when enum is used as key in a map the json format is not an expected format

[ordit1985]

Description

I would expect map to be parsed as json objects eg.
{"aa":"completed","bb":"running"}

In fact this json format is dependent on the key type. When the key type is string, the json is created as expected.
When the key type is an enum, the map is parsed as json array eg.
[["stopped","aa"],["completed","bb"]]

Reproduction steps

#include <iostream>
#include <string>
#include <map>
#include "common/include/json/json.h"

using njson = nlohmann::json;

// example enum type declaration
enum TaskState {
    TS_STOPPED,
    TS_RUNNING,
    TS_COMPLETED,
    TS_INVALID=-1,
};

// map TaskState values to JSON as strings
NLOHMANN_JSON_SERIALIZE_ENUM( TaskState, {
    {TS_INVALID, nullptr},
    {TS_STOPPED, "stopped"},
    {TS_RUNNING, "running"},
    {TS_COMPLETED, "completed"},
})

int main(int argc, char ** argv) {

	std::map<TaskState, std::string> t1 = {{TS_STOPPED, "aa"}, {TS_COMPLETED, "bb"}};
	std::map<std::string, TaskState> t2 = {{"aa", TS_COMPLETED}, {"bb", TS_RUNNING}};

	printf("enum_in_key_nohlman: %s\n", njson(t1).dump().c_str());
	printf("enum_in_val_nohlman: %s\n", njson(t2).dump().c_str());


// when enum is in key the json format changes to unexpected json format.
// enum_in_key_nohlman: [["stopped","aa"],["completed","bb"]]
// enum_in_val_nohlman: {"aa":"completed","bb":"running"}
}

Expected vs. actual results

I would expect both cases to look in same format:
enum_in_key_nohlman: {"stopped","aa","completed","bb"}
enum_in_val_nohlman: {"aa":"completed","bb":"running"}

Minimal code example

#include <iostream>
#include <string>
#include <map>
#include "common/include/json/json.h"

using njson = nlohmann::json;

// example enum type declaration
enum TaskState {
    TS_STOPPED,
    TS_RUNNING,
    TS_COMPLETED,
    TS_INVALID=-1,
};

// map TaskState values to JSON as strings
NLOHMANN_JSON_SERIALIZE_ENUM( TaskState, {
    {TS_INVALID, nullptr},
    {TS_STOPPED, "stopped"},
    {TS_RUNNING, "running"},
    {TS_COMPLETED, "completed"},
})

int main(int argc, char ** argv) {

	std::map<TaskState, std::string> t1 = {{TS_STOPPED, "aa"}, {TS_COMPLETED, "bb"}};
	std::map<std::string, TaskState> t2 = {{"aa", TS_COMPLETED}, {"bb", TS_RUNNING}};

	printf("enum_in_key_nohlman: %s\n", njson(t1).dump().c_str());
	printf("enum_in_val_nohlman: %s\n", njson(t2).dump().c_str());


// when enum is in key the json format changes to unexpected json format.
// enum_in_key_nohlman: [["stopped","aa"],["completed","bb"]]
// enum_in_val_nohlman: {"aa":"completed","bb":"running"}
}

Error messages

unexpected format
enum_in_key_nohlman: [["stopped","aa"],["completed","bb"]]

Compiler and operating system

linux

Library version

3.6.1

Validation

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

[Abastien1734]

If I wanted to work on this, would I just fork and then submit a PR? Do I need to be assigned to work on this?

[t-b]

If I wanted to work on this, would I just fork and then submit a PR?

Yes!

Do I need to be assigned to work on this?

Nope.

[amirghaz]

I'm planning to look into this issue.

[nlohmann]

@amirghaz

Call njson(t1) uses

inline void to_json(BasicJsonType& j, const CompatibleArrayType& arr)

and njson(t2) uses

inline void to_json(BasicJsonType& j, const CompatibleObjectType& obj)

It seems is_constructible_object_type_impl needs to be adjusted to detect TaskState has a conversion to string so it can be used as object key.

[amirghaz]

@nlohmann Sorry for the lack of update, some stuffs came up that prevent me from focusing on this issue. Thanks for the insight, I will look into it right away to see what can be done.

[amirghaz]

@nlohmann
I tried to modify is_compatible_object_type_impl to detect if there is any conversion to string, but there seems to be other error that arise after this modification.

The error messages that I get is:

Also, I notice that this issue only arise if we use a map object as an argument to njson. Using njson({ {TS_STOPPED, "aa"}, {TS_COMPLETED, "bb"} }).dump().c_str() directly will result in the expected conversion to a json object. Any idea why this is the case?

[nlohmann]

Passing an initializer list vs. passing a std::map use different basic_json constructors.

[bronteq]

Hi, i get the same wrong result when int key is used

std::map<int, std::string> map;
map[1] = "aaa";
map[2] = "bbb";
map[3] = "ccc";

std::string test = nlohmann::json(map).dump();
// returns "[[1,"aaa"],[2,"bbb"],[3,"ccc"]]"

[nlohmann]

Hi, i get the same wrong result when int key is used

std::map<int, std::string> map;
map[1] = "aaa";
map[2] = "bbb";
map[3] = "ccc";

std::string test = nlohmann::json(map).dump();
// returns "[[1,"aaa"],[2,"bbb"],[3,"ccc"]]"

This is expected behavior for integer keys. This issue addresses enum object keys.