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Value Types and Their Behaviors


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Value Types and Their Behaviors
Value types have changed in various ways from Managed Extensions for C++ to Visual C++ 2008.
A D V E R T I S E M E N T
In this section, we look at the CLR enum type and the value class type, together with a look at boxing and access to the boxed instance on the CLR heap, as well as a look at interior and pinning pointers. There have been extensive language changes in this area.



In This Section
Value Types and Their Behaviors
Discusses changes in the declaration and behavior of enums.
Implicit Boxing of Value Types
Discusses the motivation for implicit boxing of value types and the consequent changes in behavior.
A Tracking Handle to a Boxed Value
Discusses how implicit boxing of value types translates to a tracking handle to the boxed value object.
class="normaltext"
Discusses changes to class="normaltext", including inherited virtual methods, class default constructors, interior pointers, and pinning pointers.
CLR Enum Type
The declaration and behavior of enums has changed from Managed Extensions for C++ to Visual C++ 2008.

The Managed Extensions enum declaration is preceded by the __value keyword. The idea here is to distinguish the native enum from the CLR enum which is derived from System::ValueType, while suggesting an analogous functionality. For example:

Copy Code
__value enum e1 { fail, pass };
public __value enum e2 : unsigned short  { 
   not_ok = 1024, 
   maybe, ok = 2048 
};

The new syntax solves the problem of distinguishing native and CLR enums by emphasizing the class nature of the latter rather than its value type roots. As such, the __value keyword is discarded, replaced with the spaced keyword pair of enum class. This provides a paired keyword symmetry to the declarations of the reference, value, and interface classes:

Copy Code
enum class ec;
value class vc;
ref class rc;
interface class ic;

The translation of the enumeration pair e1 and e2 in the new syntax looks as follows:

Copy Code
enum class e1 { fail, pass };
public enum class e2 : unsigned short { 
   not_ok = 1024,
   maybe, ok = 2048 
};

Apart from this small syntactic change, the behavior of the CLR enum type has been changed in a number of ways:

  • A forward declaration of a CLR enum is no longer supported. There is no mapping. It is simply flagged as a compile-time error.
Copy Code
__value enum status; // Managed Extensions: ok
enum class status;   // new syntax: error
  • The overload resolution between the built-in arithmetic types and the Object class hierarchy has reversed between the two language versions! As a side-effect, CLR enums are no longer implicitly converted to arithmetic types.
  • In the new syntax, a CLR enum maintains its own scope, which is not the case in Managed Extensions. Previously, the enumerators were visible within the containing scope of the enum. Now, the enumerators are encapsulated within the scope of the enum.



CLR Enums are a Kind of Object

Consider the following code fragment:

Copy Code
__value enum status { fail, pass };

void f( Object* ){ Console::WriteLine("f(Object)\n"); }
void f( int ){ Console::WriteLine("f(int)\n"); }

int main()
{
   status rslt = fail;

   f( rslt ); // which f is invoked?
}

For the native C++ programmer, the natural answer to the question of which instance of the overloaded f is invoked is that of f(int). An enum is a symbolic integral constant, and it participates in the standard integral promotions which take precedence in this case. And in fact in Managed Extensions this was the instance to which the call resolves. This caused a number of surprises – not when we used them in a native C++ frame of mind – but when we needed them to interact with the existing BCL (Base Class Library) framework, where an Enum is a class indirectly derived from Object. In the Visual C++ 2008 language design, the instance of f invoked is that of f(Object^).

The way Visual C++ 2008 has chosen to enforce this is to not support implicit conversions between a CLR enum type and the arithmetic types. This means that any assignment of an object of a CLR enum type to an arithmetic type will require an explicit cast. So, for example, given

Copy Code
void f( int );

as a non-overloaded method, in Managed Extensions, the call

Copy Code
f( rslt ); // ok: Managed Extensions; error: new syntax

is ok, and the value contained within rslt is implicitly converted into an integer value. In Visual C++ 2008, this call fails to compile. To correctly translate it, we must insert a conversion operator:

Copy Code
f( safe_cast( rslt )); // ok: new syntax



The Scope of the CLR Enum Type

One of the changes between the C and C++ languages was the addition in C++ of scope within the struct facility. In C, a struct is just a data aggregate without support of either an interface or an associated scope. This was quite a radical change at the time and was a contentious issue for many new C++ users coming from the C language. The relationship between the native and CLR enum is analogous.

In Managed Extensions, an attempt was made to define weakly injected names for the enumerators of a CLR enum in order to simulate the absence of scope within the native enum. This did not prove successful. The problem is that this causes the enumerators to spill into the global namespace, resulting in difficult to manage name-collisions. In the new syntax, we have conformed to the other CLR languages in supporting scopes within the CLR enum.

This means that any unqualified use of an enumerator of a CLR enum will not be recognized by the new syntax. Let's look at a real-world example.

Copy Code
// Managed Extensions supporting weak injection
__gc class XDCMake {
public:
   __value enum _recognizerEnum { 
      UNDEFINED,
      OPTION_USAGE, 
      XDC0001_ERR_PATH_DOES_NOT_EXIST = 1,
      XDC0002_ERR_CANNOT_WRITE_TO = 2,
      XDC0003_ERR_INCLUDE_TAGS_NOT_SUPPORTED = 3,
      XDC0004_WRN_XML_LOAD_FAILURE = 4,
      XDC0006_WRN_NONEXISTENT_FILES = 6,
   };

   ListDictionary* optionList;
   ListDictionary* itagList;

   XDCMake() {
      optionList = new ListDictionary;

      // here are the problems …
      optionList->Add(S"?", __box(OPTION_USAGE)); // (1)
      optionList->Add(S"help", __box(OPTION_USAGE)); // (2)

      itagList = new ListDictionary;
      itagList->Add(S"returns", 
         __box(XDC0004_WRN_XML_LOAD_FAILURE)); // (3)
   }
};

Each of the three unqualified uses of the enumerator names ((1), (2), and (3)) will need to be qualified in the translation to the new syntax in order for the source code to compile. Here is a correct translation of the original source code:

Copy Code
ref class XDCMake {
public:
   enum class _recognizerEnum {
      UNDEFINED, OPTION_USAGE, 
      XDC0001_ERR_PATH_DOES_NOT_EXIST = 1,
      XDC0002_ERR_CANNOT_WRITE_TO = 2,
      XDC0003_ERR_INCLUDE_TAGS_NOT_SUPPORTED = 3,
      XDC0004_WRN_XML_LOAD_FAILURE = 4,
      XDC0006_WRN_NONEXISTENT_FILES = 6
   };

   ListDictionary^ optionList;
   ListDictionary^ itagList;

   XDCMake() {
      optionList = gcnew ListDictionary;
      optionList->Add("?",_recognizerEnum::OPTION_USAGE); // (1)
      optionList->Add("help",_recognizerEnum::OPTION_USAGE); //(2)
      itagList = gcnew ListDictionary;
      itagList->Add( "returns", 
         _recognizerEnum::XDC0004_WRN_XML_LOAD_FAILURE); //(3)
   }
};

This changes the design strategy between a native and a CLR enum. With a CLR enum maintaining an associated scope in Visual C++ 2008, it is neither necessary nor effective to encapsulate the declaration of the enum within a class. This idiom evolved around the time of cfront 2.0 within Bell Laboratories also in order to solve the global name pollution problem.

In the original beta release of the new iostream library by Jerry Schwarz at Bell Laboratories, Jerry did not encapsulate all the associated enums defined for the library, and the common enumerators such as read, write, append, and so on, made it nearly impossible for users to compile their existing code. One solution would have been to mangle the names, such as io_read, io_write, etc. A second solution would have been to modify the language by adding scope to an enum, but this was not practicable at the time. The middle solution was to encapsulate the enum within the class, or class hierarchy, where both the tag name and enumerators of the enum populate the enclosing class scope.) That is, the motivation for placing enums within classes, at least originally, was not philosophical, but a practical response to the global name-space pollution problem.

With the Visual C++ 2008 enum, there is no longer any compelling benefit to encapsulating an enum within a class. In fact, if you look at the System namespaces, you will see that enums, classes, and interfaces all inhabit the same declaration space.

Implicit Boxing of Value Types
The boxing of value types has changed from Managed Extensions for C++ to Visual C++ 2008.

In language design, 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 Stroustrup 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 concern itself only with the non-private implementation of its immediate base classes. In order to support a flexible initialization design for virtual inheritance, Stroustrup 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 simple code sample– and in this case, the explicit boxing is unnecessary:

Copy Code
// 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->GetLowerBound(0)),
   __box(my1DIntArray->GetUpperBound(0)) );

As you can see, there is a whole lot of boxing going on. Under Visual C++ 2008, value type boxing is implicit:

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

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


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