CRTPVisitor.cpp

#include <iostream>

/*
    Attempt at CRTP visitor sample
    DrkWithT
*/

/// Forward decls. of visitable objects
class OneBox;
class TwoBox;

/// Prints any XXBox's info
struct BoxVisitor
{
    BoxVisitor() = default;

    void visitOneBox(const OneBox& one) const;

    void visitTwoBox(const TwoBox& two) const;
};

/// CRTP compile-time interface for an XXBox
template <typename Derived>
class Base
{
private:
    [[nodiscard]] Derived* getDerivedPtr() { return static_cast<Derived*>(this); }

public:
    Base() = default;

    void acceptVisitor(const BoxVisitor& printer)
    {
        /// Here, I start double dispatch with the appropriate derived class's. The object is prompted to accept the printer.
        getDerivedPtr()->acceptVisitor(printer);
    }
};

class OneBox : public Base<OneBox>
{
private:
    int a;

public:
    constexpr OneBox(int a_)
        : a {a_} {}

    [[nodiscard]] int getA() const { return a; }

    void acceptVisitor(const BoxVisitor& printer)
    {
        // Here, the double dispatch is about to finish: the OneBox class knows by itself how to handle this visitation.
        printer.visitOneBox(*this);
    }
};

class TwoBox : public Base<TwoBox>
{
private:
    int a;
    int b;

public:
    constexpr TwoBox(int a_, int b_)
        : a {a_}, b {b_} {}

    [[nodiscard]] int getA() const { return a; }
    [[nodiscard]] int getB() const { return b; }

    void acceptVisitor(const BoxVisitor& printer)
    {
        printer.visitTwoBox(*this);
    }
};

/// Post-forward definitions of visitor's methods to help linker resolve dummy calls from forward decls.
void BoxVisitor::visitOneBox(const OneBox& one) const
{
    std::cout << "OneBox {" << one.getA() << '}' << '\n';
}

void BoxVisitor::visitTwoBox(const TwoBox& two) const
{
    std::cout << "TwoBox {" << two.getA() << ',' << two.getB() << '}' << '\n';
}

int main()
{
    // create visitable objects
    OneBox single {42};
    TwoBox couple {20, 24};

    // create printing visitor
    BoxVisitor box_display;

    // test visit methods
    box_display.visitOneBox(single);
    box_display.visitTwoBox(couple);
}