visitor_2.cpp

// http://coliru.stacked-crooked.com/a/5f6f08cff6f9a5b7
#include <iostream>

auto lit(const int value)
{
    return [value](auto visitor) { return visitor.lit(value); };    
}

template <class LHS,
          class RHS>
auto add(LHS&& lhs,
         RHS&& rhs)
{
    return [lhs,
            rhs](auto visitor) { return visitor.add(lhs(visitor),
                                                    rhs(visitor)); };
}

template <class LHS,
          class RHS>
auto mul(LHS&& lhs,
         RHS&& rhs)
{
    return [lhs,
            rhs](auto visitor) { return visitor.mul(lhs(visitor),
                                                    rhs(visitor)); };
}

struct EvalVisitor
{
    int lit(const int value) const
    {
        return value;    
    }
    
    int add(const int lhs,
            const int rhs) const
    {
        return lhs + rhs;    
    }
    
    int mul(const int lhs,
            const int rhs) const
    {
        return lhs * rhs;    
    }
};

int main()
{
    const EvalVisitor evalVisitor;
    
    const auto expression = add(lit(1),
                                mul(lit(3),
                                    lit(2)));
                                    
    std::cout << expression(evalVisitor) << std::endl;
    
    return 0;    
}