Thread: Using binary write to save a struct

Hi


I'm still reading Prata's book and there is a section where they explain how to write files using the binary mode.


They define a struct

Code:

const int LIM = 20;
struct planet
{
char name[LIM]; 
double population;
double g; 
};
planet pl;

And they say you can use the binary mode to write the whole struct at once (in text mode you need to specify every member).

Code:

ofstream fout("planets.dat",
ios_base:: out | ios_base::app | ios_base::binary);
fout.write( (char *) &pl, sizeof pl);

I don't understand why they use

Code:

fout.write( (char *) &pl, sizeof pl)

instead of just

Code:

fout.write( pl, sizeof pl)

I would assume that fout.write() requires a (char *) parameter for the buffer address. (void *) may also work.

Originally Posted by oogabooga

Firstly, passing pl wouldn't work at all since that would pass all the data in the object. You need to pass in the address of the data, not the data itself; hence the &.

The (char*) cast (or a more appropriate C++ style cast) is necessary to match the address types. The write() method knows nothing of your object and just treats it as a sequence of bytes. A sequence of bytes is essentially a char array; hence the (char*).

Good explanation