Json: error when using nlohmann::json, std::function and std::bind

[xyz1001]

What is the issue you have?

g++ 4.8.5 and nlohmann/3.7.3, error: value-initialization of reference type 'const nlohmann::basic_json<>&' when compiler example

#include <iostream>
#include <functional>
#include <map>

#include "nlohmann/json.hpp"

void Foo(const nlohmann::json &j) {
    std::cout<<j.dump()<<std::endl;
}

using Handler = std::function<void(const nlohmann::json&)>;

int main() {
    std::map<int, Handler> handlers;
    handlers.emplace(1, std::bind(Foo, std::placeholders::_1));

    for(auto &i : handlers) {
        i.second(nlohmann::json{{"data", "123"}});
    }
}

Please describe the steps to reproduce the issue.

g++ test.cpp -o main

Can you provide a small but working code example?

here is the simplest example online:

godbolt

What is the expected behavior?

compile pass

And what is the actual behavior instead?

compiler error

Which compiler and operating system are you using?

• Compiler: g++ 4.8.5, but g++ 8 and 9.1 is ok
• Operating system: cross compiler for embed linux, host is Manjaro 20.0.1, target os arch is armv7, gcc 4.8.5

Which version of the library did you use?

• [x] latest release version 3.7.3
• [x] other release - please state the version: nlohmann/3.7.0
• [ ] the develop branch

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

• [ ] yes
• [ ] no - please copy/paste the error message below
• [x] I don't try

[nlohmann]

Can you please try to compile the test suite? As the used compiler (GCC 4.8.5) is quite old, this may be a bug in its C++11 implementation.

[xyz1001]

I tried to cross build unit test and it is ok. @nlohmann

[xyz1001]

I used jsonformoderncpp/3.4.0@vthiery/stable before. This version is ok for gcc 4.8.5. However, this vesion is not maintained by conan official. nlohmann_json jsonformoderncpp

[FrancoisChabot]

This is caused by json's template<typename ValueType> operator ValueType() const;, which ends up getting invoked as part of the process of trying to match a json object to the const json& parameter of the bound function.

see #958 for discussion on the topic.

edit I managed to isolate the specific part of the bind process that's failing:

This produces the same error:

void foo() {
    nlohmann::json v;
    std::tuple<nlohmann::json&> x{v};
}

[FrancoisChabot]

In the end, this boils down to the following question:

Should the following be able to compile?

bool test = std::is_default_constructible<std::tuple<int&>>::value;

https://gcc.godbolt.org/z/6vwHuM

I'd say that the answer is evidently yes. And it's pretty obvious to me that it's GCC 4.8.5 that's broken here.

Now the only remaining question is: Is it worthwhile to work around this compiler bug or not? I tried sprinkling a few remove_reference in the code path to no avail. So the answer will most likely be of non-trivial complexity.

My take would be: if we get rid of the allegedly evil operator T(), then this goes away, otherwise, I don't think fixing this is worth the trouble.

[stale[bot]]

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.

[nlohmann]

Maybe related: #2226

[stale[bot]]

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.