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Using a Native Object in a Managed Type
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When including native types in managed types, you must reference the native types via a pointer to the object on the native heap. You cannot actually make the native type part of the layout of the class, but everything in the class layout must be a managed type since the whole object will be managed by the common language runtime. Thus, the code in Listing 6-13 is legal. Listing 6-13. Using a Native Type in a Managed Type // Forestry.cpp using namespace System; using namespace System::Runtime::InteropServices; // a native class class PlantData { private:
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CHAPTER 6 CLASSES AND STRUCTS
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wchar_t* family; wchar_t* genus; wchar_t* species; public: PlantData(const wchar_t* botanical_name) { // Let's assume this method // populates its // fields with data from the database. } }; // The following managed class contains a pointer to a native class. ref class TreeSpecies { PlantData* treedata; public: TreeSpecies(String^ genus, String^ species) { String^ botanical_name = gcnew String(genus + " " + species); // Use the Marshal class to create a pointer. // The managed class corresponding to a // pointer is IntPtr. IntPtr ip = Marshal::StringToHGlobalAnsi(botanical_name); // Cast that to the appropriate pointer type. const wchar_t* str = static_cast<const wchar_t*>(ip.ToPointer()); treedata = new PlantData(str); Marshal::FreeHGlobal( ip ); } ~TreeSpecies() { this->!TreeSpecies(); } !TreeSpecies() { if (treedata) delete treedata; } }; Don t worry too much about the details of the conversions from String to wchar_t* this is typical of the kind of type conversions you need to do when mixing managed and native code. We re simply using the Marshal class defined in the .NET Framework to create, ultimately, a
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CHAPTER 6 CLASSES AND STRUCTS
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pointer to a character array. The term marshal is a synonym for convert, although usually it suggests converting parameters from native to managed and vice versa in the context of a function call from managed to native code. However, it has come to mean converting between native and managed objects in a general sense. More information on marshaling will be discussed in 12. We include the native type PlantData as a pointer, but it would be illegal to include the native type by value. Including a pointer to the native heap creates code that cannot be verified to be safe, since the runtime has no way of knowing whether you are accessing a valid native pointer. Thus, you cannot have a native pointer or a native class in a class when compiling with /clr:safe. You can, however, use pointers when compiling with /clr:pure, because a pointer itself doesn t result in the generation of native code. The intermediate language is actually capable of representing pointers even if they are not verifiable. This is exactly what happens in C# when in an unsafe block. Finally, we include a destructor and finalizer in the type. The delete is called from the finalizer, not the destructor, and we call the finalizer from the destructor. You ll see more about this later this chapter, but in this case it s necessary to make sure that the native pointer is freed even if the destructor is never called. A more robust way to do this by writing a template class to embed the native pointer will be discussed in 12, after managed templates and some other background have been covered. Yes, it is also possible to include a managed type in a native class. To do it, you use the gcroot template in the native type, with the managed type as a template parameter (see Listing 6-14). A root is a handle that tracks a garbage-collected object. When roots exist, the object is still alive. The idea behind the name gcroot is that the pointer designates the root of a garbage-collected object on the managed heap. The gcroot template does not call the destructor on the managed object when it goes out of scope, but there is a variant, auto_gcroot, that does. Both templates are defined in the msclr namespace and require the inclusion of the appropriate header file. Listing 6-14 illustrates the basic syntax. Listing 6-14. Using a Managed Type in a Native Type // gcroot_and_auto_gcroot.cpp #include <msclr/gcroot.h> #include <msclr/auto_gcroot.h> using namespace System; using namespace msclr; // managed class R ref class R { public: void f() { Console::WriteLine("managed member function"); }
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