在CPP中，是允許一個基類通過不同的途徑重複繼承同一個祖先類的。這時祖先類裡面的成員就被拷貝多份，當外界想呼叫的時候編譯器無法決定使用哪一個副本，因而會報錯。
為了避免這種情況，CPP中產生了虛基類——Virtual base。

使用方法：在繼承訪問控制之前加上保留字：virtual

普通基類與虛基類之間的唯一區別只有在派生類重複繼承了某一基類時才表現出來。

舉個栗子：

class BASE {public: int i;};
class BASE1: virtual public BASE {  //由於使用了virtual這裡BASE變成了BASE1的一個虛基類。
 public
 
: int j;
};
class BASE2: virtual public BASE {
 public: int k;
};
class DERIVED: public BASE1, public BASE2 {
 public: int sum;
};

int main()
{
    DERIVED obj; // 宣告一個派生類物件
    obj.i = 3; // 正確：從BASE繼承的i在DERIVED中只有一份
    obj.j = 5; // 正確：使用從BASE1繼承的j
    obj.k = 7; // 正確：使用從BASE2繼承的k
    return 0;
}

若派生類有一個虛基類作為祖先類，則在派生類建構函式中需要列出對虛基類建構函式的呼叫，即顯示呼叫

再結合之前的建構函式的呼叫順序，感受一下下面這個例子：

#include "iostream"
using namespace std;
class baseA {
 public:
    baseA() {
        cout << endl << "This is baseA class." << endl;
    }
};

class baseB {
 public:
    baseB
 
() {
        cout << endl << "This is baseB class." << endl;
    }
};

class derivedA : public baseB, virtual public baseA {
 public:
    derivedA() {
        cout << endl << "This is derivedA class." << endl;
    }
};
class derivedB : public baseB, virtual public baseA {
 public:
    derivedB() {
        cout << endl << "This is derivedB class." << endl;
    }
};


class Derived : public derivedA, virtual public derivedB {
 public:
    Derived() {
        cout << endl << "This is Derived class." << endl;
    }
};

int main() {
    Derived obj;
    return 0;
}

執行結果

This is baseA class.
This is baseB class.
This is derivedB class.
This is baseB class.
This is derivedA class.
This is Derived class.