Policy class design but without making the whole user class a template

Question

Consider the following code where the Writer_I acts as an interface. Other classes which fulfil the contract of writing element types in correct form can derive

Answer 1

here are most of the errors gone except for line 41 #include #include

enum class Elem {
HEADER,
FOOTER,
};

template  class Writer_I {
public:
template  decltype(auto) write(T &&...args) {
    return static_cast(this)->template write(
        std::forward(args)...);
}
virtual ~Writer_I() {}
};

class Streams : public Writer_I {
public:
template >
void write(int a) {
    std::cout << a << std::endl;
}
template >
void write(float a) {
    std::cout << "\n-------\n" << a << std::endl;
}
};

/* Restrictions being that member functions header and footer
remain in cpp files. And creators of Calculator
can specify alternative implementations. */
class Calculator {
std::_Any_tag writer;

public:
template 
Calculator(Writer_I& writer) : writer(writer) {}

template  void write_i(T &&...args) {
    /*  */ writer(std::forward(args)); 
}

void header() {
    for (int i = 0; i < 10; i++) {
        write_i(i);
    }
}

void footer() {
    write_i(-100.0f);
}
};
int main() {
Streams streams;
Calculator calc(streams);
calc.header();
return 0;
}