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CHAPTER 6 CLASSES AND STRUCTS
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Listing 6-15. Using a Destructor and Finalizer // finalizer.cpp using namespace System; ref class R { int ID; public: R(int id) : ID(id) { Console::WriteLine("R constructor {0}", ID); } ~R() { Console::WriteLine("R destructor {0}", ID); } !R() { Console::WriteLine("R finalizer {0}", ID); } }; void MakeObjects() { R^ r; R r1(0); for (int i = 1; i < 7; i++) { r = gcnew R(i); } } int main() { MakeObjects(); // Normally, you should avoid calling GC::Collect and forcing garbage // collection rather than letting the garbage collection thread determine // the best time to collect; I do it here to illustrate a point. GC::Collect(); } Here is the output of Listing 6-15: R R R R R R R R R R constructor 0 constructor 1 constructor 2 constructor 3 constructor 4 constructor 5 constructor 6 destructor 0 finalizer 5 finalizer 6
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finalizer finalizer finalizer finalizer
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You ll notice that the destructor only got called once, and the finalizer got called six times. The destructor was for the object created in MakeObjects with stack semantics when the object went out of scope. The destructor is not called for a handle type that is not explicitly deleted. The finalizer was called when the garbage collection ran (which in this case was forced by calling GC::Collect). If you have a finalizer that does something important, you ll want your destructor to call your finalizer to make sure that the cleanup operations occur promptly rather than waiting until a garbage collection cycle occurs. A destructor call suppresses the finalizer. Now try removing the call to GC::Collect and rerunning the program. The finalizer is still called six times even though the process may have shut down. Finalizers will be run when the process ends. Finalizers are not to be used routinely; in fact, if you can avoid them, you should. A possible use is for the last-ditch cleanup of unmanaged resources in cases where you can t be sure whether the destructor is called. Examples of unmanaged resources are native file handles, device contexts, and so on. However, the .NET Framework provides wrapper classes for most of these unmanaged resources, for example, the HWnd class and the SafeHandle family of classes. When using the wrapper classes, the wrapper classes will take care of their own cleanup. Finalizers are particularly difficult to write correctly, because when they execute, their members may be disposed, in the process of finalization, or already finalized themselves. Also, to be truly robust, they need to correctly handle various rare circumstances, such as being called more than once. When the runtime invokes a finalizer, other threads are locked out automatically, so there is no need to acquire a lock within the finalizer itself. If a finalizer is implemented, you should have a destructor, and you should recommend that users of your class call that destructor, because it is very inefficient to rely on finalization to perform the cleanup operations. The basic pattern is shown in Listing 6-16. Listing 6-16. Pattern for Using a Destructor and Finalizer // destructor_and_finalizer.cpp ref class ManagedResource { public: void Free() { /* free resource */ } }; class NativeResource { public: void Free() { /* free resource */ } };
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ref class R { ManagedResource^ resource1; NativeResource* nativeResource; public: ~R() { // You may clean up managed resources that you want to free up promptly // here. If you don't, they WILL eventually get cleaned up by the garbage // collector. // If the destructor is NOT called, the GC will eventually clean // them up. resource1->Free(); this->!R(); } !R() { // Clean up unmanaged resources that the // garbage collector doesn't know how to clean up. // That code shouldn't be in the destructor because // the destructor might not get called. nativeResource->Free(); } }; You might guess from what I ve just said about the destructor suppressing the finalizer that the finalizer doesn t get called directly for objects created with stack semantics. When objects with stack semantics are destroyed at the end of a function scope, the destructor is called, but not the finalizer. Code that frees the resources should be written in the finalizer, and the destructor should call the finalizer. That way, you know your cleanup will be called regardless of whether the destructor is called or not. If it is called, the cleanup executes because the destructor calls the finalizer, and the finalizer cleans up. If it is not called, the finalizer eventually is called by the garbage collector or application shutdown process, that is, when the application domain (the CLR term for the entire space that all the application s names exist in) shuts down. In Listing 6-17, one file is opened using a native file handle, an unmanaged resource. Another file is opened using the StreamWriter class. Listing 6-17. Handling Managed and Unmanaged Resources // file_converter.cpp #include <stdio.h> #include <string.h> #include <stdlib.h> #include <vcclr.h> // for PtrToStringChars
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