node.setParents(mother, father);

node.setParents(&mother, &father);

#include <iostream>

struct node
{
    virtual ~node(){}
    
    template<class parent1, class parent2>
    void setParents(parent1&& mother, parent2&& father) 
    {
        static_assert(
            !::std::is_rvalue_reference<parent1&&>::value,
            "temporary objects 'mother' prohibited"
        );
        static_assert(
            !::std::is_rvalue_reference<parent2&&>::value,
            "temporary objects 'father' prohibited"
        );
        const auto& m = static_cast<const node&>(mother);
        const auto& f = static_cast<const node&>(father);
        this->setParents(m, f);
    }    
    virtual void setParents(const node& mother, const node& father) = 0;
};

struct label: node
{
    virtual void setParents(const node&, const node&){}
};
 
int main()
{
    label mother;
    label father;
    label child;
    node& example = child;
    
    example.setParents(mother, father);
}

#include <iostream>

struct node
{
    virtual ~node(){}
    
    template<class parent1, class parent2, dfor_lvalue(parent1), dfor_lvalue(parent2) >
    void setParents(parent1&& mother, parent2&& father) 
    {
        const auto& m = static_cast<const node&>(mother);
        const auto& f = static_cast<const node&>(father);
        this->setParents(m, f);
    }    
    virtual void setParents(const node& mother, const node& father) = 0;
};

struct label: node
{
    virtual void setParents(const node&, const node&){}
};
 
int main()
{
    label mother;
    label father;
    label child;
    node& example = child;
    
    example.setParents(mother, father);
}

#include <iostream>

struct node
{
    virtual ~node(){}
    
    template<class p1, class p2>
    void setParents(lvalue<p1>& mother, lvalue<p2>& father) 
    {
        const auto& m = static_cast<const node&>(mother);
        const auto& f = static_cast<const node&>(father);
        this->setParents(m, f);
    }    
    virtual void setParents(const node& mother, const node& father) = 0;
};

struct label: node
{
    virtual void setParents(const node&, const node&){}
};
 
int main()
{
    label mother;
    label father;
    label child;
    node& example = child;
    
    example.setParents(mother, father);
}

virtual void setParents(const node&, const node&){}

struct node
{
    virtual ~node(){}
    virtual void setParents(const node& mother, const node& father) = 0;
};