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The boxing of value types has changed from Managed Extensions for C++ to Visual C++ 2005.

Okay, so we reversed ourselves. In politics, that would likely lose us an election. In language design, it means that we imposed a philosophical position in lieu of practical experience with the feature and, in practice, it was a mistake. As an analogy, in the original multiple inheritance language design, Stroustrup decided that a virtual base class sub-object could not be initialized within a derived class constructor, and therefore the language required that any class serving as a virtual base class must define a default constructor. It is that default constructor that would be invoked by any subsequent virtual derivation.

The problem of a virtual base class hierarchy is that responsibility for the initialization of the shared virtual sub-object shifts with each subsequent derivation. For example, if I define a base class for which initialization requires the allocation of a buffer, the user-specified size of that buffer might be passed as an argument to the constructor. If I then provide two subsequent virtual derivations, call them inputb and outputb, each provides a particular value to the base class constructor. Now, when I derived an in_out class from both inputb and outputb, neither of those values to the shared virtual base class sub-object can sensibly be allowed to evaluate.

Therefore, in the original language design, Stroustrup disallowed the explicit initialization of a virtual base class within the member initialization list of the derived class constructor. While this solved the problem, in practice the inability to direct the initialization of the virtual base class proved impracticable. Keith Gorlen of the National Institute of Health, who had implemented a freeware version of the SmallTalk collection library called nihcl, was a principle voice in convincing Bjarne that he had to come up with a more flexible language design.

A principle of Object-Oriented hierarchical design holds that a derived class should only concern itself with the non-private implementation of its immediate base classes. In order to support a flexible initialization design for virtual inheritance, Bjarne had to violate this principle. The most derived class in a hierarchy assumes responsibility for all virtual sub-object initialization regardless of how deep into the hierarchy it occurs. For example, inputb and outputb are both responsible for explicitly initializing their immediate virtual base class. When in_out derives from both inputb and outputb, in_out becomes responsible for the initialization of the once removed virtual base class, and the initialization made explicit within inputb and outputb is suppressed.

This provides the flexibility required by language developers, but at the cost of a complicated semantics. This burden of complication is stripped away if we restrict a virtual base class to be without state and simply allow it to specify an interface. This is a recommended design idiom within C++. Within CLR programming, it is raised to policy with the Interface type.

Here is a real code sample doing something very simple – and in this case, the explicit boxing is mostly a lexical tax without representation.

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// Managed Extensions for C++ requires explicit __box operation
int my1DIntArray __gc[] = { 1, 2, 3, 4, 5 };
Object* myObjArray __gc[] = { 
   __box(26), __box(27), __box(28), __box(29), __box(30)

Console::WriteLine( "{0}\t{1}\t{2}", __box(0),
   __box(my1DIntArray->GetUpperBound(0)) );

As you can see, there is a whole lot of boxing going on. Under Visual C++ 2005, value type boxing is implicit:

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// new syntax makes boxing implicit
array<int>^ my1DIntArray = {1,2,3,4,5};
array<Object^>^ myObjArray = {26,27,28,29,30};

Console::WriteLine( "{0}\t{1}\t{2}", 0, 
   my1DIntArray->GetLowerBound( 0 ), 
   my1DIntArray->GetUpperBound( 0 ) );

See Also


Implicit Boxing


Value Types and Their Behaviors

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