Utility class that disallows dynamic creation

Daniel =?iso-8859-1?Q?Lidstr=F6m?= <someone@microsoft.com>
Thu, 27 Mar 2008 10:52:38 +0100

I've created a class whose purpose is to disallow dynamic creation, sort of
like boost::noncopyable. I want to make sure that clients of my classes do
not use them in an unsafe way. So I disable operator new and address-of. We
have a lot of code here that doesn't use RAII, uses new/delete scattered
throughout, etc. This is an attempt at resolving these issues from this
point on.

Here's the implementation:

struct disallow_dynamic

   // disallow operator new
   void* operator new(std::size_t);
   void* operator new(std::size_t, const std::nothrow_t&) throw();
   void* operator new[](std::size_t);
   void* operator new[](std::size_t, const std::nothrow_t&) throw();

   // disallow address-of
   const disallow_dynamic* operator&() const;
   /* */ disallow_dynamic* operator&() /* */;

This class should be used in the following way:

struct DataIO : private disallow_dynamic

Now you are only able to create DataIO instances on the stack. This is
great when combined with shared_ptr. For example, I have an interface
called IDataIO:

struct IDataIO
   virtual void Write(const std::string& str) = 0;

   virtual void Read(char* buffer, std::size_t size) = 0;

typedef boost::shared_ptr<IDataIO> IDataIOPtr;

Using this interface is a little bit awkward: typing IDataIOPtr is loong,
and I have to use ->. Just a little bit too pointer-like (new/delete is not
far away with pointers). Also, there is always the risk of someone misusing
shared_ptr by extracting the raw pointer. Let me show the nice solution:

// create a wrapper class that derives from IDataIO,
// and disallows dynamic usage
struct DataIO : IDataIO, private disallow_dynamic
   DataIO(IDataIOPtr dataIO) : mDataIO(dataIO) {}

   void Write(const std::string& str) { mDataIO->Write(str); }

   void Read(char* buffer, std::size_t size){ mDataIO->Read(buffer, size);}

   IDataIOPtr mDataIO;

Now it's impossible to misuse the IDataIO hierarchy! Here's a typical usage

   DataIO dataIO = CreateDataIO(0);
   dataIO.Write("Hej hopp");
   char buffer[1024];
   dataIO.Read(buffer, sizeof(buffer));

There is no way to obtain a pointer to IDataIO, or DataIO. No risks here!

What do you think? Is the disallow_dynamic class named properly,
implemented correctly, useful at all? Looking forward to any comments :-)


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