🚀 go-pugleaf

Alan Bellingham wrote:
> pnunn <pnunn@infoteq.com.au> wrote:
> 
> 
>>Folks.. I'm stuck on a problem that should be easy to sort out but I 
>>can't get to work.
>>
>>I have a base class defined that has a pointer to a structure in it.
>>
>>struct stateTable *table;
>>
>>and a public function setTable that takes a passed in pointer and sets 
>>table to it...
>>
>>In the derived class I have (in the constructor)
>>
>>   struct stateLine binTable[] =
>>   {
>>	   //current state, A0, A1, next state
>>	   {1, High, Low, 2},
>>	   {1, High, High, 4},
>>	   {1, Low, High, 5},
>>	   {2, Low, Low, 1},
>>	   {2, High, High, 4},
>>	   {4, High, Low, 3},
>>	   {3, Low, Low, 1},
>>	   {3, Low, High, 4},
>>	   {5, High, High, 4},
>>	   {5, Low, Low, 1}
>>   };
>>
>>   setTable(binTable);
> 
> 
> Now, if the definition of this function is

Yes, thats basically what the function does.
> 
>   void setTable(stateTable * newTable)
>   {
>      table = newTable;
>   }
> 
> then you are rightly screwed on two accounts. The first is that your
> table in the constructor is on the stack, and the moment the constructor
> completes, that stack memory gets used by other things.

Ok, that's the conclusion I came to last night.

> 
> Secondly, you are passing a pointer to stateLine to a function that
> expects a pointer to stateTable. Given that this compiles, I expect that
> a stateLine is derived from a stateTable. This is fine, ONLY so long as
> you are passing a pointer to a single element, because with a single
> element, you know better than to try indexing on that pointer. In your
> case, I expect the users of table expect it to be a pointer to an array.
> Bad news, because the pointer arithmetic assumes the base class.

Bugger, missed that one.. No, its not a derived class at all, its an 
array of stateLine elements.  Wonder why it compiled at all.

> 
> Oh, failure three - how does the code know how many elements in the
> array?

Ah, I used sizeof(table)/sizeof(*table);

> 
> Alan Bellingham

So... what should I be doing?  Guess I've got to use new to make the 
table and the elements, then pass this pointer. Is that right?  How 
should I go about this?

Ta.

Peter.