Read struct from file in C++ with multiple types

Question

I'm using the code below to read a struct from a binary file.

struct A
{
  int s;
  char cname[20];
};

int main () {
  A mystruct;

  ifstream is;
  is.open ("filename.txt", ios::binary );

  is.read ( &mystruct, sizeof(InpumystructtStruct) );
  is.close();

  return 0;
}

Suppose there are multiple structs in the binary file, of type struct A, struct B and struct C, with different size appearing in the file. If I want to read all of them in sequence, assuming I know the sequence, would this be correct?

struct A
{
  int s;
  char cname[20];
};
struct B
{
  int s;
};    
struct A
{
  char cname[20];
};

int main () {
  A mystructa;
  B mystructb;
  C mystructc;

  ifstream is;
  is.open ("filename.txt", ios::binary );

  while( is.good()  ) {
      // determine struct to use 
      is.read ( &mystruct/*put right type*/, sizeof(/*put right tupe*/) );
  }
  is.close();

  return 0;
}

wallyk · Accepted Answer · 2014-05-15 23:06:39Z

1

No, that won't work.

If you know they were written sequentially in the file:

struct {
     A a;
     B b;
     C c;
} v;

is.read (&v, sizeof v);

However, this has many problems. Not the least of which is that there could be structure padding, size of int used by the writer, endian problems, etc.

These sorts of concerns are all addressed by universal data formats such as JSON, XML, etc.

Slightly more portably, this variation of what you wrote would work, though with the same problems as before:

A mystructa;
B mystructb;
C mystructc;

is.read (&mystructa, sizeof mystructa);
is.read (&mystructb, sizeof mystructb);
is.read (&mystructc, sizeof mystructc);

edited May 15, 2014 at 23:06

answered May 15, 2014 at 19:21

wallyk

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