Just a follow-up on Runtime polymorphism without dynamic memory allocation for the reason it’s C++ and you can do all kinds of weird things. This is a fun article. I didn’t think it through.

Last time, I wanted a clear API for the caller and I created an abstraction above std::variant. I did it by returning a lambda that the caller can simply call with the needed arguments.

Then I thought “What if I can do it even more simple?”. I’d like to have the same API as the implementation with the pointer.

object->function(argument);

But without heap allocations.

The storage remains a variant. And I return a pointer to the currently selected type in the variant.

#include <cassert>
#include <variant>

struct P {
    virtual int f(int) const = 0;
    virtual ~P() = default;
};
 
struct A : P {
    int f(int in) const override {return in + 1;}
};
 
struct B : P {
    int f(int in) const override {return in + 2;};
};

struct factory {
    std::variant<A, B> object;

    P* create(char o) {  
        switch(o) {
            case 'a': object = A{}; break;
            default: object = B{}; break;
        }

        return std::visit([](auto& obj) -> P* { return &obj; }, object);
    }
};

int main() {
    factory f{};
    assert(f.create('a')->f(1) == 2);
    assert(f.create('b')->f(1) == 3);
}

 

A particular thing is that I have to use trailing return type for the lambda visitor. This is because “std::visit requires the visitor to have the same return type for all alternatives of a variant” (Clang). So I return all objects through the base class. I’m back to virtual inheritance functions.

Besides the raw pointer, I’m wondering what could go wrong with this approach.

Another one on polymorphism…

This time is about not using heap allocation while having runtime polymorphism. I will use std::variant for this, so nothing new. What got my attention is how the polymorphic objects are used if stored in a variant. This is the main topic of this short article.

The use case is a factory function that creates polymorphic objects.

Virtual inheritance

I’m taking it step by step, starting with the classic approach using virtual inheritance. For this, I need some pointers, of course.

#include <cassert>
#include <memory>

struct P {
    virtual int f(int) const = 0;
    virtual ~P() = default;
};

struct A : P {
    int f(int in) const override {return in + 1;}
};

struct B : P {
    int f(int in) const override {return in + 2;}
};

std::unique_ptr<P> factory(char o) {
    switch(o) {
        case 'a': return std::make_unique<A>();
        default: return std::make_unique<B>();
    }
}

int main() {
    assert(factory('a')->f(1) == 2);
    assert(factory('b')->f(1) == 3);
}

std::variant

The std::variant solution is to have a variant with all the possible types instead of pointers to the types. This will avoid heap allocations. And it will break the need for inheritance, having objects that are not coupled to a base class anymore. Continue reading Runtime polymorphism without dynamic memory allocation

This is my 100th article and it was celebrated.

 

As I’m a fan of polymorphism, I play with different approaches on this subject. I want to find new ways of dealing with polymorphic objects under constrained scenarios. Not all of them are great, but every time I learn something new that I should or should not apply in real situations.

This time, the self-imposed context is:

• • C++11
  • Using only the standard library
  • Using std::array to have a homogenous list of objects

I want a list of objects where each object behaves differently.

struct Object {
    int id{};
    int value{};
};

std::array<Object, 2> objects;

 

But I cannot simply add a method to Object because I want a different method attached to each Object from the array. I was spinning around attaching lambdas to those objects for a while when it hit me: I could use another object to wrap my original one.

 

The wrapper knows about Object (T) and its corresponding lambda (Function)

template <typename T, typename Function>
struct Callable {
    std::function<Function> func_;
}

and it’s callable for simple use.

template <typename T, typename Function>
struct Callable {
    void operator()() {}
}

 

I use a lambda function with its first parameter being the original object (Object), the equivalent of this (think of self in Python). And any other parameters.

using MyObject = Callable<Object, void(Object&, int)>;

MyObject object{[](Object& object, int i) {
    object.id = i;
    object.value = i;
}};

 

When I call the object’s “behavior”, I pass all the arguments except “this”.

object(1);

 

How does it happen?

Callable is based on CRTP to be an Object and to pass an instance of Object (“this”) to the lambda function. It inherits from Object (template argument T) and safely casts itself to Object, thus obtaining an instance of Object.

template <typename T, typename Function>
struct Callable : T {
    template <typename... Args>
    void operator()(Args&&... args)
    {
        func_(static_cast<T&>(*this), std::forward<Args>(args)...);
    }

    std::function<Function> func_;
};

 

What’s wrong with this approach?

There are two possible deal-breakers depending on your context:

• • std::function prevents Callable to be inlined
  • Callable adds some memory overhead

And that’s it with my learning purpose experiment. Here’s the full source code: Continue reading Polymorphism with lambda functions