将模板与多态性混合

mixing templates with polymorphism(将模板与多态性混合)

问题描述

class A
{
    friend void foo();
    virtual void print_Var() const{};

};// does not contain variable Var;


template<class T>
class B : public A
{
    T Var;
public:
    B(T x):Var(x){}
    void print_Var() const override
    {
        std::cout<<Var<<std::endl;
    }
};

void foo()
{
    std::array<std::unique_ptr<A>, 3> Arr = {
            std::make_unique<B<int>>(100),
            std::make_unique<B<int>>(20),
            std::make_unique<B<std::string>>("Hello Stackoverflow")
    };
            std::shuffle(Arr.begin(), Arr.end(), std::mt19937(std::random_device()())); // 3rd parameter generated by Clang-Tidy

    for (auto &i: Arr)
    {
        i->print_Var(); // OK
      //  auto z = i->Var   // no member named Var in A
                            // obviously base class does not contain such variable

     //   if (i->Var==20) {/* do something*