非标量类型请求

问题描述

可以有人请帮我错误

conversion from `A' to non-scalar type `B' requested

我有类A，派生自它B，但我对这些行有问题：

I have class A and derived from it B, but I have problems with these rows:

A a(1);
A *pb = new B(a);
B b = *pb;    //here I have an error

预先感谢任何帮助

class A {
protected:
    int player;
public:
    A(int initPlayer = 0);
    A(const A&);
    A& operator=(const A&);
    virtual ~A(){};
    virtual void foo();
    void foo() const;
    operator int();
};

class B: public A {
public:
    B(int initPlayer): A(initPlayer){};
    ~B(){};
    virtual void foo();
};

编辑

（我不能改变它）：

edited

I have this code and (I can't change it):

A a(1);
A *pb = new B(a);
B b = *pb;

我试图为B创建构造函数：

I tried to create constructor for B:

B::B(const A & a):
    player(a.player){}

B& B::operator=(const A& a){
    if(this == &a){
        return *this;
    }
    player = a.player;
    return *this;
}

但它给我一个错误，真的需要专业人士的帮助

but it gives me an error, really need help from professionals

推荐答案

您的问题是由于静态类型检查。当你有这条线：

Your problem is due to static type checking. When you have this line:

A *pb = new B(a);

pb 的静态类型为 A * ，它的动态类型是 B * 。虽然动态类型是正确的，编译器正在检查静态类型。

The static type of pb is A * and it's dynamic type is B *. While the dynamic type is correct, the compiler is checking the static type.

对于这个简单的代码，因为你知道 pb 总是一个B，你可以使用静态转型修复这个问题：

For this simple code, since you know the dynamic type of pb is always a B, you can fix this with a static cast:

B b = *static_cast<B *>(pb);

但是请注意，如果动态类型 pb 是 A * ，会导致未定义的行为。

But be warned that if the dynamic type of pb was an A * the cast would cause undefined behavior.

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