C++：虛繼承(virtual inheritance) 詳解

在多重繼承(multiple inheritance)中, 可能出現多個基類繼承自同一個基類, 即"菱形繼承", 會導致最頂層的基類, 被複製多次;

可以採用虛繼承(virtual inheritance)的方式, 使衍生類別(derived class)只保留最頂層基類的一個副本.

virtual, 即需要vptr(virtual table pointer), 即虛表指標, 額外佔用4個位元組;

代碼如下:

/*  * test.cpp  *  *  Created on: 2014.04.14  *      Author: Spike  */      /*eclipse cdt, gcc 4.8.1*/      #include <iostream>  #include <string>        using namespace std;        class File {private: int i[10];};        class InputFile : public File {};  class OutputFile : public File {};  class IOFile : public InputFile, public OutputFile {};        class vInputFile : virtual public File {};  class vOutputFile : virtual public File {};  class vIOFile :  public vInputFile, public vOutputFile {};        int main () {      std::cout << "File = " << sizeof(File) << std::endl;            std::cout << "InputFile = " << sizeof(InputFile) << std::endl;      std::cout << "OutputFile = " << sizeof(OutputFile) << std::endl;      std::cout << "IOFile = " << sizeof(IOFile) << std::endl;            std::cout << "virtual InputFile = " << sizeof(vInputFile) << std::endl;      std::cout << "virtual OutputFile = " << sizeof(vOutputFile) << std::endl;      std::cout << "virtual IOFile = " << sizeof(vIOFile) << std::endl;  }

輸出:

File = 40  InputFile = 40  OutputFile = 40  IOFile = 80  virtual InputFile = 44  virtual OutputFile = 44  virtual IOFile = 48

作者：csdn部落格 Spike_King

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