Troubles with using `into_toml` member function

[CDK6182CHR]

Recently I have encountered some trouble when try to use the into_toml member function to convert a user-defined struct to toml::value, using the method described in the document: link.

Minimal example

(See also the file attached)
toml11_into_toml_test.zip

Here I defined a struct with into_toml member function:

struct Config {
    int x = 1;
    double y = 2.;

    template <typename TC>
    toml::basic_value<TC> into_toml() const 
    {
         return toml::basic_value<TC>(typename toml::basic_value<TC>::table_type{
                {"x", this->x}, {"y", this->y}
            });
    }
};

Then I tried to convert it into toml::value using codes like this:

toml::value v;
v["config"] = c;

Then I get compiling error.

I found also that the type trait below gives a false value:

constexpr const auto t = toml::detail::has_into_toml_method<Config>::value;   // false

The implementation of such trait is (in V4.0.3 code, traits.hpp:81):

struct has_into_toml_method_impl
{
    template<typename T>
    static std::true_type  check(decltype(std::declval<T>().into_toml())*);
    // Other code omitted
};

And this is called similarly in the constructor of toml::basic_value.
It seems that the template argument TC (see the definition above) is not passed into the function into_toml, and the compiler has no way to deduce the TC parameter.
This may be the cause of such problem, to the best of my understanding.

On the other hand: the specialization of toml::into works well for this.

I wonder whether there is something wrong with my implementation (the definition of into_toml member function, etc), or we cannot use the into_toml member function in this case?

Test environments

The tests have been performed in the following environments and yield similar results:

• On Windows: MSVC 19.33, C++20, TOML11 V4.0.3;
• On Linux: GCC 14.1.0, C++20 and C++23, TOML11 V4.0.1

Thank you in advance.

[ToruNiina]

Thank you for detailed explanation. You are right, we need template version of has_into_toml_method to support that function.

Since non-template into_toml implementation is already there, has_template_into_toml_method should be defined separately. Then we can add a new overload of basic_value constructor that takes has_template_into_toml_method<T, TypeConfig> as SFINAE condition.

Would you like to make a pull request? I will wait your reply during this weekend.

[CDK6182CHR]

Thank you for detailed explanation. You are right, we need template version of has_into_toml_method to support that function.

Since non-template into_toml implementation is already there, has_template_into_toml_method should be defined separately. Then we can add a new overload of basic_value constructor that takes has_template_into_toml_method<T, TypeConfig> as SFINAE condition.

Would you like to make a pull request? I will wait your reply during this weekend.

OK, with pleasure. I have just created a pull request #256

[ToruNiina]

Merged and it passes. thank you!