Initialization different between gcc/clang

[rpav]

What is the issue you have?

Initialization seems to differ between clang and gcc (have not yet tried MSVC):

json j{json::object()};

std::cout << j << std::endl; 
// GCC:   [{}]
// clang: {}

I.e., in gcc this creates an array with an object as its first element; in clang it creates an object.

Please describe the steps to reproduce the issue.

Running example: https://godbolt.org/z/7r76TE

Can you provide a small but working code example?

As above.

What is the expected behavior?

Consistent initialization between compilers for the same code.

And what is the actual behavior instead?

Different values per compiler.

Which compiler and operating system are you using?

• Compiler: clang 10, gcc 10.1
• Operating system: Linux
• std: c++17

Which version of the library did you use?

• latest release version 3.7.3
• other release - please state the version: 3.6.0, 3.8.0, whatever "trunk" is on godbolt
• the develop branch

[nlohmann]

Related: #2046

[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.

[pauljurczak]

Here is another test case (using g++-8.4 with -std=gnu++17):

  auto v{json::to_bson(R"({"s": "string"})"_json)};
  auto j1 = json::from_bson(v);
  auto j2{json::from_bson(v)};
  cout << j1.dump() << endl;
  cout << j2.dump() << endl;
.........
{"s":"string"}
[{"s":"string"}]

[timoscs]

The same bug when parsing from a string, my MVE is

#include "json.hpp"

#include <string>
#include <iostream>

using namespace nlohmann;

int main() {
  std::string data{"{"
          "\"Key1\" : \"value\","
          "\"Key2\" : \"value2\""
        "}"
      };

    json j{json::parse(data)};

    for (const auto & obj : j.items()) {
      std::cout << "Found: " << obj.key() << " value " << obj.value() << "\n";
    }

    std::cout << "Key1 has " << j.at("Key1").get<std::string>() << "\n";
}

On gcc 10.2. the output is

Found: 0 value {"Key1":"value","Key2":"value2"}
terminate called after throwing an instance of 'nlohmann::detail::type_error'
  what():  [json.exception.type_error.304] cannot use at() with array
Key1 has Abort trap: 6

on clang 10.0.1:

Found: Key1 value "value"
Found: Key2 value "value2"
Key1 has value

The expected result would be at least consistent, to me the clang behaviour seems preferable. Tested on latest release, 3.9.1

[timoscs]

And as pointed to in the linked issues, it's actually about the brace initialization. If it were possible to have the clang behaviour on gcc, that would be nice, as then one can just consistently use braces, whereas now attention is needed to whether you're building an array/object with multiple keys, or initializing from an existing json.

Anyway, I can now work around this by using parentheses for the constructors where appropriate, i.e. the following works in the toy example (and can be used in my actual code):

#include  "json.hpp"

#include <string>
#include <iostream>

using namespace nlohmann;

int main() {
  std::string data{"{"
          "\"Key1\" : \"value\","
          "\"Key2\" : \"value2\""
        "}"
      };

    json j(json::parse(data));

    for (const auto & obj : j.items()) {
      std::cout << "Found: " << obj.key() << " value " << obj.value() << "\n";
    }

    std::cout << "Key1 has " << j.at("Key1").get<std::string>() << "\n";
}

[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.

[Ecordonnier]

See the comments in #2583 and https://json.nlohmann.me/home/faq/#brace-initialization-yields-arrays for an explanation as to why the bug was closed as wontfix.