Re: binary file parsing

From:

James Kanze <james.kanze@gmail.com>

Newsgroups:

comp.lang.c++

Date:

Mon, 25 May 2009 13:53:25 -0700 (PDT)

Message-ID:

<5fc19517-0544-4250-9d39-714487324d83@z7g2000vbh.googlegroups.com>

On May 25, 4:29 pm, Mateusz Loskot <see...@signature.net> wrote:

James Kanze wrote:

On May 24, 11:04 pm, Mateusz Loskot <see...@signature.net> wrote:

// Name the cast properly, to self-document the code
template<typename T>
inline char* as_buffer(T& d)
{
return static_cast<char*>(static_cast<void*>(&d));
}

I think that there are cases where It still won't work (at
least in some cases, with g++)---even though the standard
says it should. (I think that there is an option in g++
which will make it work. But since the overall approach is
broken anyway, there's no point.)

Do you mean the cast is broken or the template?
Could you explain what's broken in as_buffer?

The fact that it just copies memory, instead of formatting.

// Read binary data from input stream
template <typename T>
inline void read_n(T& d, std::istream& s, std::streamsize const& n)
{
   if (!s)
      throw std::runtime_error("input stream not readable");

   s.read(as_buffer(d), n);

   // good idea to check state of s
   // one may need to handle endianness and swap bytes
}

Example, read 4 bytes from file stream ifs

int n = 0;
read_n(n, ifs, sizeof(n));

Of course, you won't necessarily read the same value that
was written, unless it was written by the same binary,
running on the same machine.

Of course. I've indicated byte order may be an issue.

Amongst other things. Size, representation and padding are also
issues.

It should be also possible to specialize read_n function so
it's not necessary to give number of bytes explicitly.

Or just use sizeof in the function.

The only real problem is that the code only works in very
special cases.

I'm curious, because I've found it quite portable across
various implementations of C++ on Windows and Unix.

You've just not tested it.

--
James Kanze (GABI Software) email:james.kanze@gmail.com
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