overwrite adl_serializer<bool, void> to change behaviour

[Virtual-felix]

I would like to change the behaviour of how boolean are serialized, so to_json would transform boolean to string.

I tried to do an adl_serializer specialized for bolean like this

namespace nlohmann {

template<>
struct adl_serializer<bool> {
  static void to_json(json& j, bool b) {
    j = std::string(b ? "true" : "false");
  }

  static void from_json(const json& j, bool& b) {
    b = j.template get<std::string>() == "true";
  }
};

} // namespace nlohmann

Knowing that I tried to do

b = *j.get_ptr<const json::boolean_t*>()
but it was returning a null ptr (no idea wny the bool is stocked as a string)

The fact is that this try compile and work with Clang but does not compile with GCC.
GCC output:

error: partial specialization of ‘struct nlohmann::adl_serializer<T, typename std::enable_if<std::is_same<bool, _U1>::value, void>::type>’ after instantiation of ‘struct nlohmann::adl_serializer<bool, void>’ [-fpermissive]
struct adl_serializer<T, std::enable_if_t<std::is_same<bool, T>::value>> {

But I can't understand where this instantiation happen and why it happens with GCC and not Clang.
I was wondering if there is another way to change the behaviour of a boolean serialization if it is possible.

I'm using the tag 3.5 of the library.

[nickaein]

Just as a reference, I tried your code on VS 2019,

#include <iostream>
#include "json.hpp"

namespace nlohmann {
    template<>
    struct adl_serializer<bool> {
        static void to_json(json& j, bool b) {
            j = std::string(b ? "true" : "false");
        }

        static void from_json(const json& j, bool& b) {
            b = j.template get<std::string>() == "true";
        }
    };
} // namespace nlohmann

int main()
{
    nlohmann::json all;

    all["hello"] = true;
    all["bye"] = false;

    std::cout << all.dump() << std::endl;
}

It compiles without any errors/warnings and gives the below output as expected:

{"bye":"false","hello":"true"}

[Virtual-felix]

Is you VS 2019 using CLang ? On my side it does compile with Clang and work as expected, but does not compile with GCC 6.4

[nickaein]

Sorry for not being clear. I tested the code with native msvc compiler.

I've reproduced the error on GCC here which says:

prog.cc:20849:12: error: specialization of 'nlohmann::adl_serializer<bool, void>' after instantiation
     struct adl_serializer<bool, void> {
            ^~~~~~~~~~~~~~~~~~~~~~~~~~
prog.cc:20849:12: error: redefinition of 'struct nlohmann::adl_serializer<bool, void>'
prog.cc:2170:8: note: previous definition of 'struct nlohmann::adl_serializer<bool, void>'
 struct adl_serializer
        ^~~~~~~~~~~~~~

Are you getting the same error?

I wonder if it has anything to do with the fact that the library has provided from_json implementation for bool type:

json/include/nlohmann/detail/conversions/from_json.hpp

Line 67 in ee8732c

void from_json(const BasicJsonType& j, typename BasicJsonType::boolean_t& b)

But as you stated, this error doesn't happen on Clang (and msvc). This is either a compiler bug (unlikely) or some behavior that the Standard left the choice up to the compiler implementers (like this one).

[Virtual-felix]

Are you getting the same error?
Yes

I found another way to do what I wanted without having to write boolean as string. I will close this issue assuming it is a difference of behavior between the compiler, but it may hide an error I guess.

[samsaga2]

Sorry for reopen this issue.

But I have exactly the same problem. GCC & C++17 & adl_serializer gives me a redefinition error. There is something wrong in the library. The same code works correctly with clang, but that is only because it manages the problem differently.

I have fixed the compiler error resorting the includes. If you put the json header in the first place, this error disappears. There is some strange problem in the adl_serializer declaration.