[Qt-interest] howto convert QString to char*

[Andrew Hodgkinson]

On 15/07/2010 06:06, Ivan Kharin wrote:

> char *qStringCopy( const QString&  s )
> {
>    return strdup( s.toLocal8Bit().constData() );
> }

I was going to post something similar to that very early in the thread, 
but it's not all that helpful given the way the original poster wants to 
use it, or indeed, the way that most people would probably want to use 
the function. This indicates that it's not a well designed API. It's not 
the world's most efficient piece of code either, given it ends up copying 
strings all over the place.

Consider a typical use case where we want to pass a 'C' string equivalent 
of some QString into a few functions.

   someCStringHandler ( qStringCopy( someQString   ) );
   someOtherFunction  ( qStringCopy( anotherString ) );
   aThirdFunction     ( qStringCopy( thirdString   ) );

We've just leaked three strings. The caller is being given a permanently 
allocated block from the malloc heap, so the caller MUST free this block. 
Further, although most Linux variants cannot ever return NULL from 
'malloc', the process instead simply being unceremoniously killed if all 
RAM is exhausted, the Standard says that NULL may be returned from 
'malloc' if allocation fails. Some operating systems will do this, so you 
need to handle it.

Thus, the above code becomes very clumsy:

   char * cStr1 = qStringCopy( someQString   );
   char * cStr2 = qStringCopy( anotherString );
   char * cStr3 = qStringCopy( thirdString   );

   if ( cStr1 ) someCStringHandler( cStr1 );
   else // Handle allocation failure

   if ( cStr2 ) someOtherFunction( cStr2 );
   else // Handle allocation failure

   if ( cStr3 ) aThirdFunction( cStr3 );
   else // Handle allocation failure

   free( cStr1 );
   free( cStr2 );
   free( cStr3 );

At least we could start to tidy it up by throwing an exception in the 
qStringCopy function if the allocation failed, but you still have to keep 
a record of the returned pointer to give to 'free()' later. Trying to 
work around this basic API problem within the constraints of the C/C++ 
language and its handling of objects on the stack is the reason why the 
answers to the original poster's request became so complicated.

-- 
Andrew Hodgkinson, Endurance Technology
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