Ternary operator crash

[parfenuk]

This simple code crashes:
string someStr;
json j = json::object();
// the next line crashes
j["someKey"] = someStr.empty() ? nullptr : someStr;

However, if I use standard if-else condition, everything is fine:
string someStr;
json j = json::object();
// the code below works ok
if (someStr.empty()) j["someKey"] = nullptr;
else j["someKey"] = someStr;

Also if S is not empty - both versions work.
Is it a bug or feature? :)

I'm using GNU C++ 14 compiler, IDE Xcode 9.3

[nlohmann]

The types in someStr.empty() ? nullptr : someStr seems to be treated as const char* then yields j["someKey"] = (const char*)nullptr; which calls std::string(nullptr) which yields the error.

I have no idea how to tell the compiler that someStr.empty() ? nullptr : someStr should be treated as json other than to make it explicit:

j["someKey"] = someStr.empty() ? json(nullptr) : json(someStr);

[gregmarr]

A single type is deduced based on the types of the two values of the ternary operator. There is no type that works properly for nullptr and std::string here. That requirement is not there for the if/else. If you want the type to be null for an empty someStr instead of an empty string, you need to use the if/else. Otherwise, there's no point in even having the check, and you can simply do j["someKey"] = someStr;.

[parfenuk]

Yes, I indeed need the field being null for empty strings, and I was able to solve it.
Thank you, guys, for telling me two different solutions and for an information about how ternary operator works.