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CHAPTER 4 OBJECT S EMAN TICS IN C+ +/CLI
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class PlantData { public: static void PlantQuery(char* search, Recordset** records) { DBConnection connection; connection.Query( search, records); } // destructor for connection called }; A bit of a philosophical perspective is in order here. The stack and the heap have a historical origin in terms of how programming languages and memory models were implemented and evolved. There are significant life cycle differences between stack and heap objects. Stack objects are often short-lived and are freed up at the end of the block in which they are declared. They are fundamentally local variables. Heap objects could live for a lot longer and are not tied to any particular function scope. The design of C++/CLI is shaped by the idea that the notion of the semantics of a stack variable or a heap variable can be separated from the actual implementation of a given variable as actual memory on the stack or heap. Another way of looking at it is that because we have reference types that cannot live on the stack, we d like a way to have our cake and eat it, too. We d like reference types with the semantics of stack variables. With this in mind, consider the managed version of the preceding example. If you went ahead and implemented the native classes DBConnection and PlantData as managed types using a literal transliteration of the code, your code would look something like Listing 4-6. Listing 4-6. Accessing the Botany Database with Managed Classes // ManagedPlantQuery.cpp using namespace System; ref class Recordset; ref class DBConnection { public: DBConnection() { // Open the connection. // ... } Recordset^ Query(String^ search) { // Query the database, generate recordset, // and return handle to recordset. // ... }
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CHAPTER 4 OBJECT SE MAN TICS IN C++ /CLI
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~DBConnection() { // Close the connection. // ... } }; ref class PlantData { public: static Recordset^ PlantQuery(String^ search) { DBConnection^ connection = gcnew DBConnection(); return connection->Query( search ); } }; If you were to use this code in production, you would run into a problem in that the large botany database with the limited number of connections frequently runs out of available connections, so people have trouble accessing the database. Depending on the database and data access implementation, this could mean connections are refused, or a significant delay enters the system as data access code is blocked awaiting a connection. And all this because the destruction of managed objects happens not when the function exits, but only when the garbage collector feels like cleaning them up. In fact, you will find that the destructor never gets called at all in the preceding code even when the object is finally cleaned up. Instead, something called the finalizer gets called by the garbage collector to take care of the cleanup, if one exists. You ll learn more about that in 6. The ability to control when a variable goes out of scope and is destroyed is clearly necessary. Objects that open database connections or block a communication channel such as a socket should free up these resources as soon as they re no longer needed. For native C++ programmers, the solution to this problem might be to create the variable on the stack and be assured that its destructor, which frees up the resources, would be called at the end of the function. What can be done in the managed environment, when reference types cannot be created on the stack at all There are several ways of solving the problem. In the code for Listing 4-6, for example, we could have inserted an explicit delete, as in Listing 4-7. Listing 4-7. Using an Explicit Delete static Recordset^ PlantQuery(String^ search) { DBConnection^ connection = gcnew DBConnection(); Recordset^ records = connection->Query( search ); delete connection; return records; }
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