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POCalling overridden method of derived native class through wrappers (derived Managed Classes) in C++/CLI
 

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  1. POCalling overridden method of derived native class through wrappers (derived Managed Classes) in C++/CLI
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    <p>I have a tale of 6 classes: 3 managed and 3 native.</p> <p>The 3 managed classes are ManagedChildA, ManagedChildB and ManagedParent. </p> <p>ManagedChildA, ManagedChildB both inherit from ManagedParentA.</p> <p>The 3 nativeclasses are NativeChildA, NativeChildB and NativeParent. </p> <p>NativeChildA, NativeChildB both inherit from NativeParentA.</p> <p>Moreover, ManagedChildA wraps NativeChildB, ManagedChildB wraps ManagedChildB and ManagedParentA wraps NativeParentA. </p> <p>Now here the tale gows awry:</p> <p>ManagedParentA has a method called ManagedExecute() that wraps NativeParentA's NativeExecute(). When this method is called, everything runs smoothly.</p> <p>NativeChildB, ManagedChildB override ManagedExecute() to provide their own implementations, with ManagedChildA::ManagedExecute() wrapping NativeChildA::NativeExecute() and ManagedChildB::ManagedExecute() wrapping NativeChildB::NativeExecute().</p> <p>When for example the overriden ManagedExecute() of ManagedChildA is called, NativeChildA::NativeExecute() gets called albeit with a System.AccessViolation error. That is, the pointer to the original parent of NativeChildA cannot be located. </p> <p>I guess the pointer has been moved from its original address. I read on the internet, I have to pin pointers to prevent the Garbage Collector (GC) from moving memory around but I don't know what to pin, since the exception gets thrown at the native level. Any useful hints? </p> <p>Example:</p> <pre class="lang-cpp prettyprint-override"><code>//C++ -native classes class NativeFoo { public: NativeFoo(): tested(true){} virtual void execute() { std::cout &lt;&lt; "Native Foo" &lt;&lt; std::endl; } protected: bool tested; }; class NativeBarA :NativeFoo { public: NativeBarA(): NativeFoo(){} void execute() { std::cout &lt;&lt; "Native Bar A" &lt;&lt; std::endl; } }; class NativeBarB : public NativeFoo { public: NativeBarB() :NativeFoo(){} void execute() { std::cout &lt;&lt; "Native Bar B" &lt;&lt; std::endl; } }; //CLI interface public interface class IExecutable { public: Execute(); } //C++-CLI classes public ref class ManagedFoo: public IExecutable { private: NativeFoo* impl; public: ManagedFoo(): impl(NULL) { impl = new NativeFoo(); } void __clrcall Execute() { impl-&gt;execute(); } }; public ref class ManagedBarA: public ManagedFoo { private: NativeBarA* impl; public: ManagedBarA(): ManagedFoo(), impl(NULL) { impl = new NativeBarA(); } void __clrcall Execute() override { impl-&gt;execute(); } }; public ref class ManagedBarB: public ManagedFoo { private: NativeBarB* impl; public: ManagedBarB(): ManagedFoo(), impl(NULL) { impl = new NativeBarB(); } void __clrcall Execute() override { impl-&gt;execute(); } }; //Calling code [STAThread] static void Main() { ManagedFoo^ mfoo = gcnew ManagedFoo(); ManagedBarA mbarA = gcnew ManagedBarA(); ManagedBarB mbarB = gcnew ManagedBarB(); mfoo-&gt;Execute(); //OK mbarA-&gt;Execute(); //Error. Debugger sees value of tested as false mBarB-&gt;Execute(); //Error } </code></pre>
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    1. USreexmonkey
    1. USreexmonkey
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      1. VTUpMod
    1. COPointers to native classes aren't moved, that's not the problem. Could you show us short code that exhibits this problem?
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      1. USsvick
    2. COHiding base class member variables by adding a member variable with the same name in the derived class is a TERRIBLE idea. What you probably want is a getter function that casts the base class member to the derived type.
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      1. USBen Voigt
    3. COTrue Ben, but what if I want to work only through interfaces? That is, instead of: mbarA->Execute(), I want to use iexec->Execute(), where iexec is an instance variable of any class implementing IExecutable?
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      1. USreexmonkey
 

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