map with incomplete value type
c++, c++11, dictionary, libc++, std
Solution
The standard requires that, all types used in template components of the standard library must be complete, unless otherwise stated. Thus libc++'s error is correct. Using an incomplete type is undefined behavior (§17.6.4.8[res.on.functions]/2), so libstdc++ accepting it isn't wrong either.
You could use a pointer object to construct the complete type as you did. But you could also use some third-party collections library which allows incomplete types, e.g. `boost::container::map` is designed to allow incomplete types.
§17.6.4.8 Other functions / 2:
In particular, the effects are undefined in the following cases:
...
if an incomplete type (3.9) is used as a template argument when instantiating a template component, unless specifically allowed for that component.
(The only components that explicitly allow incomplete types are `declval`, `unique_ptr`, `default_delete`, `shared_ptr`, `weak_ptr` and `enable_shared_from_this`.)
Problem
I'm getting an error with the following: ``` class Test { std::map<std::string,Test> test; }; ``` The error is "Field has incomplete type 'Test'". I read a few threads with suggested this might be a bug in the version of libcxx which ships with xcode, but it wouldn't surprise me at all if I just have to change it to: ``` class Test { std::map<std::string,std::shared_ptr<Test>> test; }; ``` I just wanted to double check that this is definitely a correct error and not a bug. Cheers!