std::pair wrong serialization

[tiregram]

What is the issue you have?

My pair is not serialized as espected and a

Please describe the steps to reproduce the issue.

std::pair<std::pair<std::string, int>, std::pair<std::string, float>> p = {
      {"a", 2}, {"a", 2}}; 
  nlohmann::json j;
  j = p;
  std::cout << j << "\n"; // i get {a:2} but i want [["a":2],["a",2]]

What is the expected behavior?

i want [["a":2],["a",2]] and not {"a":2}

And what is the actual behavior instead?

i get {"a":2} and if i serialized/deserialized, I lost my data and I get a execution error:
[json.exception.type_error.304] cannot use at() with object.

Which compiler and operating system are you using?

.

• Compiler: gcc (GCC) 10.2.0
• Operating system: Linux 5.4.96-1-lts

Which version of the library did you use?

• [ x] latest release version 3.9.1

If you experience a compilation error: can you compile and run the unit tests?

• yes
• no - please copy/paste the error message below

[Ric3cir121]

MUORI COGLIONE Il mar 23 feb 2021, 10:37 Tiregram <notifications@github.com> ha scritto:

…

What is the issue you have? My pair is not serialized as espected and a Please describe the steps to reproduce the issue. std::pair<std::pair<std::string, int>, std::pair<std::string, float>> p = { {"a", 2}, {"a", 2}}; nlohmann::json j; j = p; std::cout << j << "\n"; // i get {a:2} but i want [["a":2],["a",2]] What is the expected behavior? i want [["a":2],["a",2]] and not {"a":2} And what is the actual behavior instead? i get {"a":2} and if i serialized/deserialized, I lost my data and I get a execution error: [json.exception.type_error.304] cannot use at() with object. Which compiler and operating system are you using? . - Compiler: gcc (GCC) 10.2.0 - Operating system: Linux 5.4.96-1-lts Which version of the library did you use? - [ x] latest release version 3.9.1 If you experience a compilation error: can you compile and run the unit tests </~https://github.com/nlohmann/json#execute-unit-tests>? - yes - no - please copy/paste the error message below — You are receiving this because you are subscribed to this thread. Reply to this email directly, view it on GitHub <#2655>, or unsubscribe </~https://github.com/notifications/unsubscribe-auth/AOWD6E7XX6677OSOB4HFJ2DTANZMFANCNFSM4YCENEMQ> .

[fschuetz04]

This is working as intended, objects are preferred over arrays when both are possible to create from the given data. If you explicitly want to create an array, use nlohmann::json::array:

#include <iostream>

#include "json.hpp"

int main() {
  nlohmann::json j;
  j = nlohmann::json::array({{"a", 2}, {"a", 2}});
  std::cout << j << std::endl;
}

[tiregram]

not this is a bug due to the utilization of the string as first argument
The std::tuple and std::pair systems is perfectly support as the exception of case when the first argument of pair is std::string.

In this situation json will consider it as a dict and not as a array.

[tiregram]

with this bug a serialisation/deserialisation will failled and not provide the same c++ structure.
if one of your pair have the same value your value will be lost.
Of course a know how to make a array in json. The bug is in the transfomation of std::tuple std::pair in the json equivalent data.

[nlohmann]

Lists of pairs create JSON objects if the first element is a string. This should be documented properly as this example shows that the results may be surprising. However, changing this behavior would be a breaking change.

[koniarik]

This problematic for us that have complex structures, I have a mechanism that exports a std::variant of std::tuples of various lengths, something like:

std::variant<   std::tuple<int,int,int>, std::tuple< std::string, int >, std::tuple< T, U, K, M >, .... >;

The second tuple will get serialized differently, which is problematic and counterintuitive behavior. (But I understand the backwards compability concerns)

The real problem is that even though I force it to be arrays, it is still converted to object later. That means that I can't prevent this from happening. (I would end up with one tuple being converted to object and rest being arrays)

Thanks to people on irc ##C++-general@freenode I have minimal example of the problematic situation:

#include "nlohmann/json.hpp"
#include <iostream>
#include <tuple>

using json = nlohmann::json;

struct T {
    std::tuple< std::string, int > tpl;
};

void to_json(json & j, const T & val )
{
     j = json::array({ std::get<0>(val.tpl), std::get<1>(val.tpl) });
     std::cout << "tmp res: " << j << "\n";
}

int main()
{
    T item{std::tuple< std::string, int >{"test", 123}};
    std::cout << json{item}.dump() << '\n';
}

This outputs array in tmp res and object in the latter output. In case you add just one item to the array, the conversion does not happen. The desired situation is of course that the array won't convert.

Is there some way to prevent this from happening?

[koniarik]

After some discusison on IRC, there is one way to avoid this:

std::cout << json{item}.dump() << '\n';
std::cout << json(item).dump() << '\n';

The () constructor does not convert the item into an object, {} does.
That solves my problem, but I would at least like to ask for a warning in the doc.

[nlohmann]

I added a FAQ entry for this bug: https://json.nlohmann.me/home/faq/#brace-initialization-yields-arrays