Promote idiomatic style in example code

[purpleKarrot]

The motivation of P3019 reads:

When designing a composite class, we may need an object to be stored indirectly to support incomplete types, reduce object size or support open-set polymorphism.

The usage examples in the README should:

• Focus on composite classes (Don't use indirect and polymorphic outside classes).
• Make data members private in accordance to C.9: Minimize exposure of members.
• Show that C.35 can be simplified:
  A base class destructor should be either public and virtual, or protected and non-virtual.

[camio]

Thanks for the issue!

Recursive variants are my primary use cases, but I agree that these others should be represented as well.

I'll look into applying your other suggestions as well.

[purpleKarrot]

Recursive variants are my primary use cases

Sure, indirect does not need to always be the direct data member of a composite class. A composite class like json_value will use it inside a variant, for sure. It is a good example which shows that json_value should be a wrapper and not an alias. The example could be improved by making data members private.

An example that wraps polymorphic into a composite might look like this:

class Shape
{
protected:
    // destructor of the base does not need to be accessible and may be non-virtual
    ~Shape() = default;

public:
    virtual double area() const = 0;
};

class Canvas
{
public:
    template <std::derived_from<Shape> Derived>
    void add_shape(Derived shape) {
        shapes.emplace_back(std::move(shape));
    }

    double total_area() const {
        auto areas = shapes | std::views::transform([](auto const& s) { return s->area(); });
        return std::reduce(std::begin(areas), std::end(areas), 0.0);
    }
    
private:
    std::vector<std::polymorphic<Shape>> shapes;
};

/* ... */

Canvas canvas;
canvas.add_shape(Circle{5.0});
canvas.add_shape(Square{4.0});
std::cout << "Total area: " << canvas.total_area() << '\n';