Forward declaration issue

[Andre via Digitalmars-d-learn]

Hi,

I have a strange issue with following coding.

void baz(); // forward declaration

void foo()
{
void bar()
{
baz(); // (1) without f.d. syntax error
}

void baz()
{
bar();
}

baz(); // (2) No linker error if line is removed
}

void main()
{
foo();
}

Without the forward declaration, there is a syntax error at (1)
With the forward declaration there is no syntax error but
a linker error at (2). This linker error disappears if line at (2)
is removed.
It looks like a bug, is it?

Kin regards
Andre

Re: Forward declaration issue

[Jonathan M Davis via Digitalmars-d-learn]

On Friday, December 04, 2015 08:12:05 Andre via Digitalmars-d-learn wrote:
> Hi,
>
> I have a strange issue with following coding.
>
> void baz(); // forward declaration
>
> void foo()
> {
>   void bar()
>   {
>   baz(); // (1) without f.d. syntax error
>   }
>
>   void baz()
>   {
>   bar();
>   }
>
>   baz(); // (2) No linker error if line is removed
> }
>
> void main()
> {
>   foo();
> }
>
> Without the forward declaration, there is a syntax error at (1)
> With the forward declaration there is no syntax error but
> a linker error at (2). This linker error disappears if line at (2)
> is removed.
> It looks like a bug, is it?

You cannot use symbols before you declare them in a function (even if
they're nested functions), and you can't forward declare them. When you
declare baz outside of foo, bar is now trying to use a different baz from
the one that you declare after it. Rather, it's trying to use one that's at
the module-level, not a nested function. And you never defined that baz. So,
you get a linker error when you use it. What's going on would be clearer if
you used distinct names:


void module_baz();

void foo()
{
void bar() { module_baz(); }
void baz() { bar(); }

baz();
}


While that may not be what you're trying to do, it's what you're actually
doing. Mutually recursive nested functions aren't possible in D.

- Jonathan M Davis

Re: Forward declaration issue

[Artur Skawina via Digitalmars-d-learn]

On 12/04/15 09:12, Andre via Digitalmars-d-learn wrote:
> Hi,
> 
> I have a strange issue with following coding.
> 
> void baz(); // forward declaration
> 
> void foo()
> {
> void bar()
> {
> baz(); // (1) without f.d. syntax error
> }
> 
> void baz()
> {
> bar();
> }
> 
> baz(); // (2) No linker error if line is removed
> }
> 
> void main()
> {
> foo();
> }
> 
> Without the forward declaration, there is a syntax error at (1)
> With the forward declaration there is no syntax error but
> a linker error at (2). This linker error disappears if line at (2)
> is removed.
> It looks like a bug, is it?

No, it's how D is designed -- inside functions the order of
declarations matters (and forward declarations don't work).

Your version wrongly declares another `baz` at module scope,
and, as there's no definition, you end up with the linker error.

Two workarounds:

1) Templatize the functions:

   void foo()
   {
  void bar()()
  {
  baz();
  }

  void baz()()
  {
  bar();
  }

  baz();
   }

2) Use a struct:

   void foo()
   {
  struct Hack {
 void bar()
 {
 baz();
 }

 void baz()
 {
 bar();
 }
  }

  Hack hack;

  hack.baz();
   }

artur

Re: Forward declaration issue

[Andre via Digitalmars-d-learn]

On Friday, 4 December 2015 at 09:51:30 UTC, Artur Skawina wrote:

No, it's how D is designed -- inside functions the order of 
declarations matters (and forward declarations don't work).


Your version wrongly declares another `baz` at module scope, 
and, as there's no definition, you end up with the linker error.


Two workarounds:

1) Templatize the functions:

   void foo()
   {
  void bar()()
  {
  baz();
  }

  void baz()()
  {
  bar();
  }

  baz();
   }

2) Use a struct:

   void foo()
   {
  struct Hack {
 void bar()
 {
 baz();
 }

 void baz()
 {
 bar();
 }
  }

  Hack hack;

  hack.baz();
   }

artur


Thanks for the clarifications and the example.

Kind regards
André