C++核心准则C.138:使用using为派生类生成重载函数集合‍

#include <iostream>
class B {
public:
    virtual int f(int i) {
        std::cout << "f(int): "; return i; 
    }
    virtual double f(double d) {
        std::cout << "f(double): "; return d; 
    }
    virtual ~B() = default;
};

class D: public B {
public:
    int f(int i) override 
    {
        std::cout << "f(int): "; return i + 1; 
    }
};

int main()
{
    D d;
    std::cout << d.f(2) << '\n';   // prints "f(int): 3"
    std::cout << d.f(2.3) << '\n'; // prints "f(int): 3"
}

class D: public B {
public:
    int f(int i) override 
    {
        std::cout << "f(int): "; return i + 1; 
    }
    using B::f; // exposes f(double)
};

template <class... Ts>
struct Overloader : Ts... {
    using Ts::operator()...; // exposes operator() from every base
};