with and use

with and use

[David Rasmussen]

First of all, what does this message from GNAT mean:

"with" can only appear in context clause

?

Secondly, where is with and use typically used, and what is the 
difference between them?

/David

Re: with and use

[Jean-Pierre Rosen]

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"David Rasmussen" <pinkfloydhomer@yahoo.com> a �crit dans le message news: 3D2C2FF9.4020300@yahoo.com...
> First of all, what does this message from GNAT mean:
>
> "with" can only appear in context clause
>
A context clause is a clause that appears *before* a compilation unit. A with clause cannot appear *inside* anything else, so this
basically means that you have to move the with clause on top of the unit.

> Secondly, where is with and use typically used, and what is the
> difference between them?
>
A with clause means that a compilation unit needs the services from another *compilation unit* (i.e. a package, subprogram or
generic which is separately compiled).

A use clause factors out a *package*, i.e. you don't have to repeat the package name in front of every element.
Since a package can (but needs not) be a compilation unit, there are cases where both clauses can be applied, but they are really
orthogonals.

--
---------------------------------------------------------
           J-P. Rosen (rosen@adalog.fr)
Visit Adalog's web site at http://www.adalog.fr

Re: with and use

[David Rasmussen]

Jean-Pierre Rosen wrote:
 > "David Rasmussen" <pinkfloydhomer@yahoo.com> a �crit dans le message
 > news: 3D2C2FF9.4020300@yahoo.com...
 >
 >> First of all, what does this message from GNAT mean:
 >>
 >> "with" can only appear in context clause
 >>
 >
 > A context clause is a clause that appears *before* a compilation
 > unit. A with clause cannot appear *inside* anything else, so this
 > basically means that you have to move the with clause on top of the
 > unit.
 >

Thanks!

 >
 >> Secondly, where is with and use typically used, and what is the
 >> difference between them?
 >>
 >
 > A with clause means that a compilation unit needs the services from
 > another *compilation unit* (i.e. a package, subprogram or generic
 > which is separately compiled).
 >
 > A use clause factors out a *package*, i.e. you don't have to repeat
 > the package name in front of every element. Since a package can (but
 > needs not) be a compilation unit, there are cases where both clauses
 > can be applied, but they are really orthogonals.
 >

So with corresponds roughly to an #include of some header file that 
defines prototypes etc. for some other compilation unit in C++, and use 
corresponds roughly to "using namespace Whatever", assuming that the 
things included were in a seperate namespace?

"with" and only appear at the top of a unit. Where can "use" appear?

/David

Re: with and use

[Jean-Pierre Rosen]

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"David Rasmussen" <pinkfloydhomer@yahoo.com> a �crit dans le message news:
> So with corresponds roughly to an #include of some header file that
> defines prototypes etc. for some other compilation unit in C++, and use
> corresponds roughly to "using namespace Whatever", assuming that the
> things included were in a seperate namespace?
Roughly :-)

> "with" and only appear at the top of a unit. Where can "use" appear?
>
At any place a declaration can appear, and like any declaration, its effects extends from where it is declared to the corresponding
"end". In addition, it can also appear in a context clause.

--
---------------------------------------------------------
           J-P. Rosen (rosen@adalog.fr)
Visit Adalog's web site at http://www.adalog.fr

Re: with and use

[Jeffrey D. Cherry]

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David Rasmussen <pinkfloydhomer@yahoo.com> wrote in news:3D2C3A69.8070207
@yahoo.com:

> Jean-Pierre Rosen wrote:
> > "David Rasmussen" <pinkfloydhomer@yahoo.com> a �crit dans le message
> > news: 3D2C2FF9.4020300@yahoo.com...
> >

<snip>

> >> Secondly, where is with and use typically used, and what is the
> >> difference between them?
> >>
> >
> > A with clause means that a compilation unit needs the services from
> > another *compilation unit* (i.e. a package, subprogram or generic
> > which is separately compiled).
> >
> > A use clause factors out a *package*, i.e. you don't have to repeat
> > the package name in front of every element. Since a package can (but
> > needs not) be a compilation unit, there are cases where both clauses
> > can be applied, but they are really orthogonals.
> >
> 
> So with corresponds roughly to an #include of some header file that 
> defines prototypes etc. for some other compilation unit in C++, and use 
> corresponds roughly to "using namespace Whatever", assuming that the 
> things included were in a seperate namespace?
> 
> "with" and only appear at the top of a unit. Where can "use" appear?
> 
> /David
> 

You are quite correct here to associate the Ada "use" clause with the C++ 
"using namespace" using-directive.  You are also reasonably correct regarding 
the "with" clause, but I would qualify the C++ equivalent a bit further.  I 
would say the Ada "with" clause more closely corresponds to a #include of 
some header file that defines a namespace containing function prototypes 
and/or other declarations.

The Ada "with" clause should appear in what would be the C++ global scope if 
C++ were limited to a single class, namespace, or function per translation 
unit.  The Ada "use" clause can appear in what would be the C++ global scope 
or in the C++ block scope.

One final note would be that the Ada "use" clause is only valid for packages.  
It cannot be used for a "with"ed subpgrogram or generic unit.  If the generic 
unit is a package, the "use" clause can be issued after the instantiation.  
For example:

   with Some_Generic_Package;

   procedure Main is

      package Instance is new Some_Generic_Package( ... );
      use Instance;

   begin -- Main
      null;
   end Main;

This shows the "with" clause pulling a generic package (a C++ template unit) 
into the Main procedure's context, instantiating the package in the 
declarative region, and then making the contents of the new package directly 
visible via a "use" clause (in a C++ block scope).

Hope that helps!


-- 
Regards,
Jeffrey D. Cherry
Senior IV&V Analyst
Northrop Grumman Information Technology

Re: with and use

[Gautier direct_replies_not_read]

> > "with" and only appear at the top of a unit. Where can "use" appear?

>At any place a declaration can appear, and like any declaration, its 
>effects extends from where it is declared to the corresponding
>"end". In addition, it can also appear in a context clause.

Maybe these examples could help to understand the "why".

(1)

with GL, GLU, GLUT, GLOBE_3D, Game_control, Level_Manager;

procedure GloDemo1 is
  ...
  -- Here I don't want to open visibility
  ...
  procedure Display is
    use GL, GLOBE_3D; -- <-- Here but just here I want to open
                      -- visibility of the full GL bindings
  begin
    -- No need of "GL."

    Clear( COLOR_BUFFER_BIT or DEPTH_BUFFER_BIT );

    PushMatrix;
    Translate ( -eye(0), -eye(1), -eye(2) );

    Rotate( rot(0), 1.0, 0.0, 0.0 );
    Rotate( rot(1), 0.0, 1.0, 0.0 );
    Rotate( rot(2), 0.0, 0.0, 1.0 );

    ...

    PopMatrix;

    GL.Flush;
    GLUT.SwapBuffers;
  end Display;
  ...

(2)
with Ada.Text_IO; use Ada.Text_IO;

procedure xxx(f:File_Type) is
begin
  null;
end xxx;

(3)
with Ada.Text_IO;

procedure xxx(f:Ada.Text_IO.File_Type) is
begin
  null;
end xxx;

____________________________________________________________
Gautier  --  http://www.mysunrise.ch/users/gdm/index.htm#Ada

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Re: with and use

[Ted Dennison]

"Jean-Pierre Rosen" <rosen@adalog.fr> wrote in message news:<aghg1g$r9b$1@s1.read.news.oleane.net>...
> "David Rasmussen" <pinkfloydhomer@yahoo.com> a �crit dans le message news:
> > So with corresponds roughly to an #include of some header file that
> > defines prototypes etc. for some other compilation unit in C++, and use
> > corresponds roughly to "using namespace Whatever", assuming that the
> > things included were in a seperate namespace?
> Roughly :-)

Yeah, in the same way that a hacksaw roughly corresponds to a
chainsaw: You generally use them for the same kinds of tasks, but
thinking of them as the *exact* same thing can be hazardous to your
health.


> > "with" and only appear at the top of a unit. Where can "use" appear?
> >
> At any place a declaration can appear, and like any declaration, its effects 
> extends from where it is declared to the corresponding
> "end". In addition, it can also appear in a context clause.

The use clause is very contraversial, as it effectivly flattens a
namespace that was (possibly) designed to be hierarchal. I generally
suggest that beginners avoid the "use" clause (although not "use
type"). Practice a while using full dot notation until you are
competent enough with Ada to intelligently decide where you stand on
the issue.