Re: [Boost-users] Library Interface Design

From: Scott Meyers (usenet_at_[hidden])
Date: 2006-09-14 20:21:17

David Abrahams wrote:
> I'd much rather develop a library of mock objects for testing.  Okay a
> conforming mock stream may be a little work to write, but you write it
> once and you're done.

I take this to mean that you have not found it 
necessary/desirable/useful to create separate test and client 
interfaces.  Okay.

My understanding is that the ability to drop in mock objects requires 
programming to an interface that can be either a "real" object or a mock 
object, which in turn suggests using either a template parameter or a 
base class.  Suppose, for example, you have this:

   class BigHonkinHairyClass {
   ...                            // expensive to construct
   };

and you want to implement some function f that takes a 
BigHonkinHairyClass as a parameter.  The straightforward declaration 
would be

   void f(BigHonkinHairyClass& bhhc);   // maybe const, doesn't matter

But now it's hard to drop in a mock.  So I assume you'd modify the 
interface to be either

   template<typename T>
   void f(T& bhhc);               // T must more or less model
                                  // BigHonkinHairyClass

or this:

   class BigHonkinHairyBase {
   ...                            // interface to program against --
   };                             // uses virtuals

   class BigHonkinHairyClass: public BigHonkinHairyBase { ... };

   class MockHonkinHairyClass: public BigHonkinHairyBase { ... };

   void f(BigHonkinHairyBase& bhhc);

Is that correct?  In other words, you'd come up with an interface that 
let clients do what they wanted but that was also mock-friendly for 
testing purposes?  That is, you'd take testability into account when 
designing your client interface?

Thanks,

Scott