[computer-go] C++ parameter passing

[Stefan Nobis]

Peter Drake <drake at lclark.edu> writes:

> I'd like to pass a large array by constant reference. I tried this:

> void remove_from_zobrist_hash(BoardState & state, color_t color, const uint64_t &
> zobrist_numbers[][])

Hmmm... my C++ is a bit rusty, but it seems, this is one of the
pitfalls of the declaration syntax (and I hope, I get everythin
right). Especially arrays and references are a bit hairy.

One of the problems: In some cases C++ distinguishes between arrays
and pointers, in some cases not... and references don't make this
easier. If C++ would allow assignment between arrays, the dimensions
would have to match, so

  int a[3];
  int b[5];
  b = a;

would be an error. If we play with references another detail is
imporant: The type precedence. In most cases arrays and pointers are
the same in C++, so

  int a[];
  int* b;

are quite the same. Where to put the reference sign? If you try

  int &a[];

you get an error, because arrays of references, which is what is
declared here, are not allowed in C++. The type declarators like * and
& are part of the type, so the type is 'reference to int' and from
this type are the array elements. What to do? Correct the precedence
with parens:

  int (&a)[];

Now the type is a reference to an array of ints. Will this do? No, not
quite. This is a real array type and in this case dimensions are
imporant:

  int a[3];
  int (&r)[3] = a;  // OK, dimensions match
  int (&s)[5] = a;  // error, dimension mismatch
  int (&t)[] = a;   // error, dimension mismatch

In C++ it's only allowed not to set the array dimensions, when the
declaration is in fact a pointer declaration (int* === int[]). So in
your case, if you really want a const reference to an array, you have
to specify the dimension:

  void f(const int (&x)[dim1][dim2]);

and dim1 and dim2 have to be compile time constants.

> Is there another way to do this?

You may pass it as const pointer, then you have to specify all
dimensions but the first:

  void f(const int (*const x)[dim2]);

--
Until the next mail...,
Stefan.
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