c++ - In-class initialization of static data members -

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in c++, static members may not initialized in class body these exceptions:

  • static members of const integral type can be
  • static members of constexpr literal type must be

can explain why these exceptions?

also, holds:

even if const static data member initialized in class body, member ordinarily should defined outside class definition.

this never understood @ all. what's point of definition?

just trying intuitions here.

why can there initializer in class definition?

concerning 2 exceptions const , constexpr static data members:

[class.static.data]/3

[ note: in both these cases, member may appear in constant expressions. — end note ]

i.e. initializer, may use them in constant expressions, e.g.

struct s {     static std::size_t const len = 10;     int arr[len]; }; std::size_t const s::len;

if len wasn't initialized in class definition, compiler couldn't know value in next line define length of arr.

one argue allowing initializers of non-const, non-constexpr static data members in class definition, interfere initialization order:

[basic.start.init]/2

definitions of explicitly specialized class template static data members have ordered initialization. other class template static data members (i.e., implicitly or explicitly instantiated specializations) have unordered initialization. other non-local variables static storage duration have ordered initialization.

that is, order of definitions including initializers important. order of (dynamic) initialization of non-local objects defined within translation unit, reason why there has definition including initializer non-const, non-constexpr static data members.

what's point of definition?

this has been answered in comments imo. might want add odr, is, name external linkage, static data member must (only) defined in 1 translation unit (if it's odr-used). it's programmer choose translation unit.


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