c++ - 为什么 C++ 不支持跨作用域重载？

Question

我相信这里已经给出了最佳答案:Why does an overridden function in the derived class hide other overloads of the base class?

但我有点困惑，特别是声明:

In order to override this behavior, an explicit action is required from the user: originally a redeclaration of inherited method(s) (currently deprecated), now an explicit use of using-declaration.

假设我有以下程序:

#include <iostream>
using namespace std;
class Base
{
public:
    int f(int i)
    {
        cout << "f(int): ";
        return i+3;
    }
};
class Derived : public Base
{
public:
    double f(double d)
    {
        cout << "f(double): ";
        return d+3.3;
    }
};
int main()
{
    Derived* dp = new Derived;
    cout << dp->f(3) << '\n';
    cout << dp->f(3.3) << '\n';
    delete dp;
    return 0;
}

我有两个问题:

1. 我可以假设，对于派生类对象，int f(int i) 函数根本不存在。这不是因为名字隐藏而继承的。

2. 如果我必须在派生类中使用这个函数，我必须在派生类中重新定义它吗？

Answer 1

1. Can I assume, w.r.t derived class object, the int f(int i) function does not exist at all. This is not inherited because of name hiding.

它是继承的，它只是...隐藏，如果不指定范围(unqualified name lookup)是找不到的。你可以指定它显式地使用范围解析运算符 :: ( qualified name lookup ) :

dp->Base::f(3);

2. If I have to use this function in Derived class, I have to define it again in derived class?

正如引用的答案所说，您可以通过“明确使用 using-declaration”来实现。

class Derived : public Base
{
public:
    using Base::f;
    ...
};

编辑(来自评论的补充问题)

3. If it's name hidden, that means I can declare it again ? Same name, same parameters?

是的，你可以。它仍然是名字隐藏。

4. If yes, what if I also added using Base::f along with the new declaration? Will it result in double definition?

不，这不是双重定义。 Using declaration只是将名称引入派生类范围。而在派生类中声明的成员函数会隐藏从基类中引入的成员函数，仍然是名称隐藏。 (请注意，您仍然可以通过 dp->Base::f(3); 调用基类之一。)

If the derived class already has a member with the same name, parameter list, and qualifications, the derived class member hides or overrides (doesn't conflict with) the member that is introduced from the base class.