问题描述
在 C++ 中，Type ** 到 Type const ** 的转换是被禁止的.此外，不允许将 derived ** 转换为 Base **.
In C++, Type ** to Type const ** conversion is forbidden. Also, conversion from derived ** to Base ** is not allowed.
为什么这些转换是 wtong ?还有其他不能发生指针转换的例子吗?
Why are these conversions wtong ? Are there other examples where pointer to pointer conversion cannot happen ?
有没有办法解决:如何将指向 Type 类型的非常量对象的指针转换为指向 Type 类型的 const 对象的指针code>，因为 Type ** --> Type const ** 不成功?
Is there a way to work around: how to convert a pointer to pointer to a non-const object of type Type to a pointer to pointer to const object of type Type, since Type ** --> Type const ** does not make it ?
推荐答案
Type * to const Type* 是允许的:
Type t;
Type *p = &t;
const Type*q = p;
*p可以通过p修改，但不能通过q修改.
*p can be modified through p but not through q.
如果 Type ** 到 const Type** 转换被允许，我们可能有
If Type ** to const Type** conversion were allowed, we may have
const Type t_const;
Type* p;
Type** ptrtop = &p;
const Type** constp = ptrtop ; // this is not allowed
*constp = t_const; // then p points to t_const, right ?
p->mutate(); // with mutate a mutator,
// that can be called on pointer to non-const p
最后一行可能会改变 const t_const ！
The last line may alter const t_const !
derived ** 到 Base ** 的转换，Derived1 和 Derived2 类型派生时会出现问题来自相同的 Base.那么，
For the derived ** to Base ** conversion, problem occurs when Derived1 and Derived2 types derive from same Base. Then,
Derived1 d1;
Derived1* ptrtod1 = &d1;
Derived1** ptrtoptrtod1 = &ptrtod1 ;
Derived2 d2;
Derived2* ptrtod2 = &d2;
Base** ptrtoptrtobase = ptrtoptrtod1 ;
*ptrtoptrtobase = ptrtod2 ;
并且一个 Derived1 * 指向一个 Derived2.
and a Derived1 * points to a Derived2.
将 Type ** 设为指向 const 的指针的正确方法是将其设为 Type const* const*.
Right way to make a Type ** a pointer to pointer to a const is to make it a Type const* const*.
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