英文:
Why is this cross-cast not allowed?
问题
我得到以下错误消息：
错误: 无法使用 static_cast 从 'Base1 *' 转换到 'Base2 *'，它们之间没有继承关系
编译器应该有足够的信息来确定可以从 Derived 到达 Base2，而不需要使用 RTTI（dynamic_cast），并让我这样做：
Derived* foo3 = static_cast<Derived*>(foo1);
Base2* foo2 = foo3;
为什么不允许这样做呢？（可以争论编译器不知道 foo1 是否为 Derived 类型，但即使从例如 Base1 转换到 Derived 时，static_cast 也不会检查类型。）
英文:
Consider this simple example:
struct Base1 {};
struct Base2 {};
struct Derived : public Base1, public Base2 {};
int main()
{
Derived foo;
Base1* foo1 = &foo;
Base2* foo2 = static_cast<Base2*>(foo1);
}
I get:
Error: static_cast from 'Base1 *' to 'Base2 *', which are not related by inheritance, is not allowed
The compiler should have enough information to figure out that Base2 can be reached from Derived without RTTI (dynamic_cast) and having me to do:
Derived* foo3 = static_cast<Derived*>(foo1);
Base2* foo2 = foo3;
Why isn't this allowed? (One could argue that the compiler doesn't know if foo1 is of Derived type, but static_cast doesn't check the type anyway even when converting from Base1 to Derived for example)
Note: This question is similiar to mine, but it's not quite the same because here we are cross-casting base classes, not derived ones
答案1
得分: 3
static_cast 会失败，因为非正式地说，Base1 和 Base2 没有关联。
然而，如果你的类是多态的，dynamic_cast 将起作用：你可以通过添加虚拟析构函数来实现：
struct Base1 {virtual ~Base1() = default;};
struct Base2 {virtual ~Base2() = default;};
struct Derived : Base1, Base2 {};
int main()
{
Derived foo;
Base1* foo1 = &foo;
Base2* foo2 = dynamic_cast<Base2*>(foo1);
}
在处理组合（即接口）时，从 Base1 到 Base2 的这种转型是惯用的。
英文:
A static_cast will fail since, informally speaking, Base1 and Base2 are not related.
However a dynamic_cast will work if your classes are polymorphic: which you can achieve by adding virtual destructors:
struct Base1 {virtual ~Base1() = default;};
struct Base2 {virtual ~Base2() = default;};
struct Derived : Base1, Base2 {};
int main()
{
Derived foo;
Base1* foo1 = &foo;
Base2* foo2 = dynamic_cast<Base2*>(foo1);
}
This casting from Base1 to Base2 is idiomatic when working with composition (i.e. interfaces).
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