[英]static_cast and reference to pointers
Can anyone tell me why this doesn't compile: 任何人都可以告诉我为什么这不编译:
struct A { };
struct B : public A { };
int main()
{
B b;
A* a = &b;
B* &b1 = static_cast<B*&>(a);
return 0;
}
Now, if you replace the static cast with: 现在,如果你用以下方法替换静态强制转换:
B* b1 = static_cast<B*>(a);
then it does compile. 然后它确实编译。
Edit: It is obvious that the compiler treats A*
and B*
as independent types, otherwise this would work. 编辑:很明显,编译器将
A*
和B*
视为独立类型,否则这将起作用。 The question is more about why is that desirable? 问题更多的是为什么这是可取的?
B
is derived from A
, but B*
isn't derived from A*
. B
来自A
,但B*
不是来自A*
。 A pointer to a B
is not a pointer to an A
, it can only be converted to one. 指向
B
的指针不是指向A
的指针,它只能转换为一个指针。 But the types remain distinct (and the conversion can, and often will, change the value of the pointer). 但是类型仍然不同(转换可以,而且经常会改变指针的值)。 A
B*&
can only refer to a B*
, not to any other pointer type. A
B*&
只能引用B*
,而不能引用任何其他指针类型。
非常量左值引用(B *&)不能绑定到不相关的类型(A *)。
You are trying to cast an A*
to a B*
. 您正在尝试将
A*
转换为B*
。 This is the wrong way around and not very useful. 这是错误的方式,并不是很有用。 You probably want to store a pointer to derived in a pointer to base, which is useful and doesn't even need a cast.
您可能希望将指针存储在指向base的指针中,这很有用,甚至不需要强制转换。
I suppose a dynamic_cast
might work here, but the result is implementation defined if I'm not mistaken. 我想动态
dynamic_cast
可能在这里工作,但如果我没有弄错的话,结果就是定义的实现。
Handling of references is something the compiler does for you, there should be no need to cast to reference. 引用的处理是编译器为您做的事情,不需要转换为引用。
If we refactor the code to: 如果我们将代码重构为:
B b;
A* a = &b;
B* b_ptr = static_cast<B*>(a);
B*& p1 = b_ptr;
It will compile. 它会编译。
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