c++ - static_cast - 为什么它在这里工作？

Question

我有以下代码片段

class base
{
public:
    virtual void MyMethod()
    {
        std::cout << "Base MyMethod" << "\n";
    }
};


class der : public base
{
public:
    virtual void MyMethod()
    {
        std::cout << "Derived MyMethod" << "\n";
    }
};


void foo(base* b)
{
    der* d = static_cast<der*>(b);
    d->MyMethod();
}

int main()
{
    base* b = new der();
    foo(b);
}

现在我的问题是为什么 static_Cast 在这里工作。我读到 static_casts 不能通过多态类型进行转换。那么为什么上面的例子有效 - 我在这里遗漏了什么吗？我期待动态转换只能解决这样的问题，因为它们应该与多态类型一起使用？谁能举例说明静态转换会失败而动态转换会通过？

Answer 1

“Now my question is why is static_cast working here.”

没有理由不工作。类型通过类派生相关，编译器知道这一点。本质上，static_cast 仅限于执行或撤消任何隐式转换，并且您确实有从 der* 到 base* 的隐式转换。

“I read that static_casts cannot cast through polymorphic types.”

那只是胡说八道。

“[snip] Could anyone give an example where static cast would fail and dynamic cast would pass?”

struct A { virtual ~A(){} };
struct B { virtual ~B(){} };

struct T: A, B {};

auto main()
    -> int
{
    T o;
    A& oA = o;
    //B& oB = static_cast<B&>( oA );    //! This won't compile, unrelated types.
    B& oB = dynamic_cast<B&>( oA );
}