Using custom string.

[armatti]

I've tried using custom containers with nlohmann::json. ObjectType and ArrayType can be swapped, but changing StringType breaks creating json from most types.

Here's a minimal, non-functional string implementation, that builds as long as you don't try to create a json from other types than string literals: https://godbolt.org/z/GTvn8T

If I read correctly nlohmann::detail::detector fails to find a matching to_json function, but I don't understand how changing StringType has anything to do with it. Should StringType be replaceable? Does it then need a custom serializer or what's the trick?

[stale]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.

[nlohmann]

I can reproduce your findings, but not yet understood the error.

[nlohmann]

I could create a minimal wrapper around std::string with which I could compile your example:

class my_string {
  public:
    using value_type = std::string::value_type;
    using size_type = std::string::size_type;

    my_string() {}
    my_string(const value_type* s) : _s(s) {}
    my_string(std::initializer_list<value_type> ilist) : _s(ilist) {}
    ~my_string() {}

    const value_type* data() const noexcept {
        return _s.data();
    }

    size_type size() const noexcept {
        return _s.size();
    }

    void clear() noexcept {
        _s.clear();
    }

    void push_back(size_type ch) {
        _s.push_back(ch);
    }

    friend bool operator<(const my_string& lhs, const my_string& rhs)
    {
        return lhs._s < rhs._s;
    }

  private:
    std::string _s;
};

I am not sure whether it is complete, but it helped me find a problem in your code. After removing the constructor

my_string(char){}

your code compiles.

[armatti]

Sorry for not updating the issue sooner. Yes, thanks, removing that constructor does help. Additional gripe was with nlohmann::detail::int_to_string which was solved by defining an overload of the function with the custom string_type.

Looks like int_to_string was reported also in #2059 and commit aeef507 linked as a fix, but not sure how that is supposed to work: https://godbolt.org/z/aEWbM6joo

[gregmarr]

@armatti you're right that the referenced commit does not do what it was intended to do. ADL is for the "arguments" of the function, not the return type. However, looks like you can use ADL to replace this function instead, which is what it sounds like you did.

void int_to_string(my_string& str, std::size_t value)
{
    str = {};
}

FYI @dota17

[armatti]

As problems could be solved without modifying nlohmann::json, I'm happy to close this issue.