To avoid some linking errors

[Walter Bright]

On 10/29/2012 2:28 PM, Jesse Phillips wrote:
 > On Sunday, 28 October 2012 at 13:39:26 UTC, bearophile wrote:
 >> This code compiles with no errors, and then later the linker gives a "Symbol
 >> Undefined":
 >>
 >>
 >> abstract class A {
 >>     public void foo();
 >> }
 >> class B : A {}
 >> void main() {}
 >
 > Interestingly, adding abstract to the method will result in no linker error for
 > compiling.

That's because by saying "abstract" you're telling the compiler that there is no 
implementation for A.foo(), which is fundamentally different from saying that 
A.foo() is defined elsewhere. The compiler inserts a 0 in the vtbl[] slot for 
it, even though it won't let you try to call it.

 > And if a new B is created then a compiler error is provided instead
 > of a linker error.

Abstract types are really a different thing from "it's defined somewhere else."

 > I'm for getting some line numbers over the less informative linker error.

The object file format does not support line numbers for symbol references and 
definitions. None of the 4 supported ones (OMF, ELF, Mach-O, MsCoff) have that. 
Even the symbolic debug info doesn't have line numbers for references, just for 
definitions.