nlohmann::json::array 'push_back' is ambiguous

[ceandrade]

Description

This is an issue related to issue #235

I'm trying to keep the insertion order on nested objects. Using the following solution, we lose the nested insertion order:

#include "third_part/json/json.hpp"
#include <iostream>
#include <iomanip>

using namespace std;

int main() {
    nlohmann::ordered_json json;

    json["first"] = "first arg";
    json["second"] = "second arg";

    // Now, I have a list of tuples, shcih keys' order I need to preserve.
    auto my_tuples = nlohmann::json::array();

    my_tuples.push_back({
        {"node", "a"},
        {"old_value", 0},
        {"new_value", 1}
    });
    my_tuples.push_back({
        {"node", "b"},
        {"old_value", 10},
        {"new_value", 20}
    });

    json["my_tuples"] = my_tuples;

    cout << "First try:\n" << setw(2) << json << "\n";

    return 0;
}

This is the output:

{
  "first": "first arg",
  "second": "second arg",
  "my_tuples": [
    {
      "new_value": 1,
      "node": "a",
      "old_value": 0
    },
    {
      "new_value": 20,
      "node": "b",
      "old_value": 10
    }
  ]
}

Note that in the nested list, we have the keys in alphabetical order rather than insertion order.

Now, I tried to use a nested nlohmann::ordered_json in these ways:

#include "third_part/json/json.hpp"
#include <iostream>
#include <iomanip>

using namespace std;

int main() {
    nlohmann::ordered_json json;

    json["first"] = "first arg";
    json["second"] = "second arg";

    // Now, I have a list of tuples, shcih keys' order I need to preserve.
    auto my_tuples = nlohmann::json::array();

    my_tuples.push_back(nlohmann::ordered_json({
        {"node", "a"},
        {"old_value", 0},
        {"new_value", 1}
    }));

    auto tmp = nlohmann::ordered_json({
        {"node", "b"},
        {"old_value", 10},
        {"new_value", 20}
    });
    my_tuples.push_back(tmp);

    json["my_tuples"] = my_tuples;

    cout << "First try:\n" << setw(2) << json << "\n";

    return 0;
}

However, the compiler complains in both cases:

$ g++ -std=c++17 -O3 -I. test/test_json.cpp -o test_json
test/test_json.cpp: In function 'int main()':
test/test_json.cpp:20:7: error: call of overloaded 'push_back(nlohmann::ordered_json)' is ambiguous
   20 |     }));
      |       ^
In file included from test/test_json.cpp:1:
./third_part/json/json.hpp:19986:10: note: candidate: 'void nlohmann::basic_json<ObjectType, ArrayType, StringType, BooleanType, NumberIntegerType, NumberUnsignedType, NumberFloatType, AllocatorType, JSONSerializer, BinaryType>::push_back(nlohmann::basic_json<ObjectType, ArrayType, StringType, BooleanType, NumberIntegerType, NumberUnsignedType, NumberFloatType, AllocatorType, JSONSerializer, BinaryType>&&) [with ObjectType = std::map; ArrayType = std::vector; StringType = std::__cxx11::basic_string<char>; BooleanType = bool; NumberIntegerType = long long int; NumberUnsignedType = long long unsigned int; NumberFloatType = double; AllocatorType = std::allocator; JSONSerializer = nlohmann::adl_serializer; BinaryType = std::vector<unsigned char>]'
19986 |     void push_back(basic_json&& val)
      |          ^~~~~~~~~
./third_part/json/json.hpp:20019:10: note: candidate: 'void nlohmann::basic_json<ObjectType, ArrayType, StringType, BooleanType, NumberIntegerType, NumberUnsignedType, NumberFloatType, AllocatorType, JSONSerializer, BinaryType>::push_back(const nlohmann::basic_json<ObjectType, ArrayType, StringType, BooleanType, NumberIntegerType, NumberUnsignedType, NumberFloatType, AllocatorType, JSONSerializer, BinaryType>&) [with ObjectType = std::map; ArrayType = std::vector; StringType = std::__cxx11::basic_string<char>; BooleanType = bool; NumberIntegerType = long long int; NumberUnsignedType = long long unsigned int; NumberFloatType = double; AllocatorType = std::allocator; JSONSerializer = nlohmann::adl_serializer; BinaryType = std::vector<unsigned char>]'
20019 |     void push_back(const basic_json& val)
      |          ^~~~~~~~~
./third_part/json/json.hpp:20051:10: note: candidate: 'void nlohmann::basic_json<ObjectType, ArrayType, StringType, BooleanType, NumberIntegerType, NumberUnsignedType, NumberFloatType, AllocatorType, JSONSerializer, BinaryType>::push_back(const typename nlohmann::basic_json<ObjectType, ArrayType, StringType, BooleanType, NumberIntegerType, NumberUnsignedType, NumberFloatType, AllocatorType, JSONSerializer, BinaryType>::object_t::value_type&) [with ObjectType = std::map; ArrayType = std::vector; StringType = std::__cxx11::basic_string<char>; BooleanType = bool; NumberIntegerType = long long int; NumberUnsignedType = long long unsigned int; NumberFloatType = double; AllocatorType = std::allocator; JSONSerializer = nlohmann::adl_serializer; BinaryType = std::vector<unsigned char>; typename nlohmann::basic_json<ObjectType, ArrayType, StringType, BooleanType, NumberIntegerType, NumberUnsignedType, NumberFloatType, AllocatorType, JSONSerializer, BinaryType>::object_t::value_type = std::pair<const std::__cxx11::basic_string<char>, nlohmann::basic_json<> >]'
20051 |     void push_back(const typename object_t::value_type& val)
      |          ^~~~~~~~~
./third_part/json/json.hpp:20082:10: note: candidate: 'void nlohmann::basic_json<ObjectType, ArrayType, StringType, BooleanType, NumberIntegerType, NumberUnsignedType, NumberFloatType, AllocatorType, JSONSerializer, BinaryType>::push_back(nlohmann::basic_json<ObjectType, ArrayType, StringType, BooleanType, NumberIntegerType, NumberUnsignedType, NumberFloatType, AllocatorType, JSONSerializer, BinaryType>::initializer_list_t) [with ObjectType = std::map; ArrayType = std::vector; StringType = std::__cxx11::basic_string<char>; BooleanType = bool; NumberIntegerType = long long int; NumberUnsignedType = long long unsigned int; NumberFloatType = double; AllocatorType = std::allocator; JSONSerializer = nlohmann::adl_serializer; BinaryType = std::vector<unsigned char>; nlohmann::basic_json<ObjectType, ArrayType, StringType, BooleanType, NumberIntegerType, NumberUnsignedType, NumberFloatType, AllocatorType, JSONSerializer, BinaryType>::initializer_list_t = std::initializer_list<nlohmann::detail::json_ref<nlohmann::basic_json<> > >]'
20082 |     void push_back(initializer_list_t init)
      |          ^~~~~~~~~
test/test_json.cpp:27:28: error: call of overloaded 'push_back(nlohmann::basic_json<nlohmann::ordered_map>&)' is ambiguous
   27 |     my_tuples.push_back(tmp);
      |                            ^
In file included from test/test_json.cpp:1:
./third_part/json/json.hpp:19986:10: note: candidate: 'void nlohmann::basic_json<ObjectType, ArrayType, StringType, BooleanType, NumberIntegerType, NumberUnsignedType, NumberFloatType, AllocatorType, JSONSerializer, BinaryType>::push_back(nlohmann::basic_json<ObjectType, ArrayType, StringType, BooleanType, NumberIntegerType, NumberUnsignedType, NumberFloatType, AllocatorType, JSONSerializer, BinaryType>&&) [with ObjectType = std::map; ArrayType = std::vector; StringType = std::__cxx11::basic_string<char>; BooleanType = bool; NumberIntegerType = long long int; NumberUnsignedType = long long unsigned int; NumberFloatType = double; AllocatorType = std::allocator; JSONSerializer = nlohmann::adl_serializer; BinaryType = std::vector<unsigned char>]'
19986 |     void push_back(basic_json&& val)
      |          ^~~~~~~~~
./third_part/json/json.hpp:20019:10: note: candidate: 'void nlohmann::basic_json<ObjectType, ArrayType, StringType, BooleanType, NumberIntegerType, NumberUnsignedType, NumberFloatType, AllocatorType, JSONSerializer, BinaryType>::push_back(const nlohmann::basic_json<ObjectType, ArrayType, StringType, BooleanType, NumberIntegerType, NumberUnsignedType, NumberFloatType, AllocatorType, JSONSerializer, BinaryType>&) [with ObjectType = std::map; ArrayType = std::vector; StringType = std::__cxx11::basic_string<char>; BooleanType = bool; NumberIntegerType = long long int; NumberUnsignedType = long long unsigned int; NumberFloatType = double; AllocatorType = std::allocator; JSONSerializer = nlohmann::adl_serializer; BinaryType = std::vector<unsigned char>]'
20019 |     void push_back(const basic_json& val)
      |          ^~~~~~~~~
./third_part/json/json.hpp:20051:10: note: candidate: 'void nlohmann::basic_json<ObjectType, ArrayType, StringType, BooleanType, NumberIntegerType, NumberUnsignedType, NumberFloatType, AllocatorType, JSONSerializer, BinaryType>::push_back(const typename nlohmann::basic_json<ObjectType, ArrayType, StringType, BooleanType, NumberIntegerType, NumberUnsignedType, NumberFloatType, AllocatorType, JSONSerializer, BinaryType>::object_t::value_type&) [with ObjectType = std::map; ArrayType = std::vector; StringType = std::__cxx11::basic_string<char>; BooleanType = bool; NumberIntegerType = long long int; NumberUnsignedType = long long unsigned int; NumberFloatType = double; AllocatorType = std::allocator; JSONSerializer = nlohmann::adl_serializer; BinaryType = std::vector<unsigned char>; typename nlohmann::basic_json<ObjectType, ArrayType, StringType, BooleanType, NumberIntegerType, NumberUnsignedType, NumberFloatType, AllocatorType, JSONSerializer, BinaryType>::object_t::value_type = std::pair<const std::__cxx11::basic_string<char>, nlohmann::basic_json<> >]'
20051 |     void push_back(const typename object_t::value_type& val)
      |          ^~~~~~~~~
./third_part/json/json.hpp:20082:10: note: candidate: 'void nlohmann::basic_json<ObjectType, ArrayType, StringType, BooleanType, NumberIntegerType, NumberUnsignedType, NumberFloatType, AllocatorType, JSONSerializer, BinaryType>::push_back(nlohmann::basic_json<ObjectType, ArrayType, StringType, BooleanType, NumberIntegerType, NumberUnsignedType, NumberFloatType, AllocatorType, JSONSerializer, BinaryType>::initializer_list_t) [with ObjectType = std::map; ArrayType = std::vector; StringType = std::__cxx11::basic_string<char>; BooleanType = bool; NumberIntegerType = long long int; NumberUnsignedType = long long unsigned int; NumberFloatType = double; AllocatorType = std::allocator; JSONSerializer = nlohmann::adl_serializer; BinaryType = std::vector<unsigned char>; nlohmann::basic_json<ObjectType, ArrayType, StringType, BooleanType, NumberIntegerType, NumberUnsignedType, NumberFloatType, AllocatorType, JSONSerializer, BinaryType>::initializer_list_t = std::initializer_list<nlohmann::detail::json_ref<nlohmann::basic_json<> > >]'
20082 |     void push_back(initializer_list_t init)
      |          ^~~~~~~~~

So, this is close to issue #235, although we have a series of template resolution that looks like a bit different here.

Is this real bug? Can I keep the inserting order on nested objects in some way.

Thanks,

Carlos

Reproduction steps

Please, compile the given code using GNU G++ (MacPorts gcc10 10.3.0_1) or Clang++ (Apple clang version 12.0.5 (clang-1205.0.22.11))

Expected vs. actual results

Expected the nested keys in the inserted order. Actual results are not produced due to compiling error.

Minimal code example

#include "third_part/json/json.hpp"
#include <iostream>
#include <iomanip>

using namespace std;

int main() {
    nlohmann::ordered_json json;

    json["first"] = "first arg";
    json["second"] = "second arg";

    // Now, I have a list of tuples, shcih keys' order I need to preserve.
    auto my_tuples = nlohmann::json::array();

    my_tuples.push_back(nlohmann::ordered_json({
        {"node", "a"},
        {"old_value", 0},
        {"new_value", 1}
    }));

    auto tmp = nlohmann::ordered_json({
        {"node", "b"},
        {"old_value", 10},
        {"new_value", 20}
    });
    my_tuples.push_back(tmp);

    json["my_tuples"] = my_tuples;

    cout << "First try:\n" << setw(2) << json << "\n";

    return 0;
}

Error messages

$ g++ -std=c++17 -O3 -I. test/test_json.cpp -o test_json
test/test_json.cpp: In function 'int main()':
test/test_json.cpp:20:7: error: call of overloaded 'push_back(nlohmann::ordered_json)' is ambiguous
   20 |     }));
      |       ^
In file included from test/test_json.cpp:1:
./third_part/json/json.hpp:19986:10: note: candidate: 'void nlohmann::basic_json<ObjectType, ArrayType, StringType, BooleanType, NumberIntegerType, NumberUnsignedType, NumberFloatType, AllocatorType, JSONSerializer, BinaryType>::push_back(nlohmann::basic_json<ObjectType, ArrayType, StringType, BooleanType, NumberIntegerType, NumberUnsignedType, NumberFloatType, AllocatorType, JSONSerializer, BinaryType>&&) [with ObjectType = std::map; ArrayType = std::vector; StringType = std::__cxx11::basic_string<char>; BooleanType = bool; NumberIntegerType = long long int; NumberUnsignedType = long long unsigned int; NumberFloatType = double; AllocatorType = std::allocator; JSONSerializer = nlohmann::adl_serializer; BinaryType = std::vector<unsigned char>]'
19986 |     void push_back(basic_json&& val)
      |          ^~~~~~~~~
./third_part/json/json.hpp:20019:10: note: candidate: 'void nlohmann::basic_json<ObjectType, ArrayType, StringType, BooleanType, NumberIntegerType, NumberUnsignedType, NumberFloatType, AllocatorType, JSONSerializer, BinaryType>::push_back(const nlohmann::basic_json<ObjectType, ArrayType, StringType, BooleanType, NumberIntegerType, NumberUnsignedType, NumberFloatType, AllocatorType, JSONSerializer, BinaryType>&) [with ObjectType = std::map; ArrayType = std::vector; StringType = std::__cxx11::basic_string<char>; BooleanType = bool; NumberIntegerType = long long int; NumberUnsignedType = long long unsigned int; NumberFloatType = double; AllocatorType = std::allocator; JSONSerializer = nlohmann::adl_serializer; BinaryType = std::vector<unsigned char>]'
20019 |     void push_back(const basic_json& val)
      |          ^~~~~~~~~
./third_part/json/json.hpp:20051:10: note: candidate: 'void nlohmann::basic_json<ObjectType, ArrayType, StringType, BooleanType, NumberIntegerType, NumberUnsignedType, NumberFloatType, AllocatorType, JSONSerializer, BinaryType>::push_back(const typename nlohmann::basic_json<ObjectType, ArrayType, StringType, BooleanType, NumberIntegerType, NumberUnsignedType, NumberFloatType, AllocatorType, JSONSerializer, BinaryType>::object_t::value_type&) [with ObjectType = std::map; ArrayType = std::vector; StringType = std::__cxx11::basic_string<char>; BooleanType = bool; NumberIntegerType = long long int; NumberUnsignedType = long long unsigned int; NumberFloatType = double; AllocatorType = std::allocator; JSONSerializer = nlohmann::adl_serializer; BinaryType = std::vector<unsigned char>; typename nlohmann::basic_json<ObjectType, ArrayType, StringType, BooleanType, NumberIntegerType, NumberUnsignedType, NumberFloatType, AllocatorType, JSONSerializer, BinaryType>::object_t::value_type = std::pair<const std::__cxx11::basic_string<char>, nlohmann::basic_json<> >]'
20051 |     void push_back(const typename object_t::value_type& val)
      |          ^~~~~~~~~
./third_part/json/json.hpp:20082:10: note: candidate: 'void nlohmann::basic_json<ObjectType, ArrayType, StringType, BooleanType, NumberIntegerType, NumberUnsignedType, NumberFloatType, AllocatorType, JSONSerializer, BinaryType>::push_back(nlohmann::basic_json<ObjectType, ArrayType, StringType, BooleanType, NumberIntegerType, NumberUnsignedType, NumberFloatType, AllocatorType, JSONSerializer, BinaryType>::initializer_list_t) [with ObjectType = std::map; ArrayType = std::vector; StringType = std::__cxx11::basic_string<char>; BooleanType = bool; NumberIntegerType = long long int; NumberUnsignedType = long long unsigned int; NumberFloatType = double; AllocatorType = std::allocator; JSONSerializer = nlohmann::adl_serializer; BinaryType = std::vector<unsigned char>; nlohmann::basic_json<ObjectType, ArrayType, StringType, BooleanType, NumberIntegerType, NumberUnsignedType, NumberFloatType, AllocatorType, JSONSerializer, BinaryType>::initializer_list_t = std::initializer_list<nlohmann::detail::json_ref<nlohmann::basic_json<> > >]'
20082 |     void push_back(initializer_list_t init)
      |          ^~~~~~~~~
test/test_json.cpp:27:28: error: call of overloaded 'push_back(nlohmann::basic_json<nlohmann::ordered_map>&)' is ambiguous
   27 |     my_tuples.push_back(tmp);
      |                            ^
In file included from test/test_json.cpp:1:
./third_part/json/json.hpp:19986:10: note: candidate: 'void nlohmann::basic_json<ObjectType, ArrayType, StringType, BooleanType, NumberIntegerType, NumberUnsignedType, NumberFloatType, AllocatorType, JSONSerializer, BinaryType>::push_back(nlohmann::basic_json<ObjectType, ArrayType, StringType, BooleanType, NumberIntegerType, NumberUnsignedType, NumberFloatType, AllocatorType, JSONSerializer, BinaryType>&&) [with ObjectType = std::map; ArrayType = std::vector; StringType = std::__cxx11::basic_string<char>; BooleanType = bool; NumberIntegerType = long long int; NumberUnsignedType = long long unsigned int; NumberFloatType = double; AllocatorType = std::allocator; JSONSerializer = nlohmann::adl_serializer; BinaryType = std::vector<unsigned char>]'
19986 |     void push_back(basic_json&& val)
      |          ^~~~~~~~~
./third_part/json/json.hpp:20019:10: note: candidate: 'void nlohmann::basic_json<ObjectType, ArrayType, StringType, BooleanType, NumberIntegerType, NumberUnsignedType, NumberFloatType, AllocatorType, JSONSerializer, BinaryType>::push_back(const nlohmann::basic_json<ObjectType, ArrayType, StringType, BooleanType, NumberIntegerType, NumberUnsignedType, NumberFloatType, AllocatorType, JSONSerializer, BinaryType>&) [with ObjectType = std::map; ArrayType = std::vector; StringType = std::__cxx11::basic_string<char>; BooleanType = bool; NumberIntegerType = long long int; NumberUnsignedType = long long unsigned int; NumberFloatType = double; AllocatorType = std::allocator; JSONSerializer = nlohmann::adl_serializer; BinaryType = std::vector<unsigned char>]'
20019 |     void push_back(const basic_json& val)
      |          ^~~~~~~~~
./third_part/json/json.hpp:20051:10: note: candidate: 'void nlohmann::basic_json<ObjectType, ArrayType, StringType, BooleanType, NumberIntegerType, NumberUnsignedType, NumberFloatType, AllocatorType, JSONSerializer, BinaryType>::push_back(const typename nlohmann::basic_json<ObjectType, ArrayType, StringType, BooleanType, NumberIntegerType, NumberUnsignedType, NumberFloatType, AllocatorType, JSONSerializer, BinaryType>::object_t::value_type&) [with ObjectType = std::map; ArrayType = std::vector; StringType = std::__cxx11::basic_string<char>; BooleanType = bool; NumberIntegerType = long long int; NumberUnsignedType = long long unsigned int; NumberFloatType = double; AllocatorType = std::allocator; JSONSerializer = nlohmann::adl_serializer; BinaryType = std::vector<unsigned char>; typename nlohmann::basic_json<ObjectType, ArrayType, StringType, BooleanType, NumberIntegerType, NumberUnsignedType, NumberFloatType, AllocatorType, JSONSerializer, BinaryType>::object_t::value_type = std::pair<const std::__cxx11::basic_string<char>, nlohmann::basic_json<> >]'
20051 |     void push_back(const typename object_t::value_type& val)
      |          ^~~~~~~~~
./third_part/json/json.hpp:20082:10: note: candidate: 'void nlohmann::basic_json<ObjectType, ArrayType, StringType, BooleanType, NumberIntegerType, NumberUnsignedType, NumberFloatType, AllocatorType, JSONSerializer, BinaryType>::push_back(nlohmann::basic_json<ObjectType, ArrayType, StringType, BooleanType, NumberIntegerType, NumberUnsignedType, NumberFloatType, AllocatorType, JSONSerializer, BinaryType>::initializer_list_t) [with ObjectType = std::map; ArrayType = std::vector; StringType = std::__cxx11::basic_string<char>; BooleanType = bool; NumberIntegerType = long long int; NumberUnsignedType = long long unsigned int; NumberFloatType = double; AllocatorType = std::allocator; JSONSerializer = nlohmann::adl_serializer; BinaryType = std::vector<unsigned char>; nlohmann::basic_json<ObjectType, ArrayType, StringType, BooleanType, NumberIntegerType, NumberUnsignedType, NumberFloatType, AllocatorType, JSONSerializer, BinaryType>::initializer_list_t = std::initializer_list<nlohmann::detail::json_ref<nlohmann::basic_json<> > >]'
20082 |     void push_back(initializer_list_t init)
      |          ^~~~~~~~~

$ clang++ -std=c++17 -O3 -I. test/test_json.cpp -o test_json
test/test_json.cpp:16:15: error: call to member function 'push_back' is ambiguous
    my_tuples.push_back(nlohmann::ordered_json({
    ~~~~~~~~~~^~~~~~~~~
./third_part/json/json.hpp:19986:10: note: candidate function
    void push_back(basic_json&& val)
         ^
./third_part/json/json.hpp:20019:10: note: candidate function
    void push_back(const basic_json& val)
         ^
./third_part/json/json.hpp:20051:10: note: candidate function
    void push_back(const typename object_t::value_type& val)
         ^
./third_part/json/json.hpp:20082:10: note: candidate function
    void push_back(initializer_list_t init)
         ^
test/test_json.cpp:27:15: error: call to member function 'push_back' is ambiguous
    my_tuples.push_back(tmp);
    ~~~~~~~~~~^~~~~~~~~
./third_part/json/json.hpp:19986:10: note: candidate function
    void push_back(basic_json&& val)
         ^
./third_part/json/json.hpp:20019:10: note: candidate function
    void push_back(const basic_json& val)
         ^
./third_part/json/json.hpp:20051:10: note: candidate function
    void push_back(const typename object_t::value_type& val)
         ^
./third_part/json/json.hpp:20082:10: note: candidate function
    void push_back(initializer_list_t init)
         ^
2 errors generated.

Compiler and operating system

Mac OS X Big Sur, GNU G++ (MacPorts gcc10 10.3.0_1) or Clang++ (Apple clang version 12.0.5 (clang-1205.0.22.11))

Library version

Stable 3.10.5

Validation

• The bug also occurs if the latest version from the develop branch is used.
• I can successfully compile and run the unit tests.

[falbrechtskirchinger]

You're mixing nlohmann::json and nlohmann::ordered_json. Don't.

Specifically, here:

auto my_tuples = nlohmann::json::array();

Use:

auto my_tuples = nlohmann::ordered_json::array();

[ceandrade]

Oh, I see. Indeed, very simple! Thank for the extremely fast reply!