问题
Is it possible to make class type, which is:
- trivially move-constructible, but not trivially copy-constructible, but still copy-constructible
- trivially copy-constructible, but not trivially move-constructible, but still move-constructible
- trivially copy-assignable, but not trivially move-assignable, but still move-assignable
- trivially move-assignable, but not trivially copy-assignable, but still copy-assignable
- trivially copy/move-constructible, but not trivially copy/move-assignable, but still copy/move-assignable and vice versa
- etc (if any)
?
How they would look like?
The question make sense for developing, say, std::variant-like or std::tuple-like class.
The question is arised after looking through this article.
回答1:
To answer what I think is the heart of your question:
The question make sense for developing, say, std::variant-like or std::tuple-like class
There are 3 kinds of class-types that are of concern here viz.
- Owner Types
- Regular Types
- Other types (Functions, Mutexes)
In the following assume both assign & construct suffixes are present for move/copy. I will refer to non-trivial operations as clone-:
Owner Types: These are Movable & Clone-able ( deep copy or shallow copy are the choices to be made)
Stereotypical examples are std::unique_ptr, std::shared_ptr
Regular Types(int-like): These are Movable & Copy-able. Stereotypical examples are int, std::vector. A lot more details for these types can be found at What is a "Regular Type" in the context of move semantics? & in Elements Of Programming.
Other types (Functions, Mutexes etc.): These types refuse to be put into one of the clean classes above eg. Functions may refer to a pointer, a lambda with captures etc.
As far as all the other combinations of trivially copy-able/movable are concerned while you can create such classes I think they are more like curiosities rather things you will see in any useful scenario.
I have put std::vector in Regular Types rather than Owner Types even though std::vector manages memory since its interface is designed to be as close to a regular type as possible. You can often design a Regular interface for a type which is superficially non-regular. In such scenarios reasoning about code is easier if you use a regular interface.
回答2:
- yes
- yes
- yes
- yes
You'll need to be more clear on 5.
Trivial means (among other things) not user-provided. So simply explicitly writing the default implementation takes something from trivial to non-trivial.
http://eel.is/c++draft/class#class!trivially copyable
http://eel.is/c++draft/class.copy#25
来源：https://stackoverflow.com/questions/39607635/trivially-moveable-but-not-trivially-copyable