schmidt@bonnie.ics.uci.edu (Douglas C. Schmidt) (08/26/88)
Hi,
I've been browsing through Stroustrup's C++ book and came
across the following puzzling construct on page 34:
Vec& sum = *new Vec(s);
I don't believe that this use of the new allocator (preceded by the
'*') is described anywhere else in the book. Could someone please
explain the meaning of this, in the context of the function
depicted below?
Vec operator+(Vec a, Vec b) {
int s = a.size();
if (s != b.size()) error("foo");
Vec& sum = *new Vec(s);
int *sp = sum.v;
int *ap = a.v;
int *bp = b.v;
while (s--) *sp++ = *ap++ + *bp++;
return sum;
}
thanks!
Doug Schmidt
--
schmidt@bonnie.ics.uci.edu (ARPA)
"If our behavior is strict, we do not need fun." -Zippy th' Pinhead
"If our behavior is struct, we do not need defun." -Anonkanner@Apple.COM (Herbert Kanner) (08/26/88)
In article <646@paris.ICS.UCI.EDU> schmidt@bonnie.ics.uci.edu (Douglas C. Schmidt) writes: >Hi, > > I've been browsing through Stroustrup's C++ book and came >across the following puzzling construct on page 34: > >Vec& sum = *new Vec(s); > >I don't believe that this use of the new allocator (preceded by the >'*') is described anywhere else in the book. Could someone please >explain the meaning of this, in the context of the function >depicted below? > >Vec operator+(Vec a, Vec b) { > int s = a.size(); > if (s != b.size()) error("foo"); > Vec& sum = *new Vec(s); > int *sp = sum.v; > int *ap = a.v; > int *bp = b.v; > while (s--) *sp++ = *ap++ + *bp++; > return sum; >} > It is probably impossible to give a short explanation that will be understandable. If my explanation makes no sense to you, study section 3.2.6 on the "new" operator and section 2.3.10 on references. Also, I suggest you simply not worry about anything that is not understood in chapter 1 until you have read the rest of the book three times. Having said that: Vec& sum = *new Vec(s); is explained as follows. sum is declared as a reference to Vec--that is what Vec& is. The assigment is an initialization. Initialization of a reference to Vec must by assigning a Vec. The "new" operator returns a pointer to the requested type, namely a pointer to Vec. The "*" dereferences this pointer, yielding the Vec that is required. Herb Kanner Apple Computer, Inc. {idi, ios, nsc}!apple!kanner kanner@apple.apple.com
rich@jpl-devvax.JPL.NASA.GOV (Richard Pettit) (08/27/88)
> >Vec& sum = *new Vec(s); Lets start from the top. Reference is just generally confusing. Consider Pascal: (forgive the formatting) program plugh (output); var i : integer; procedure xyzzy (var a : integer); begin a := 3; end; begin xyzzy(i); end. Note that in this example, the value "3" will be "copied back" into variable "i" upon return from xyzzy. This is because "a" is a REFERENCE variable. Now look at the C++ version: void xyzzy(int &a) { a = 3; } main() { int i; xyzzy(i); } This does the same thing. The C version: void xyzzy(int *a) { *a = 3; } main() { int i; xyzzy(&i); } makes explicit use of the pointer. The REFERENCE notation in C++ simply eliminates the need for making pointers to everything. Variables which are made reference variables are treated like regular non-pointer variables (like Pascal). The C++ declaration: Vec& sum = *new Vec(s); is such because new "returns" a pointer. The REFERENCE variable "sum" is treated syntactically like a structure, and the value assigned to it must also be a structure. Hence the de-reference on the "new" allocator. Just as the notation Vec a; Vec &b = a; assigns "a reference to `a'" to "b", the previous statement assigns "a reference to an anonymous heap space variable" to "sum". I've probably confused the issue even worse. Rich -- "Macrosolutions to Megaproblems" -- Voivod rich@jpl-devvax.Jpl.Nasa.Gov