Code fails with linker error. Why?

[John Colvin via Digitalmars-d-learn]

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:
>>> class ShapeSurface(T) {
>>> public:
>>> 	int formula();
>>
>> that means you have a definition of formula elsewhere (which 
>> the linker tries to find, but obviously fails. What you want is
>>
>> class ShapeSurface(T) {
>> public:
>> 	abstract int formula();
>
> Thanks, but still. The compiler shall not let that code pass 
> down to the linker. It has everything it needs to not do that, 
> or it shall have. Linker errors shall be simply because the 
> libraries are not in place (either not installed, either 
> linking path not correctly configured, either broken versions 
> of those libraries).

Sorry, but that's just not how it works. There is no requirement 
for the definition of a function to be found in the same 
compilation unit as it's declaration.

// a.d
void foo();
void main() { foo(); }

// b.d
import std.stdio;
void foo() { writeln("Hello World"); }

$ ls
a.d     b.d

$ dmd b.d -c

$ ls
a.d     b.d     b.o

$ dmd a.d b.o

$ ls
a       a.d     a.o     b.d     b.o

$ ./a
Hello World


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.