multiple-inheritance | 易学教程

PyQt5: Issues with a Mixin with a metaclass within a derived QWidget class

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问题 I'm having issues trying to add a mixin with a metaclass to a class whose base is a QWidget. I'm aware that PyQt5 supports cooperative multiple inheritance and if my MixIn class has no metaclass then things work fine. However, if it has a metaclass - whether it be the pyqtWrapperType metaclass shared by QWidgets or a derived metaclass, then I receive the following error: Process finished with exit code -1073741819 (0xC0000005) The code for the rest of the script runs, but the QWidget does not

Why is this the same even when object pointers differ in multiple inheritance?

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问题 When using multiple inheritance C++ has to maintain several vtables which leads to having "several views" of common base classes. Here's a code snippet: #include "stdafx.h" #include <Windows.h> void dumpPointer( void* pointer ) { __int64 thisPointer = reinterpret_cast<__int64>( pointer ); char buffer[100]; _i64toa( thisPointer, buffer, 10 ); OutputDebugStringA( buffer ); OutputDebugStringA( "\n" ); } class ICommonBase { public: virtual void Common() = 0 {} }; class IDerived1 : public

C++: Memory layout of classes using inheritance

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问题 I know how data will be packed is not specified by the standard. I was just trying to get an idea about the memory layout of classes ( esp. how dynamic_cast<void*> guarantees to return a pointer to the start of the most derived class). I could not think of any explanation about the output of the following code: struct A{ int a;}; struct B{ int b;}; struct C: public A, public B { int c;}; struct D:public C {int d;}; int main(){ D* ob=new D; A* a = ob; B* b = ob; C* c = ob; } Printing the

Why do IUnknown* pointers retrieved through different interfaces of the same COM object have the same value?

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问题 I have the following hierarchy of COM interfaces and a class implementing them: interface IX : public IUnknown{}; interface IY : public IUnknown{}; class CA: public IX, public IY{}; Here class CA effectively inherits from IUnknown twice. We know there are two vtable pointers in class CA - one is pointed to IX and another is pointed to IY . So IUnknown stored in the IX subobject is different from IUnknown stored in the IY subobject. Yet when we call IX::QueryInterface() or IY::QueryInterface()

Why the size of empty class that is derived from two empty classes is 2?

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问题 What I know is that size of empty class is 1 just to conform to the standard that does not allow objects (and classes thereof) of size to be 0. Here, I am getting size of a derived class D as 2. Why behavior is different in this case given that there is no data member or virtual pointer inherited from class B and C to D? #include<iostream> using namespace std; class A {}; class B : public A {}; class C : public A {}; class D : public B, public C {}; int main() { cout<<"Size of D is: "<<sizeof

Choosing which base class to override method of

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问题 Given the following: class Observer { public: virtual void Observe(Parameter p) = 0; }; template<size_t Tag> class TaggedObserver : public Observer { }; class Thing : public TaggedObserver<0>, TaggedObserver<1> { public: virtual void Observe(Parameter p) override; }; Thing::Observe overrides both TaggedObserver<0>::Observe and TaggedObserver<1>::Observe . Is there a way to provide a different override for each base class? Rationale: I want the class to be able to observe two notification

What is the proper approach to swap and copy idiom in virtual inheritance?

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问题 Consider classic virtual inheritance diamond hierarchy. I wonder to know what is the right implementation of copy and swap idiom in such hierarchy. The example is a little artificial - and it is not very smart - as it would play good with default copy semantic for A,B,D classes. But just to illustrate the problem - please forget about the example weaknesses and provide the solution. So I have class D derived from 2 base classes (B<1>,B<2>) - each of B classes inherits virtually from A class.

Implement two functions with the same name but different, non-covariant return types due to multiple abstract base classes

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问题 If I have two abstract classes defining a pure virtual function with the same name, but different, non-covariant return types, how can I derive from these and define an implementation for both their functions? #include <iostream> class ITestA { public: virtual ~ITestA() {}; virtual float test() =0; }; class ITestB { public: virtual ~ITestB() {}; virtual bool test() =0; }; class C : public ITestA, public ITestB { public: /* Somehow implement ITestA::test and ITestB::test */ }; int main() {

Is it better to use `#ifdef` or inheritance for cross-compiling?

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问题 To follow from my previous question about virtual and multiple inheritance (in a cross platform scenario) - after reading some answers, it has occurred to me that I could simplify my model by keeping the server and client classes, and replacing the platform specific classes with #ifdefs (which is what I was going to do originally). Will using this code be simpler? It'd mean there'd be less files at least! The downside is that it creates a somewhat "ugly" and slightly harder to read Foobar

python multi inheritance with parent classes have different init()

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Here both B and C are derived from A , but with different __init__() parameters. My question is how to write the correct/elegant code here to initialize self.a,self.b,self.c1,self.c2 in the following example? Maybe another question is--is it a good coding practice to do this variable setting in __init()__ function or it is better to use simpler __init__() function, and do set() function for each class later, which seems not as simple as to just do it in __init()__ ? class A(object): __init__(self,a): self.a=a class B(A): __init__(self,a,b): super(B,self).__init__(a) self.b=b class C(A): __init