Comparing CComBSTR to NULL

Question

Lately, I was working on a code and found following code:

CComBSTR userName;
SUCCEED(getUserName(userName));
if(userName == NULL) ...

The last line alerts me, since I'm comparing an object against NULL which is not intuitive. But after a glance on MSDN, it is perfectly supported:

bool operator ==( const CComBSTR& bstrSrc ) const throw( ); 
bool operator ==( LPCOLESTR pszSrc ) const; 
bool operator ==( LPCSTR pszSrc    ) const;     
bool operator ==(  int nNull  ) const throw( );

So my question is, why would API design of CComBSTR allow such a comparison? Just to be more tolerant to errors?

BSTR is a pointer type. Pointers can be NULL. – Hans Passant May 10 '13 at 07:29 — Hans Passant, May 10 '13 at 07:29

score 1 · Accepted Answer · answered May 10 '13 at 08:18

1

A smart pointer is designed and intended to mimic a raw pointer as much as possible, simply provide automatic memory management but otherwise be transparent to outside code. That means overriding the ->, =, and & operators, implementing conversion operations, etc. This way, the rest of the code can treat the smart pointer as if it were a real pointer in almost all aspects.

Imagine someone started with this code:

BSTR userName;
SUCCEED(getUserName(&userName));
if(userName == NULL) ...

Then wanted to upgrade to a smart pointer:

CComBSTR userName;
SUCCEED(getUserName(&userName));
if(userName == NULL) ...

See how that works? Only one line changed.

answered May 10 '13 at 08:18

Remy Lebeau

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Days ago, somebody replied that empty BSTR semantically can be null pointer, "". I think that is where I get wrong. Thank you very much for your help! – Archer May 13 '13 at 02:59
I posted that answer based on http://stackoverflow.com/questions/171641/ ; but your question here is about `NULL` check on `CComBSTR` so I think this answer is more appropriate. – Afriza N. Arief May 13 '13 at 06:45

score 0 · Answer 2 · answered May 10 '13 at 07:11

First you should find the definition of the class CComBSTR to confirm whether you use the windows system API not your own method. When we develop COM, maybe your development system redefine the Class. when I use the API of the windows, there is no problem. good luck

Comparing CComBSTR to NULL

2 Answers2