{} uses copy constructor, while = does not

[jllansford]

Just a little annoyance I encountered while writing to_json functions for a large number of in house classes. Ideally I'd like it if the {} operator behaved the same as the = operator.

Given the following code:

using json = nlohmann::json;

class A {
private:
	std::string          _val;

	// by including the mutex we've implicitly deleted our copy constructor 
	std::recursive_mutex _mutex;

public:
	A() : _val("Hello World") {};

	friend void to_json(json &j, const A &a) {
		j = {
			{"val", a._val}
		};
	}
};



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

	json j = {};
	A a;

        // This line compiles and runs completely fine.
        j["data"]["A"] =  a;

        //this line will fail to compile, due to the implicitly deleted A::A(const A&)
	j["data"] = {{"A", a}};
	
        std::cout << j.dump() << std::endl;
	return 0;
}

[theodelrieu]

Could you paste the complete error output? Knowing where the copy happens would help a lot :)

[nlohmann]

Xcode:

main.cpp:36:24: error: call to implicitly-deleted copy constructor of 'A'
    j["data"] = {{"A", a}};
                       ^
main.cpp:13:26: note: copy constructor of 'A' is implicitly deleted because
      field '_mutex' has an inaccessible copy constructor
    std::recursive_mutex _mutex;
                         ^
./json.hpp:6785:22: note: passing argument to parameter 'args' here
    json_ref(Args... args)
                     ^
1 error generated.

[theodelrieu]

Oops, missing forwarding references it seems

[nlohmann]

Aha? I have no idea how to fix this.

[theodelrieu]

I'll fix it by tonight, if there is no surprises with json_ref.

[nlohmann]

Thanks a lot! I have no time right now to wrap my head around this...

[nlohmann]

@jllansford PR #807 fixed the issue. Could you please check if this works for you?

[jllansford]

I've been away for a few days but got around to testing the fix today and it works great.

Thanks for the prompt reply and fix.