Binary files

[jorgen]

Hello! I'm trying to learn how to use binary files instead of text files (supposedly they will take up less space?) and I have the following small test program:

int main() {

	int i;
	double d;
	char c;
	ofstream bout;
	bout.open("bina.dat", ios::binary);
	for(i=0;i<10;i++) {
		d=i*0.5;
		bout.write((const char *) &d, 4);
	}
	bout.close();
	
	ifstream bin;
	bin.open("bina.dat", ios::binary);
	while(bin.read((char *) &d, 4)) cout << d << endl;
	bin.close();

	return 0;
}

But when I run it it prints out 4.5 ten times (a similar one just printing out i , i.e. 0,...,9 worked fine). Any idea what the problem is?

Also, is there a way to condense line 9 and 10 into one line, that is to make it faster by doing the calculation and printing it on one go?

Jorgen

EDIT: small mistake

[Peter87]

Are you sure that sizeof(double) == 4?

[jorgen]

Noooooooo that might be 16:)

Thanks!

[Peter87]

It could be 16 but it is probably 8. Instead if guessing you can use sizeof(double) in the code.

[jorgen]

Ha, I always thought it was 16. sizeof(double) is safer I guess but in the end I will use this in a code where speed is crucial, and that would be a tiny bit slower right? (for a lot and lot of repetitions)

[Peter87]

No it would not be slower.

[jorgen]

Ok. Is that because the compiler optimizes it so the program doesn't have to find the size of double every time, or something like that?

[Peter87]

The value of sizeof(double) is known at compile time. It's not much of an optimization.

[jorgen]

Ok. I have a lot to learn.. On a related note, any idea why the following doesn't work?

void test(ofstream *bout) {
	int i;
	double d;
	for(i=0;i<10;i++) {
		d=i*0.5;
		*bout.write((const char *) &d, sizeof(double));
	}
	*bout.close();
	
	ifstream bin;
	bin.open("bina.dat", ios::binary);
	while(bin.read((char *) &d, sizeof(double))) cout << d << endl;
	bin.close();
}

int main() {

	ofstream bout;
	bout.open("bina.dat", ios::binary);
	test(&bout);


	return 0;
}

Somehow passing the ofstream to a function I get the error " Request for member 'write' in 'bout', which is of non-class type 'std::ofstream*' ." I don't understand why ofstream now can't be used like this.

[Peter87]

*bout.write((const char *) &d, sizeof(double));
is evaluated as
*(bout.write((const char *) &d, sizeof(double)));
so you have to put parentheses around *bout to make it work.
(*bout).write((const char *) &d, sizeof(double));
or you can use the arrow syntax (usually more convenient)
bout->write((const char *) &d, sizeof(double));

[jorgen]

That worked, thanks a lot!