C++ 虛擬繼承詳解：菱形繼承與記憶體配置

菱形繼承問題

兩個基類別共同繼承同一頂層基類別，最底層派生類別會包含兩份頂層基類別子物件，造成資料重複與存取模糊。

虛擬繼承解決

中間層以 virtual public Base 繼承，確保最底層派生類別僅有一份頂層基類別。

程式碼與尺寸對比

/* test.cpp
 * Created on: 2014.04.14
 * Author: Spike
 */
#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() {
    cout << "File = " << sizeof(File) << endl;           // 40
    cout << "InputFile = " << sizeof(InputFile) << endl;   // 40
    cout << "OutputFile = " << sizeof(OutputFile) << endl;  // 40
    cout << "IOFile = " << sizeof(IOFile) << endl;         // 80 (兩份 File)
    cout << "virtual InputFile = " << sizeof(vInputFile) << endl; // 44 (File + vptr)
    cout << "virtual OutputFile = " << sizeof(vOutputFile) << endl; // 44
    cout << "virtual IOFile = " << sizeof(vIOFile) << endl;     // 48 (一份 File + 2 vptr)
}

輸出

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

記憶體配置差異

• 非虛擬：IOFile 含兩個 File 子物件（80 bytes）。
• 虛擬：vIOFile 含一個 File 子物件 + 兩個虛擬基類別指標（48 bytes）。