CODEWITHSUNDEEP
c++polymorphismvirtual
Eduard Rostomyan
Eduard Rostomyan

Reputation: 6546

Is overriding a function with derived class argument considered name hiding?

I is this considered name hiding?
As according to the output it is:

class A
{
public:
    A(){}
    virtual bool f(A& a) {
        std::cout << "A" << std::endl;
        return true;
    }
};

class B : public A
{
    std::string s;
public:
    B(){}
    bool f(B& a) {
        std::cout << "B" << std::endl;
        return s == a.s;
    }
};

int main()
{
    A* a = new B;
    B b;
    a->f(b);
}

This prints A, and if I want to achieve the polymorphic behaviour, I need to make the derived class's f's argument the same as the base classes. But in that case I don't have access to s in the derived class function. Is this a design deadlock or I am messing up the OOD principles?

Upvotes: 0

Views: 60

Answers (1)

Jarod42
Jarod42

Reputation: 217275

It seems you want multiple dispatch.

Double dispatch pattern is a solution.

With c++17, we have std::variant which might do the dispatching for us:

class A;
class B;

using VariantA = std::variant<A*, B*, C*>;
using VariantConstA = std::variant<const A*, const B*, const C*>;

class A
{
public:
    virtual ~A() = default;
    virtual VariantA AsVariant() { return this; }
    virtual VariantConstA AsVariant() const { return this; }

    friend bool operator==(const A&, const A&);
};

class B : public A
{
    std::string s;
public:
    B(std::string s) : s(s) {}
    VariantA AsVariant() override { return this; }
    VariantConstA AsVariant() const override { return this; }

    friend bool operator==(const B& lhs, const B& rhs);
};

// class C

struct EqualVisitor
{
    template <typename T>
    bool operator() (const T& lhs, const T& rhs) const { return lhs == rhs; }

    template <typename T1, typename T2>
    bool operator() (const T1&, const T2&) const { return false; }
};


bool AreEqual(const A& lhs, const A& rhs)
{
    return std::visit(EqualVisitor{}, lhs.AsVariant(), rhs.AsVariant());
}

Demo

Upvotes: 2

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