Question

I came across the statement:

outbal.write( (char*) &acc , sizeof( struct status ) );

outbal is an object of ofstream and status is a type.

Therefore:

struct status {
  // code
};

status acc;

Talking about the second line I don’t understand the first argument, which is (char*) &acc. What are we doing and how?

Answer 1

(char*)&acc if the address of the variable acc, cast to a char pointer so as to be compatible with ostream::write(). It is that variable that is being written, to the outbal stream, for a length of sizeof(struct status).

ostream::write takes a memory address and a length and will output that memory to the specified stream. In other words, you’re simply outputting the entire memory contents of the acc variable.

Your code is similar to:

struct xyz {int a; float b; void *c};

ostream os("myfile.dat");

struct xyz abc; // 'struct' not technically needed in C++

os.write (  (char *)abc,      sizeof (struct xyz));
//        <<-memory addr->>   <<-----size----->>