派生类和基类之间的指针到指针的转换？ [英] Conversion of pointer-to-pointer between derived and base classes?

问题描述

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在最后一个赋值语句中产生以下错误：

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. 为什么没有隐式转换Child ** to Base **？


2. 我可以使用C风格的转换或 reinterpret_cast 使用这些铸件意味着丢弃所有类型的安全。有没有什么我可以添加到类定义，使这些指针隐式转换，或至少短语的转换，允许我使用 static_cast ？

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，您可以这样写：

If this was allowed, you could write this:

*bb = new Base;

并且 c Base 的实例。错误。

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