Re: Question on undefined behaviour

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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 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.

              ----

Then, is it not true that the following sequence is undefined?

DWORD dwKeyValue;
if(ReadRegistryDword("SomeKeyName", dwKeyValue) && dwKeyValue != 0)

  {
  puts("Key value non zero");
  }

My guess is that the answer is YES. Undefined behaviour (similar to var =
++var + var).

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If A1 == YES then the follow-up question is this:

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)?

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)?

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");
  }

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Thank you in advance for your time and patience,

- Alan Carre