core.sys.posix.unistd link error

[Jonathan M Davis]

On Sunday, September 22, 2013 13:52:54 Ruslan Mullakhmetov wrote:
> But now i need to sort out what the difference between
> // global scope
> 
> int a = 10;

That directly initializes the variable at compile time, meaning that whatever 
is used to initialize the variable must be callable at compile time. And the 
value must be able to be set at compile time and then be carried over to 
runtime. That will work with int, but it does not work with most stuff that's 
on the heap (like classes or AAs) - arrays would be the major exception to 
that, since they can be set at compile time (and I believe that it was 
recently changed so that immutable classes could be set at compile time, but 
not const or mutable ones - implementing that is rather complicated, and it 
may or may not ever happen). Over time, what you can do at compile time with 
CTFE (Compile Time Function Evaluation) has improved, but there are still 
restrictions, and some things will never be possible (e.g. I/O or calling C 
functions).

> and
> 
> int a;
> 
> static this()
> {
>   a = 10;
> }

That does not set the variable at compile time. Rather, the static constructor 
sets it at runtime. So, this has none of the restrictions that directly 
initializing a module or static variable does. However, it does have the 
downside that two modules that have static constructors can't import each 
other (either directly or indirectly), because then the runtime wouldn't know 
which order to run them in. If you do that, you'll get an exception at runtime 
complaining about a circular import (which sucks, but unfortunately, the 
circular import can't always be detected at compile time - thanks in part to 
.di files - so runtime detection is the best that can be done). So, while 
static constructors can be really nice, you do have to avoid having modules 
that use them import each other, which means either being careful about how 
your modules import each other or avoiding static constructors. Which is 
easier depends on your code.