Phew, a lot of classes were affected by this. The rationale for the
changes are as follows:
- When a type X declares any constructor, the default constructor is
not declared, so there is no need for X() = delete there. In fact,
there is brutal difference between those two: deleted members
participate in overload resolution, but not-declared members do not!
- When a type X wants to be non-copyable (e.g. to be only movable, like
threads, unique_ptrs, and so on), you can either do it by inheriting
from boost::noncopyable (the old way), or better declare both (!) the
copy constructor _and_ the copy assignment operator as deleted:
X(X const&) = delete;
X& operator=(X const&) = delete;
We had tons of types with deleted copy constructors that were lacking
a corresponding deleted copy assignment operator, making them still
copyable and you wouldn't even notice (read: scary)!
References:
- http://accu.org/content/conf2014/Howard_Hinnant_Accu_2014.pdf
- http://www.boost.org/doc/libs/master/libs/core/doc/html/core/noncopyable.html
Note: I know, I'm quoting Hinnant's extraordinary slides a lot, but
getting the sematic right here is so incredibly important.
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