Felix Petriconi Felix Petriconi - 2 months ago 6
C++ Question

How to implement an abstract method when abstract class is used in a variadic context

How to implement in the following code the abstract base class in a generic case. The code is simplified from a library I am working on. So an explicit implementation for int and double is not an option.

template <typename T>
struct Foo
{
virtual void send(T t) = 0;
};

template <typename...T>
struct Bar : Foo<T>...
{
void send(T t) override { // does not compile because
// abstract method not implemented
}
};

int main() {
// example usage
Bar<int, double> b;

b.send(1);
b.send(2.3);
}


Many thanks in advance.

Edit: Added virtual to abstract method.

Answer

What about the following example?

First of all, I think you need define virtual the send() method in Foo (if you want it pure virtual).

Next, you can declare a intermediate template class (Foo2) where implement the override send()

Last, you can use a template send() method in Bar to select the correct virtual send() method.

#include <iostream>

template <typename T>
struct Foo
 { virtual void send(T t) = 0; };

template <typename T>
struct Foo2 : Foo<T>
 {
   void  send(T) override
    { std::cout << "sizeof[" << sizeof(T) << "] " << std::endl; }
 };

template <typename...T>
struct Bar : Foo2<T>...
 {
   template <typename U>
   void send (U u)
    { Foo2<U>::send(u); }
 };

int main()
 {
   Bar<int, double> b;

   b.send(1);    // print sizeof[4]
   b.send(2.3);  // print sizeof[8]
 }