Re: how to implement a function in a different D source file

[mipri via Digitalmars-d-learn]

On Tuesday, 26 November 2019 at 03:06:52 UTC, Omar wrote:
Hey, I'm very interested in this programming language, I 
already prefer it to C++ and if only i had adopted it years 
ago, but that's beside the point.

I read a bunch of tuts today and the only thing I'm really 
stuck on at the moment is how to actually implement a function 
in a different file.

the page here https://dlang.org/spec/function.html
suggests you can implement a function in a different file, and 
a different tutorial somewhere else (or maybe the same site or 
even same page idr) mentioned the endeavour of 
no-bodied-functions as a way of presenting a black-box type of 
interface.

But with the whole module namespace thing I'm a bit at a loss 
how to do this.
Say I have main.d ex.d and ex2.d and I want to import ex, and 
call a function it
declares, and implement the function in ex2.d and link the 
objects at the end so the linker is happy. I don't know how to 
do this and I can't find a straigh answer anywhere or any 
examples.

so ?

First, here's exactly what you're asking for:

  $ cat ex1.d
  enum string admin = "Dave";

  $ cat ex2.d
  import ex1;

  string greeting() {
      return "Hello, " ~ admin;
  }

  $ cat main.d
  import ex2;

  void main() {
      import std.stdio: writeln;
      writeln(greeting);
  }

Three files that all import what they need, and can be
compiled separately and then linked.

  $ dmd -c ex1
  $ dmd -c ex2
  $ dmd -c main
  $ dmd -ofmain main.o ex2.o ex1.o
  $ ./main
  Hello, Dave

And dmd isn't cheating, as you can see by skipping the first
two compiles:

  $ rm *.o
  $ dmd -c main
  $ dmd -ofmain main.o
  /usr/bin/ld: main.o: in function `_Dmain':
  main.d:(.text._Dmain[_Dmain]+0x5): undefined reference to 
`_D3ex28greetingFZAya'
  collect2: error: ld returned 1 exit status
  Error: linker exited with status 1

But you could also just compile everything together:

  $ rm *.o
  $ dmd -ofmain *.d
  $ ./main
  Hello, Dave

If all of your code is in D, my received understanding is that
this is a completely reasonable option.

You could also use dub as your build system:

  Package recipe format (sdl/json) [json]:
  Name [main]:
  Description [A minimal D application.]:
  Author name [mipri]:
  License [proprietary]: MIT
  Copyright string [Copyright © 2019, mipri]:
  Add dependency (leave empty to skip) []:

This'll create a new directory named 'main', and you can build
and run a 'hello world' by running 'dub'.

After moving some files around...

  $ cat source/movie/references/names.d
  module movie.references.names;

  enum string admin = "Dave";

  $ cat source/movie/references/lines.d
  module movie.references.lines;

  string cantDoThat() {
      import movie.references.names: admin;
      return "I'm afraid I can't do that, " ~ admin;
  }

  $ cat source/app.d
  void main() {
      import movie.references.lines: cantDoThat;
      import std.stdio: writeln;
      writeln(cantDoThat);
  }

Building and running it:

  $ dub
  Performing "debug" build using dmd for x86_64.
  main ~master: building configuration "application"...
  Linking...
  Running ./main
  I'm afraid I can't do that, Dave