C ++：可以多态复制构造函数工作？ [英] C++: Could Polymorphic Copy Constructors work?

问题描述

请考虑：

class A
{
public:
    A( int val ) : m_ValA( val ) {}
    A( const A& rhs ) {}
    int m_ValA;
};

class B : public A
{
public:
    B( int val4A, int val4B ) : A( val4A ), m_ValB( val4B ) {}
    B( const B& rhs ) : A( rhs ), m_ValB( rhs.m_ValB ) {}
    int m_ValB;
};

int main()
{
    A* b1 = new B( 1, 2 );
    A* b2 = new A( *b1 ); // ERROR...but what if it could work?
    return 0;
}

如果new A（ b1）能够解析为创建新的B副本并返回A

Would C++ be broken if "new A( b1 )" was able to resolve to creating a new B copy and returning an A?

这会有用吗？

推荐答案

你需要这个功能，还是只是一个思想实验？

Do you need this functionality, or is this just a thought experiment?

如果你需要这样做，常见的成语是有一个 Clone 方法：

If you need to do this, the common idiom is to have a Clone method:

class A
{
public:
    A( int val ) : m_ValA( val ) {}
    A( const A& rhs ) {}
    virtual A *Clone () = 0;
    int m_ValA;
};

class B : public A
{
public:
    B( int val4A, int val4B ) : A( val4A ), m_ValB( val4B ) {}
    B( const B& rhs ) : A( rhs ), m_ValB( rhs.m_ValB ) {}
    A *Clone() { return new B(*this); }
    int m_ValB;
};

int main()
{
    A* b1 = new B( 1, 2 );
    A* b2 = b1->Clone();
    return 0;
}

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