Code fails with linker error. Why?

[John Colvin via Digitalmars-d-learn]

On Saturday, 4 October 2014 at 10:37:40 UTC, eles wrote:
> On Saturday, 4 October 2014 at 10:27:18 UTC, John Colvin wrote:
>> On Saturday, 4 October 2014 at 04:02:46 UTC, eles wrote:
>>> On Friday, 3 October 2014 at 15:47:33 UTC, John Colvin wrote:
>>>> On Friday, 3 October 2014 at 15:44:16 UTC, eles wrote:
>
>> So the compiler has no way of knowing whether you've forgotten 
>> to include a definition, or whether it's simply sitting in 
>> some other object file / library / whatever. The linker, 
>> however, can know, hence the linker error.
>
> No "extern" required?...

nope, no extern required. The extern you are referring to is the 
storage class http://dlang.org/declaration.html#extern and is for 
variables.

> This seem to allow an entire class of problems, by linking 
> against long-time forgotten functions...

It's the core functionality that allows you to link to a function 
in a library without having the entire source available. You have 
a declaration, but the implementation is in another object file / 
library.

D is slightly tighter than C wrt all this. In C, you can just 
call functions without even having a declaration, the compiler 
will assume the argument types from what you passed in. A 
forgotten #include and compiling without warnings = horrific bugs 
in C. In D the declaration is required.