Jan 10

Managed Wrappers and Hidden Interdependencies

C#, C++ 2 Comments »

Let’s say you have the following unmanaged code:

  1. #pragma unmanaged
  2.  
  3. class Stream { … }; // Conceptual stream class
  4.  
  5. class StreamWriter
  6. {
  7. public:
  8.     StreamWriter(Stream* pStream) : m_pStream(pStream) {}
  9.     ~StreamWriter() { /* Use m_pStream in some way */ }
  10.  
  11.     …
  12. private:
  13.     Stream* m_pStream;
  14. };
  15.  
  16. void f()
  17. {
  18.     Stream stream;
  19.     StreamWriter streamWriter(&stream);
  20.  
  21.     // Use streamWriter
  22.  
  23.     // streamWriter is destroyed
  24.     // stream is destroyed
  25. }

Note that StreamWriter’s destructor uses m_pStream (perhaps by flushing the stream). This means that the order of destruction is important — StreamWriter must be destroyed before its underlying Stream is.

Now let’s try to write and use some simple managed C++ wrappers for these classes:

  1. #pragma managed
  2.  
  3. public __gc class ManagedStream
  4. {
  5. public:
  6.     ManagedStream() : m_pStream(new Stream) {}
  7.  
  8.     // NOTE: This is a finalizer, not a determinstic destructor  
  9.     ~ManagedStream() { delete m_pStream; }
  10.  
  11. public private: // Make accessible by ManagedStreamWriter
  12.     Stream __nogc* m_pStream;
  13. };
  14.  
  15. public __gc class ManagedStreamWriter
  16. {
  17. public:
  18.     ManagedStreamWriter(ManagedStream* pStream) :
  19.         m_pStreamWriter(new StreamWriter(pStream->m_pStream)) {}
  20.  
  21.     // NOTE: This is a finalizer, not a determinstic destructor  
  22.     ~ManagedStreamWriter() { delete m_pStreamWriter; }
  23.  
  24. private:
  25.     StreamWriter __nogc* m_pStreamWriter;
  26. };
  27.  
  28. void f()
  29. {
  30.     ManagedStream stream = __gc new ManagedStream();
  31.     ManagedStreamWriter streamWriter =
  32.         __gc new ManagedStreamWriter(stream);
  33.  
  34.     // Use streamWriter
  35.  
  36.     // GC will clean up stream and streamWriter
  37. }

See the problem?

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