From: Warren Young
Date: December 5 2008 2:47pm
Subject: Re: Problem with Null string type
List-Archive: http://lists.mysql.com/plusplus/8238
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On Dec 5, 2008, at 2:19 AM, Brad Hubbard wrote:

> If I want to display the data in it->mobileNumber is the preferred  
> method to do it like this?
>
> if( it->mobileNumber == mysqlpp::null )
> {
> something_that_requires_a_c_string = "";
> }
> else
> {
> something_that_requires_a_c_string = it- 
> >mobileNumber.data.c_str(); // or it->mobileNumber.data
> }

If that's appropriate code for your application, it implies SQL NULL  
has no special significance to you, so you should drop it from the DB.

If you can't change the DB, use the NullIsBlank behavior.  If you do  
that, the code can be:

	something_that_requires_a_c_string = sql_char(it- 
 >mobileNumber).c_str();

You need the explicit conversion in there because C++ won't implicitly  
do two casts in a row.

Actually, this might work, too:

	something_that_requires_a_c_string = sql_char(it->mobileNumber);

...because sql_char has a const char* operator.

If SQL NULL does mean something different from blank in your  
application, then I question whether the quoted code above is even  
appropriate.  Maybe different parts of your application have more  
stringent requirements than others, but you should think about which  
case applies, not just blindly cast away NULLness.

> The only example using char based data does this;
>
>           if (it->description != mysqlpp::null) {
>               cout << " (" << it->description << ")";
>
> but I presume it can do that because of an overloaded "<<" operator?

Not exactly.  It works because Null<foo> can convert itself to foo, so  
you can use Null<foo> anywhere you need a foo.  This doesn't help you  
because you need a double conversion.