Re: Question on undefined behaviour
Alan Carre wrote:
----------------------------
Please note:
Before I get the book thrown at me, let me state that I openly admit I am
not an expert on the C++ standard. I'm not even a novice!
It seems every time someone says something here, eventually somebody else
comes back with one of these:
"Listen, if you'd just simply read the standard you'd know from ISO/C99
++2003 2.5.1[a.32], 8.6.5.1[1.65] that you're completely wrong!"
I wish I had the time to sit down and read every last detail about the
language but I don't.
That's fine, Alan. You don't have read all the things yourself. You
just need to trust those who "throw the book" at you and their
interpretation of "the book". Right? It's either you read and try to
understand it yourself (and ask for clarification if something needs it)
or you rely on others.
----------------------------
That being said, I have a technical question and a conditional question
depending on the answer.
Q1.
Suppose we had a function that took as an argument a buffer (reference or
pointer) and returned a boolean value indicating whether or not the buffer
was successfully filled with the requested data. Something like:
bool ReadRegistryDword(const char* strKeyName, DWORD& dwKeyValue);
or
bool ReadRegistryDword(const char* strKeyName, DWORD* pdwKeyValue);
I assume these are essentially equivalent except that the latter might
concievably be passed NULL for the value part.
Sounds good.
----
Then, is it not true that the following sequence is undefined?
DWORD dwKeyValue;
Is 'dwKeyValue' left uninitialised by mistake?
if(ReadRegistryDword("SomeKeyName", dwKeyValue) && dwKeyValue != 0)
{
puts("Key value non zero");
}
The sequence is not undefined (why do you think it is?). If the
function 'ReadRegistryDword' returns 'false' the expression after the
'&&' is not going to be evaluated. The logical AND operator is special
in that sense. It provides a sequence point (all side effects of the
left-hand side expression must take place before it) and it also follows
the rules of short-circuit evaluation (i.e. if left expression is
'false', the right expression is not even evaluated).
My guess is that the answer is YES. Undefined behaviour (similar to var =
++var + var).
Nope.
-------------------------
If A1 == YES then the follow-up question is this:
I guess this is irrelevant then.
Q2:
Shouldn't it be considered bad practice to define a function which takes as
input an "__out" value and returns anything that can be used in a
conditional expression (such as bool or HRESULT etc)?
I don't think so.
Wouldn't it be safer to pass in both the buffer and a reference to a bool
(or HRESULT) indicating success (ie. instead of returning said value)?
That's overkill.
void ReadRegistryDword(LPCTSTR strKeyName, DWORD& dwKeyValue, bool&
bSuccess);
With the above function it is impossible for me to proceed to make use of
dwKeyValue until a sequence point has been reached, after which dwKeyValue
will be valid if and only if bSuccess is true.
bool bSuccess;
DWORD dwKeyValue;
ReadRegistryDword("SomeKeyName", dwKeyValue, bSuccess);
// Well defined conditional
if(bSuccess && dwKeyValue != 0)
{
puts("Key value non zero");
}
That's totally unnecessary considering my answer to Q1.
-------------------------
Thank you in advance for your time and patience,
You're most welcome.
- Alan Carre
V
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