派生类和基类之间指针到指针的转换?

2021-12-13 00:00:00 inheritance pointers c++ subtyping

关于以下 C++ 程序:

Regarding the following C++ program:

class Base { };

class Child : public Base { };

int main()
{   
    // Normal: using child as base is allowed
    Child *c = new Child();
    Base *b = c;

    // Double pointers: apparently can't use Child** as Base**
    Child **cc = &c;
    Base **bb = cc;

    return 0;
}

GCC 在最后的赋值语句上产生以下错误:

GCC produces the following error on the last assignment statement:

error: invalid conversion from ‘Child**’ to ‘Base**’

我的问题分为两部分:

  1. 为什么没有从 Child** 到 Base** 的隐式转换?
  2. 我可以使用 C 样式转换或 reinterpret_cast 使这个示例工作.使用这些强制转换意味着抛弃所有类型安全.有什么我可以添加到类定义中以使这些指针隐式转换的，或者至少以一种允许我使用 static_cast 的方式来表达转换?
  1. Why is there no implicit conversion from Child** to Base**?
  2. I can make this example work with a C-style cast or a reinterpret_cast. Using these casts means throwing away all type safety. Is there anything I can add to the class definitions to make these pointers cast implicitly, or at least phrase the conversion in a way that allows me to use static_cast instead?

推荐答案

如果允许，你可以这样写:

If this was allowed, you could write this:

*bb = new Base;

而且c 最终会指向Base 的一个实例.不好.

And c would end up pointing to an instance of Base. Bad.

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