boost::variant apply static_visitor to certain types

Question

I have the following variant:

typedef boost::variant<int, float, bool> TypeVariant;

And I want to create a visitor to convert a int or float type to a bool type.

struct ConvertToBool : public boost::static_visitor<TypeVariant> {

    TypeVariant operator()(int a) const {
        return (bool)a;
    }

    TypeVariant operator()(float a) const {
        return (bool)a;
    }
};

However this is giving me the error message:

'TypeVariant ConvertToBool::operator ()(float) const': cannot convert argument 1 from 'T' to 'float'

What is the correct way of allowing a visitor to only apply to certain types?

Answer 1

Just include the missing overload:

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#include <boost/variant.hpp>
#include <iostream>

using TypeVariant = boost::variant<int, float, bool>;

struct ConvertToBool {
    using result_type = TypeVariant;
    TypeVariant operator()(TypeVariant const& v) const {
        return boost::apply_visitor(*this, v);
    }

    TypeVariant operator()(int   a) const { return a != 0; }
    TypeVariant operator()(float a) const { return a != 0; }
    TypeVariant operator()(bool  a) const { return a; }
} static constexpr to_bool{};

int main() {
    using V = TypeVariant;
    
    for (V v : {V{}, {42}, {3.14f}, {true}}) {
        std::cout << v << " -> " << std::boolalpha << to_bool(v) << "\n";
    }
}

Generalize

In more general cases you can supply a catch-all template overload:

template <typename T> TypeVariant operator()(T const& a) const {
    return static_cast<bool>(a);
}

In fact in your trivial case that's all you needed anyways:

Live On Coliru

#include <boost/variant.hpp>
#include <iostream>

using TypeVariant = boost::variant<int, float, bool>;

struct ConvertToBool {
    using result_type = TypeVariant;
    TypeVariant operator()(TypeVariant const& v) const {
        return boost::apply_visitor(*this, v);
    }

    template <typename T> TypeVariant operator()(T const& a) const {
        return static_cast<bool>(a);
    }
} static constexpr to_bool{};

int main() {
    using V = TypeVariant;

    for (V v : { V{}, { 42 }, { 3.14f }, { true } }) {
        std::cout << v << " -> " << std::boolalpha << to_bool(v) << "\n";
    }
}

Still prints

0 -> false
42 -> true
3.14 -> true
true -> true