Re: Class invariants and implicit move constructors (C++0x)

Consider a class with two containers, where the sum of the sizes of the
containers is cached. The class invariant is that as long as the cache is
claimed to be up to date, the sum of the sizes of the containers is accurately
cached:

   class Widget {
   public:
     ...
   private:
     std::vector<int> v1;
     std::vector<int> v2;
     mutable std::size_t cachedSumOfSizes;
     mutable bool cacheIsUpToDate;

     void checkInvariant() const
     { assert(!cacheIsUpToDate || cachedSumOfSizes == v1.size()+v2.size()); }
   };

Assume that checkInvariant is called at the beginning and end of every public
member function. Further assume that the class declares no copy or more
operations, i.e., no copy or move constructor, no copy or move assignment
operator.

Suppose I have an object w where v1 and v2 have nonzero sizes, and
cacheIsUpToDate is true. Hence cachedSumOfSizes == v1.size()+v2.size(). If w
is copied, the compiler-generated copy operation will be fine, in the sense that
w's invariant will remain fulfilled. After all, copying w does not change it in
any way.

But if w is moved, the compiler-generated move operation will "steal" v1's and
v2's contents, setting their sizes to zero. That same compiler-generated move
operation will copy cachedSumOfSizes and cacheIsUpToDate (because moving
built-in types is the same as copying them), and as a result, w will be left
with its invariant unsatisfied: v1.size()+v2.size() will be zero, but
cachedSumOfSizes won't be, and cacheIsUpToDate will remain true.

When w is destroyed, the assert inside checkInvariant will fire when it's called
from w's destructor. That means that the compiler-generated move operation for
Widget broke the Widget invariant, even though the compiler-generated copy
operations for Widget left it intact.

The above scenario suggests that compiler-generated move operations may be
unsafe even when the corresponding compiler-generated copy operations are safe.
  Is this a valid analysis?

Scott

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