Dub / Derelict confusion

[Paul via Digitalmars-d-learn]

On Friday, 21 November 2014 at 01:22:34 UTC, Mike Parker wrote:
> On 11/21/2014 5:57 AM, Paul wrote:
>
>>
>> This is a tad off topic now but I'm struggling to get a window 
>> on screen
>> as SDL_CreateWindow is failing, here's what I've got:
>>
>> try{
>>     DerelictSDL2.load();
>> }catch(Exception e){
>>     writeln("Failed to load DerelictSDL2 :( %s", e.msg);
>>     return;
>> }
>
> Before getting to your issue, I'd like to point out that this 
> is a needless redundancy. You are adding anything by catching 
> an exception here and printing to the console that the library 
> didn't load. The exception message will tell you that already. 
> I suggest you just remove the try..catch and let the exception 
> propagate. The only reason to catch a DerelictException here is 
> if you want to do something other than print the message to the 
> console, like display a message box or write to a log file.
>
>
>>
>> try{
>>     SDL_Init(SDL_INIT_EVERYTHING);
>> }catch(Exception e){
>>      writeln("Can't init SDL :( %s", e.msg);
>>      return;
>> }
>>
>
> SDL_Init isn't goint to throw an exception. It can't. It's a 
> function in a C library and C doesn't know anything about D 
> exceptions. If you look at the documentation for SDL_Init[1], 
> you will find that it returns a negative number on failure and 
> 0 on success.
>
> if( SDL_Init( SDL_INIT_EVERYTHING ) < 0 ) {
>  // See below for how to handle this
> }
>
>>
>> SDL_Window *testWindow;
>> //temporary magic numbers ahoy
>> testWindow = SDL_CreateWindow( "Test Window", 
>> SDL_WINDOWPOS_UNDEFINED,
>> SDL_WINDOWPOS_UNDEFINED, 800, 600, SDL_WINDOW_RESIZABLE );
>> if( testWindow == null ) {
>>     writeln("Window could not be created!");
>>     return;
>> }     else {
>>     SDL_Surface* testSurface;
>>     testSurface = SDL_GetWindowSurface(testWindow);
>> }
>>
>> Everything seems to initialise ok but SDL_CreateWindow always 
>> returns
>> null. For some reason I can't use DerelictException which I 
>> guess might
>> help me dig deeper. AFAIK there are no issues with this 
>> machine's
>> hardware - I can certainly run other SDL-based apps on it.
>>
>
> Again, DerelictExceptions aren't going to help you here. They 
> are only thrown when the load function fails (to manipulate a 
> DerelictException directly, import derelict.util.exception.) To 
> get an error message out of SDL, you need to call SDL_GetError, 
> which returns a const( char )*.
>
> void printSDLError( string msg ) {
>   import std.conv : to;
>   import std.stdio : writefln;
>
>   writefln("%s: %s", msg, to!string( SDL_GetError() ));
> }
>
> ...
> if( testWindow == null )
> {
>   printSDLError( "Failed to create window." );
>   return;
> }
>
> I prefer to wrap it up in a subclass of Error. Others may 
> prefer Exception, but when I throw an SDLError I don't intend 
> to catch it. I really want the app to exit.
>
> class SDLError : Error
> {
>     public static string errStr() @property
>     {
>         import std.conv : to;
>         import derelict.sdl2.sdl : SDL_GetError;
>         return to!string( SDL_GetError() );
>     }
>
>     public this( string msg, string file = __FILE__, size_t 
> line = __LINE__ )
>     {
>         import std.string : format;
>
>         auto fmsg = format( "%s: %s", msg, errStr );
>         super( fmsg, file, line );
>     }
> }
>
> Now it becomes:
>
> DerelictSDL2.load();
>
> if( SDL_Init( SDL_INIT_EVERYTHING ) < 0 )
> {
>   throw new SDLError( "Failed to initialize SDL" );
> }
>
> testWindow = SDL_CreateWindow( ... );
> if( !testWindow )
> {
>   throw new SDLError( "Failed to create window" );
> }
>
> Whichever approach you choose, SDL_GetError will shed light on 
> why any SDL call failed as long as you call it immediately upon 
> failure.
>
> [1] 
> https://wiki.libsdl.org/SDL_Init?highlight=%28%5CbCategoryInit%5Cb%29%7C%28CategoryEnum%29%7C%28CategoryStruct%29

Thank you for that extensive reply :D As you can tell I'm not 
very familiar with exceptions (or D!) so I need to digest that 
little lot. Outcome to follow... :D