Re: Is C++ used in life-critical systems?

From:
Michael Doubez <michael.doubez@free.fr>
Newsgroups:
comp.lang.c++
Date:
Thu, 13 Jan 2011 06:08:49 -0800 (PST)
Message-ID:
<b21a1ab2-aa5e-4faf-8a0b-38301bad2f51@f20g2000vbc.googlegroups.com>
On 13 jan, 12:26, Richard Kettlewell <r...@greenend.org.uk> wrote:

Michael Doubez <michael.dou...@free.fr> writes:

Richard Kettlewell <r...@greenend.org.uk> wrote:

James Kanze <james.ka...@gmail.com> writes:

On Jan 12, 2:30 am, Adam Skutt <ask...@gmail.com> wrote:

You're free to create a FILE,


Are you? I don't have my copy of the C standard available to
check, but I was pretty sure that just a "struct FILE;" in
stdio.h was all that was required. (Most implementations do
provide a more complete specification, in order to implement
some of the smaller functions as macros, but this is not
required by the standard.)


7.19.1 requires that FILE 'is an object type'. Object types are
disjoint from incomplete types (6.2.5).

So I think you're free to write 'FILE x;' if you really want...


So it seems although you are not guaranteed to have the real
underlying structure (and probably you don't).


No, other way round. It cannot be an incomplete type.


I can use polymorphism and have FILE as a complete type:

In header:

struct _Base_FILE
{
 // Nothing
};
typedef _Base_FILE FILE;

And in implementation:

struct _File_FILE
{
   struct _Base_FILE _base;
   // other stuff related to file handling
};

struct _Stdio_FILE
{
  struct _Base_FILE _base;
  // other stuff related to standard I/O handling
};

And then, allocation function will provide either
  * a file stream

FILE* fopen(...)
{
  _File_FILE* f = (_File_FILE*) malloc(...);

  return &f->_base;
};

  * a stdio stream
_Stdio_FILE _stdout;
FILE* stdout = &_stdout._base;

In practice, I expect the base will contain function pointers to hande
the real underlying type and f*() function will delegate it to those
members..

In the Standard,
27.8.1(Note 305): In C FILE must be a typedef. In C++ it may be a
typedef or other type name.


I don't see how that removes the requirement that it be an object type.


If it can be any type name, it can be an incomplete type. AKAIS, in C+
+, it could be a void which is not an object type.

--
Michael

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