Implicit conversion issues

[ukhegg]

Hi!
I'm using your library in my projects and i really like it.
But on the other hand it's basic_object<> implicit conversions from string is a source of bugs and long debugging sessions. I had two such sessions today.
Take a look. Some time ago a had a settings_provider class and a test mock

class settings_provider{
   virtual std::string get_settings(int id) const;
};  
class settings_provider_mock : public settings_provider{
   MOCK_CONST_METHOD1(get_settings, std::string(_));
};

Then,when using this mock,i setup it to return some json-formatted string

 EXPECT_CALL(*sp_mock ,get_settings(_)).WillOnce(Return("{ \"key\"=\"value\" }"));

Some time later i changed settings_provider and corresponding mock interface to

class settings_provider{
   virtual basic_json<> get_settings(int id) const;
};
class settings_provider_mock : public settings_provider{
   MOCK_CONST_METHOD<N>(get_settings, basic_json<>(_));
};

But the EXPECT_CALL continued to compile,but now get_settings returns json object with value "{ "key"="value" }".
So, my suggestion is to make conversion from string(and maybe other types) explicit, to avoid such errors in future.

[theodelrieu]

Hi @ukhegg.

I don't think we can mark the constructor explicit (especially since there is only one constructor for almost every type now), this would break everyone's code.

I know that implicit conversions are a source of bugs, but in this case the _json suffix seems to be the best way to fix that kind of issues

[szikra]

I also run into a similar issue when first tried doing something like this:
string smsg((char*)buff, len); json jmsg{smsg}; or json jmsg = smsg; or something.

I think what would help, is adding a note in the documentation here

// create object from string literal
json j = "{ \"happy\": true, \"pi\": 3.141 }"_json;

// or explicitly
auto j3 = json::parse("{ \"happy\": true, \"pi\": 3.141 }");

Like:

// Note the '_json' at the end of the string 
json j = "{ \"happy\": true, \"pi\": 3.141 }";  /// will not parse std::string, you need to use json::parse for that

[nlohmann]

Implicit conversion is one of the major aspect that makes the library usable. Unfortunately, here the "wrong" string-to-JSON conversion has been chosen. I shall add a comment to the documentation to make this clearer.