Hi,
On Wed, Aug 06, 2014 at 01:19:44PM -0400, David Malcolm wrote:
> In many circumstances, is_a_helper <T>::test assumes that the pointer is
> non-NULL, but sometimes you have a pointer of type T that can be NULL.
>
> Earlier versions of this patch kit made numerous uses of the ternary
> operator to handle nullable pointers e.g.:
>
> return insn ? as_a <rtx_insn *> (insn) : NULL;
>
> but this was ugly. Instead, introduce an as_a_nullable<T> variant that
> adds a check for NULL, so the above can be written simply as:
>
> return as_a_nullable <rtx_insn *> (insn);
A tiny bikeshedding, disregard it if you don't like it: however, the
"nullable" part of the name suggests the pointer can be NULLed in the
process. I think that variants of functions that survive NULL
arguments are traditionally called "safe" in gcc, so what about
"safe_as" (or safely_as)?
Martin
>
> gcc/
> * is-a.h (template<T, U> as_a_nullable <U *p>) New function.
> ---
> gcc/is-a.h | 24 ++++++++++++++++++++++++
> 1 file changed, 24 insertions(+)
>
> diff --git a/gcc/is-a.h b/gcc/is-a.h
> index a14e344..f50e62c 100644
> --- a/gcc/is-a.h
> +++ b/gcc/is-a.h
> @@ -46,6 +46,14 @@ TYPE as_a <TYPE> (pointer)
>
> do_something_with (as_a <cgraph_node *> *ptr);
>
> +TYPE as_a_nullable <TYPE> (pointer)
> +
> + Like as_a <TYPE> (pointer), but where pointer could be NULL. This
> + adds a check against NULL where the regular is_a_helper hook for TYPE
> + assumes non-NULL.
> +
> + do_something_with (as_a_nullable <cgraph_node *> *ptr);
> +
> TYPE dyn_cast <TYPE> (pointer)
>
> Converts pointer to TYPE if and only if "is_a <TYPE> pointer".
> Otherwise,
> @@ -185,6 +193,22 @@ as_a (U *p)
> return is_a_helper <T>::cast (p);
> }
>
> +/* Similar to as_a<>, but where the pointer can be NULL, even if
> + is_a_helper<T> doesn't check for NULL. */
> +
> +template <typename T, typename U>
> +inline T
> +as_a_nullable (U *p)
> +{
> + if (p)
> + {
> + gcc_checking_assert (is_a <T> (p));
> + return is_a_helper <T>::cast (p);
> + }
> + else
> + return NULL;
> +}
> +
> /* A generic checked conversion from a base type U to a derived type T. See
> the discussion above for when to use this function. */
>
> --
> 1.8.5.3
>
