[LLVMdev] Adding DenseMap::FindAndConstruct with a default value

[Owen Anderson]

Assuming the default value is not a valid entry in your map (for  
instance, if you're using pointers), you can do:

Foo& entry = DenseMap[Key]
if (entry == DefaultValue)
	entry = constructNewValue();

... // Use entry

--Owen


On Jun 5, 2008, at 7:49 AM, Matthijs Kooijman wrote:

> Hi All,
>
> I've been fiddling around with a DenseMap to store cached copies of  
> some
> result. In short, I'm doing the following:
>
> It = Map.find(Key)
> if (It != Map.end() && It->second != Unknown)
> 	Return It->second;
>
> // do_stuff
>
> return Map[Key] = Result;
>
> However, I this requires two lookups in the hash table, which is not  
> so nice.
> Currently, there is no way to write this down so you only do one  
> lookup.
> Intuitively, you'd write:
>
> ValueT &Value = Map[Key];
> if (Value != Unknown)
> 	return Value;
>
> // do_stuff
>
> return Value = Result;
>
> However, I'm using an enum as a map value, so when you do Map[Key]  
> while Key
> is not yet in the map, the new value will be unitialized (I can't  
> define a
> constructor on an enum, right?).
>
> So, to solve this, I propose adding a second version of
> DenseMap::FindAndConstruct, which has an extra argument for the  
> default value.
> So, it would do the same thing as the original (return a reference  
> to the
> value from the map, adding it if it's not already in), but instead  
> of using
> ValueT() as a default value, it would use an argument.
>
> Any objections to adding this? Suggestions for alternatives?
>
> Gr.
>
> Matthijs
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