Re: [boost] Supressing warnings

From: Reece Dunn (msclrhd_at_[hidden])
Date: 2006-06-27 09:37:13

Oliver Kullman wrote:
> > NOTE: Instead of disabling the warnings, you can remove the warning by
> > updating the code. For example:
> > 
> > void foo( int bar ){} // unreferenced parameter
> > void foo( int bar ){ bar; } // no warning!
> >
> 
> this is FALSE:

No - it is one way to prevent the bug.

> void foo(int){} 

This doesn't work if you have:

/** @brief something
   * @param bar Oh dear, this is missing!
   */
void foo( int bar ){}

Either not specifying bar or commenting it out will mean that Doxygen will not
be able to identify the bar parameter.

> > bool foo(){ BOOL bar = TRUE; return bar; } // performance warning
> > bool foo(){ BOOL bar = TRUE; return bar != FALSE; } // no warning
> 
> this is a perversion.

Agreed. I was giving *examples* of *possible ways* of removing the warnings
without using pragmas. I wasn't suggesting that this should be the way the code
is written!

> > class foo
> > {
> >    bar & m_bar;
> >    foo( bar & b ): m_bar( b ){}
> > 
> >    // disable the no default compiler assignment operator warning:
> >    inline foo & operator=( const foo & );
> > };
> 
> again this is a perversion: the C++ should follow the LANGUAGE, and the special
> handling of the *special member function* is important part of C++.

No. You miss the issue here. The problem is that m_bar is a *reference* and
*you can't assign a reference*!

> In the above example, first "inline" shouldn't be there, and then it's not
> about warnings, but about semantics: either we want the implicitely declared
> copy assignment operator, and then we *must not* add that line, or we don't
> want it, and then we *must* add the line --- there is no choice.

The inline *should* be there. This is like deriving from boost::noncopyable which
uses the above trick to prevent copying. In this case, I don't think it is possible to
use noncopyable as this is about the default assignment operator in foo. By adding
the line:

   inline foo & operator=( const foo & );

I am telling the compiler that I am providing my own assignment operator, so
the compiler doesn't automatically generate one. This is the classic way to
implement non-copyable behaviour (as well as providing a similar copy constructor).

In fact, this highlights a potential issue in the code. The standard says that you
can't change a reference, so the assignment operator will copy values not what
is being pointed to. This can lead to a bug if you assign one foo to another. In
this case the warning has identified a potential problem and the above will identify
any places where that problem is manifest.

Also, explicitly disabling the variable not used warning will mask this bug:

   class foo
   {
      int bar;
      foo( int bar ): bar( bar ) {} // oops! This will be picked up by warnings!
   };

- Reece
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