多重和虚拟继承的类型转换

Question

为什么虚拟继承编译

static_pointer_cast

时出错？

#include <iostream>
#include <memory>

struct Top {
    int a = 1;
};

struct Left : public Top {
    int b = 2;
};

struct Right : public Top {
    int c = 3;
};

struct Bottom : public Left, public Right {
    int d = 4;
};

int main() {
    std::shared_ptr<Right> pright = std::make_shared<Bottom>();
    std::shared_ptr<Top> ptop = pright;
    auto p = std::static_pointer_cast<Right>(ptop); // work well
    std::cout << "c: " << p->c << std::endl;
}

链接

static_pointer_cast

适用于多重继承。

#include <iostream>
#include <memory>

struct Top {
    int a = 1;
};

struct Left : virtual public Top {
    int b = 2;
};

struct Right : virtual public Top {
    int c = 3;
};

struct Bottom : public Left, public Right {
    int d = 4;
};

int main() {
    std::shared_ptr<Right> pright = std::make_shared<Bottom>();
    std::shared_ptr<Top> ptop = pright;
    auto p = std::static_pointer_cast<Right>(ptop); // compile error!
    std::cout << "c: " << p->c << std::endl;
}

链接

为什么虚拟继承编译

static_pointer_cast

时出错？