c++ - union 行为

Question

是否有可能从 union 中获得以下行为？

class A : public Base //Base is an abstract base class.
{
  public:
    A( int );
    virtual void foo() //A virtual function from Base.
    {
      //I don't want to have to specify if it's oneOption or twoOption.
      // I would like it to just call whichever one's is defined in mData.
      mData.foo();
    }
  private:
    union B
    {
      C oneOption; //A class which inherits from Base.
      D twoOption; //Another class which inherits from Base.
    } mData;
};

基本上，我想要一个包含派生类 union 的类。然后我想根据我的 Union 实现所有虚函数基类。

如果它太令人困惑，我可以尝试重新措辞。

谢谢

Answer 1

union 成员不允许有构造函数。

你可以这样做:

void PickC() { new (oneOption) C; }
void PickD() { new (twoOption) D; }
C * GetC() { return (C*) oneOption; }
D * GetD() { return (D*) twoOption; }

private:

union B
{
  char oneOption[ sizeof(C) ]; //A class which inherits from Base.
  char twoOption[ sizeof(D) ]; //Another class which inherits from Base.
} mData;

但它真的很丑。