The D language specification under "Global and static
initializers" [1], says the following:
The Initializer for a global or static variable must be
evaluatable at compile time. Whether some pointers can be
initialized with the addresses of other functions or data
is implementation defined. Runtime initialization can be
done with static constructors.
What is the rationale of making this implementation defined?
What's the range of possible behaviors? Are there any
circumstances in which a pointer can't be initialized with an
address of a function or data? If so, couldn't a subset of cases
have an explicitly defined, portable behavior?
As far as I've tested this, DMD, GDC and LDC can handle static
initialization of pointers to functions or data (except that LDC
fails if function pointer(s) are obtained via
__traits(getUnitTests, module_name)), even across separately
compiled modules, which is consistent with a similar feature of C
and C++.
IIUC, the C standard always allows such initialization. In 6.6
Constant expressions, N1570 [2] says:
7 More latitude is permitted for constant expressions
in initializers. Such a constant expression shall be,
or evaluate to, one of the following:
— an arithmetic constant expression,
— a null pointer constant,
— an address constant, or
— an address constant for a complete object type
plus or minus an integer constant expression.
and
9 An address constant is a null pointer, a pointer to an
lvalue designating an object of static storage duration,
or a pointer to a function designator; it shall be created
explicitly using the unary & operator or an integer constant
cast to pointer type, or implicitly by the use of an
expression of array or function type. The array-subscript []
and member-access . and -> operators, the address & and
indirection * unary operators, and pointer casts may be used
in the creation of an address constant, but the value of an
object shall not be accessed by use of these operators.
[1] https://dlang.org/spec/declaration.html#global_static_init
[2] http://iso-9899.info/n1570.html
