padding between the members of the class, while arrays are not padded.

Answer 3

I think that what you're trying to do would be easier (and clearer/cleaner) with an union.