On Wed, Aug 13, 2014 at 12:01 PM, Martin Jambor <mjam...@suse.cz> wrote:
> 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)?
Ok, I resisted at first starting the bike-shedding but I agree on
the badness of "nullable" (what does that mean anyway?).
as_a_safe or safe_as_a both work for me.
Richard.
>
> 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
>>
