fread/fwrite problems!

fread/fwrite problems!

Karadoc ~~

Member #2,749

September 2002

Yes, members of structs are never reordered; polymorphism in C relies on it!

struct BASE_CLASS
{
  int a, b, c;
};
struct DERIVED_CLASS
{
  int a, b, c;
  float extra_data;
};

Polymorhism in C is done by type casting to and from stuff that looks like that.

eg.

float my_function(struct BASE_CLASS *foo)
{
  float bar = foo->a * foo->b;
  if (foo->c == 5)
  {
    bar*=((DERIVED_CLASS*)foo)->extra_data;
  }
  return bar;
}

-----------

Michael Jensen

Member #2,870

October 2002

Erm that doesn't seem relibable. Did I miss something or is the struct automatically padded? And how much padding is it safe to rely on, is there?

1struct sta
2{
3  int a, b, c;
4};
5 
6struct stb
7{
8  int a, b, c;
9  int d[5120];
10};
11 
12void this_shouldnt_work_should_it() //?
13{
14  sta A;
15  stb * B = (stb)&A;
16 
17  B->d[4000] = 15;
18}

How can one be gauranteed that enough padding will be added?

HoHo

Member #4,534

April 2004

That one definitely shouldn't work. If it compiles then it should segfault if you try writing to the char array

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Bob

Free Market Evangelist

September 2000

Quote:

Since doubles/floats are defined specifically by IEEE, why arn't they platform/etc dependant?

In addition to what gillius said, not all computers use IEEE floats.

--
- Bob
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Michael Jensen

Member #2,870

October 2002

Quote:

should segfault

Right, that's what I'd expact.

By those principals -- Why can what Karadoc~~ posted be guaranteed to work?

da_flo

Member #1,907

February 2002

Karadoc~~'s code is garanteed to work if c==5 only the type of the object is DERIVED_CLASS. It would be more clear if the variable was named 'type'. It's just a matter of design. It's up to the programmer to ensure the types are right.
To perform polymorphism in C you have to keep track of the type of the object anyway.

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Michael Jensen

Member #2,870

October 2002

Quote:

if (foo->c == 5)

Ahh, I didn't see that before.

Evert

Member #794

November 2000

Quote:

Why can what Karadoc~~ posted be guaranteed to work?

You can acces the elements common to sta and stb, but you cannot acces elements specific to stb by casting sta to stb.

Think about it: sta is the base, stb is a derived.

Michael Jensen

Member #2,870

October 2002

Right that's what I thought, then I saw Karadocs code and I thought he was implying that you could -- I didn't see the c==5 thing....