C++ 定义到基类的转换

Question

我很难为基类定义和使用转换运算符。请考虑以下事项:

class base 
{
public:
    base(const base&);
    base& operator=(const base&);
    //some stuff
};
class child : public base
{
public:
    operator base() const;
    //some more stuff
};

int main()
{
    child c;
    base b=c;
    b=c;
}

如果我尝试将子项转换为基数，则永远不会调用 operator base()(即使我进行了显式转换)。相反，直接调用基类的复制构造函数或赋值运算符，而不进行转换。

当 child 被分配(或复制构造)到 base 时，如何调用 operator base()？

Answer 1

基类的转换函数在语法上是合法的，但永远不会被调用。来自标准草案 n3337:

12.3.2 Conversion functions [class.conv.fct] §1

[...] A conversion function is never used to convert a (possibly cv-qualified) object to the (possibly cv-qualified) same object type (or a reference to it), to a (possibly cv-qualified) base class of that type (or a reference to it), or to (possibly cv-qualified) void. [...]

该语言具有不同的从派生到基础的转换机制。了解 object slicing .简而言之 - 它会自动为您完成。