No user defined conversion when using standard variants and visitor pattern

Question

Could you please help me figure out why is this not working i.e. refering to the comment in the code //I need to do this but I can't. I thought this the goal!? I have no idea why this is not working, it's inspired by examples I have seen online.

#include <variant>
#include <iostream>

template<typename... args>
class Visitor //: public boost_base_visitor<double>...
{
public:
    virtual ~Visitor() = default;

    virtual double visit(typename std::variant<args...> visitable)
    {
        auto op = [this](typename std::variant<args...> visitable) -> double { return this->apply(visitable); };
        return std::visit(std::ref(op), visitable);
    }

    virtual double apply(typename std::variant<args...> visitable) = 0;

    Visitor() = default;
};

class SubVisitor : public Visitor<std::string, double>
{
public:
    virtual ~SubVisitor() = default;
    SubVisitor() : Visitor<std::string, double>() {};
    
    virtual double apply(std::variant<std::string, double> visitable) override
    {
        //return process(visitable); //I need to do this but I can't. I thought this the goal!
        return process(std::get<std::string>(visitable)); //I DON'T KNOW IF THIS IS REALLY A STRING??
    };

    virtual double process(std::string visitable)
    {
        std::cout << "STRING HANDLED" << std::endl;
        return 0.0;
    }

    virtual double process(double visitable)
    {
        std::cout << "DOUBLE HANDLED" << std::endl;
        return 1.0;
    }
};

int main(int argc, char* argv[])
{
    SubVisitor v;
    v.apply("dd");
    //v.apply(1.0); //This will fail as we only handle string?? What is the purpose of variant then?
    return 1;
}

I am getting error when uncommenting the process function above:

Error C2664: 'double SubVisitor::process(std::string)': cannot convert argument 1 from 'std::variantstd::string,double' to 'std::string'

You have to uncomment the line ```//return process(visitable); //I need to do this but I can't. I thought this the goal!```. Could you please please delete your comment it's shadowing the issue and people will just pass by :) Thanks so much for checking! — Vero, Dec 03 '22 at 14:24
@Vero not sure what's the problem? you can use `std::visit` to do operation, or `std::get_if` if you want to know if it's a string. — apple apple, Dec 03 '22 at 14:30
Repro: [Link](https://godbolt.org/z/M1qscoYxE). Could you please help solve? Thanks so much! — Vero, Dec 03 '22 at 14:31
Please do not post code that works. If you have code that does NOT work, post it with no change. Otherwise it is not clear what your problem is and I need to do an unknown number of edits in order to reproduce it. — n. m. could be an AI, Dec 03 '22 at 15:56

apple apple · Answer 1 · 2022-12-03T14:41:50.997

you can use std:visit to operate on the variant

class SubVisitor{
    virtual double apply(std::variant<std::string, double> visitable) override
    {
        // std::visit expect `operator()`, not `process`
        // so wrap `this` inside a lambda here
        return std::visit(            
            [this](auto&& v){return process(v);},
            visitable
        );
    };
}

or if you want, you can also check the type manually

class SubVisitor{
    virtual double apply(std::variant<std::string, double> visitable) override
    {
        if(auto* s = std::get_if<std::string>(&visitable))
            return process(*s);
        
        else if(auto* d = std::get_if<double>(&visitable))
            return process(*d);

        throw std::bad_variant_access();
    }
};

score 1 · Answer 2 · answered Dec 03 '22 at 15:00

in addition to the answer, imo the base class should be something like

template<typename... args>
class Visitor{
public:
    Visitor() = default;
    virtual ~Visitor() = default;

    // this should be non-virtual
    double visit(std::variant<args...> visitable)
    {
        // dispatch via the customization point `this->apply`
        return this->apply(visitable);
    }
    virtual double apply(std::variant<args...> visitable) = 0;
};

No user defined conversion when using standard variants and visitor pattern

2 Answers2

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