VS 2017 15.6 (March 2018) doesn't seem to understand
`*DeclVal<SharedFontList*>()` anymore.
To work around this issue, the pointed-to type is now extracted in a separate
struct, for which we provide a specialization for raw pointers, so we don't
encounter the shaky `*DeclVal<T*>()` statement anymore.
MozReview-Commit-ID: FuslManbfdB
--HG--
extra : rebase_source : be3813aa9a028e6891cb3de1fc4faae5bde0348e
MakeNotNull is similar to UniquePtr, in that it combines the infallible
allocation and construction of an object on the heap and wraps the (raw or
smart) pointer into a NotNull.
It skips the unnecessary null check from WrapNotNull, and removes the usual
naked 'new' used in many WrapNotNull calls.
MozReview-Commit-ID: UwCrhDnkUg
--HG--
extra : rebase_source : 5a027165fc17ed748783c7ffda03eb421865ad6e
This patch implements mozilla::NotNull, which is similar but not identicial to
gsl::not_null.
The current draft(?) implementation of gsl::not_null is at
https://github.com/Microsoft/GSL/blob/master/include/gsl.h.
The main difference is that not_null allows implicit conversion from T to
not_null<T>. In contrast, NotNull only allows explicit conversion from T to
NotNull<T> via WrapNotNull().
The rationale for this is that when converting from a less-constrained type to
a more constrained type, implicit conversion is undesirable. For example, if I
changed a function f from this:
f(int* aPtr);
to this:
f(gsl::not_null<int*> aPtr);
no call sites would have to be modified. But if I changed it to this:
f(mozilla::NotNull<int*> aPtr);
call sites *would* need to be modified. This is a good thing! It forces the
author to audit the call sites for non-nullness, and encourages them to
back-propagate NotNull throughout the code.
The other difference between not_null and NotNull is that not_null disables
pointer arithmetic, which means it cannot be used with array pointers. I have
not implemented this restriction for NotNull because it seems arbitrary and
unnecessary.
