llvm dev - Jun 2008 - [LLVMdev] Adding DenseMap::FindAndConstruct with a default value

> 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();
The problem here is that the DefaultValue is undefined. However, Chris
suggested that the default value, ValueT(), is not undefined but simply zero.
However, on IRC someone was quite positive that it would become undefined.
Anyone that knows for sure (preferably from the language standard, not only
from experience :-)

Gr.

Matthijs
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On Fri, Jun 6, 2008 at 12:27 AM, Matthijs Kooijman <matthijs at stdin.nl>
wrote:
>> 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();
> The problem here is that the DefaultValue is undefined. However, Chris
> suggested that the default value, ValueT(), is not undefined but simply
zero.
This is correct; from the standard: "The expression T(), where T is a
simple-type-specifier (7.1.5.2) for a non-array complete object type
or the (possibly cv-qualified) void type, creates an rvalue of the
specified type, which is value-initialized."  (Section 5.2.3
[expr.type.conv] in the C++0x draft standard; I don't have the
published standard on hand.) And value-initialization is defined to be
zero-initialization for scalar types. (Section 8.5 [dcl.init] in the
C++0x draft.)
> However, on IRC someone was quite positive that it would become undefined.
The reason this is kind of confusing is that in a lot of places, the
standard says something along the lines of "if this object is a
non-POD type, value-initialize it; otherwise, leave it uninitialized"
(examples include automatic variables and class members).

-Eli

Hi Eli,

thanks for the clarification, this saves me from implementing extra code :-)

Gr.

Matthijs
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