Andrei Avram

This is a follow-up of the Executing tasks based on static polymorphism article, which I recommend to be read for the full picture of what is about to come, as it offers details on why I study this approach and how I implemented it (compile-time iteration of a tuple).

My first attempt on C++ compile-time polymorphism is designed around a task struct. The requirement for a task is to implement an Execute method that will perform some work. This requires that the task struct is mine. Otherwise, if there’s some information provided by another library through some struct, I can wrap it in a task that has the required Execute method.

Inspired by some of Sean Parent’s talks about runtime polymorphism and some of its issues, I found another way of implementing static polymorphism. One that does not have any requirement for the input structs; they don’t need to implement a method nor they must be wrapped in other structs.

Along with the requirements in the previous article, I add these ones:

• • A container with multiple objects of different types so I have a list of items
  • For each object in the container, something different must be performed depending on its type (by iterating all objects, not handling it manually)
  • Objects in the container are provided by someone else and cannot be changed
  • C++11/14 compatible

This approach starts with some input structs in a tuple (the container); references to the objects constructed of the structs to prevent copies:

namespace input {
    struct A {
        int a;
    };

    struct B {
        int b;
    };
}

using Objects = std::tuple<input::A&, input::A&, input::B&, input::A&>;

Continue reading A more decoupled approach on static polymorphism

If you want to skip reading and get to the code, then see some kind of C++ static polymorphism in the GitHub repository.

I’m a fan of tasks. I enjoy each time I implement any kind of system that executes tasks, from very simple ones (based on a list of tasks executed on a single thread) to multithreaded ones with features like cancellation and dynamic concurrency level.

My first encounter with the idea of having a list of objects that I iterate and call a function for each one of them was when I had to implement a feature that dynamically restricted users’ access to some content based on various conditions. You can think about a lot of if statements. I don’t like if statements, I struggle to avoid them because each one of them brings new responsibility, test cases, and maintenance costs. And I really enjoy seeing product and management people happy when their requirements are implemented at reasonable times. And being productive is a lot about code.

A way to decouple code is, of course, to separate concerns. And if you’re in a case with multiple concerns serving the same concept, you could use a polymorphic approach. You can have an interface for a task, some tasks implementations, and something to run all the tasks when needed. Everything you’ll see in this article is a light version because I’m focusing on the idea, not on details.

#include <array>
#include <iostream>
#include <memory>

class Task {
   public:
    virtual void Execute() = 0;
};

template <typename T>
struct Executor {
    T& tasks;

    void Execute()
    {
        for (auto& task : tasks) {
            task->Execute();
        }
    }
};

class A : public Task {
   public:
    void Execute() override { std::cout << 1; }
};

class B : public Task {
   public:
    void Execute() override { std::cout << 2; }
};

int main()
{
    using Tasks = std::array<std::unique_ptr<Task>, 2>;
    Tasks tasks{
        std::make_unique<A>(),
        std::make_unique<B>(),
    };

    Executor<Tasks> executor{tasks};
    executor.Execute();
}

Now I’ll add some constraints for learning purposes. Sometimes performance matters and you can’t always use the latest C++ standards, so I’ll go with these:

• • C++ 11
  • No heap allocations
  • Minimum stack allocations
  • No virtual methods

And I’m entering the world of templates because this can help me achieve compile-time polymorphism (C++ static polymorphism), and that’s what I’m aiming for. Continue reading Executing tasks based on static polymorphism