[Rcpp-devel] Initialization through constructor or through assignment?

[Dirk Eddelbuettel]

On 21 October 2011 at 15:12, Douglas Bates wrote:
| Perhaps this is an indication that I should read some sections of "C++
| Annotations" again.  I am trying to remember the pros and cons of
| initializing a class instance in C++ through assignment or through the
| copy constructor.  So, for example, if I have a class "foo" and a
| function "bar" that returns a foo.  Am I better off writing
| 
| foo   baz(bar());
| 
| or
| 
| foo   baz = bar();
| 
| I have tended to prefer the former, but not for any good reasons, at
| least as far as I can remember.
| 
| As a much more trivial example, in C I would write
| 
| int one = 1;
| 
| but in C++ I tend to write
| 
| int one(1);
| 
| Any comments one way or the other?

For the C example, I think the compiler optimizes that the right way even if
we write it in a suboptimal way.  For the initialization, I will gladly admit
that I do not know either.   I always suspect the second 'copy' form is
worse.  But when I (trying to be empirical ;-) set up a test, it seems to
win, albeit by a tiny, tiny, tiny margin that may not matter all that much.

R> suppressMessages(require(inline))
R> suppressMessages(require(rbenchmark))
R> suppressMessages(require(microbenchmark))
R> 
R> ctrFun <- cxxfunction(signature(xs="numeric"), plugin="Rcpp", body='
+    Rcpp::NumericMatrix x(xs);
+    return R_NilValue;
+ ')
R> 
R> cpyFun <- cxxfunction(signature(xs="numeric"), plugin="Rcpp", body='
+    Rcpp::NumericMatrix x = Rcpp::NumericMatrix(xs);
+    return R_NilValue;
+ ')
R> 
R> M <- matrix(seq(1.0, 100.0^2, by=1.0), 100, 100)
R> benchmark(ctrFun(M), cpyFun(M), order="relative", replications=1e6)
       test replications elapsed relative user.self sys.self user.child sys.child
2 cpyFun(M)      1000000   5.358  1.00000      5.34     0.00          0         0
1 ctrFun(M)      1000000   5.387  1.00541      5.35     0.01          0         0
R> microbenchmark(ctrFun(M), cpyFun(M), times=1e6)
Unit: nanoseconds
       expr  min   lq median   uq      max
1 cpyFun(M) 1211 1314   1358 1426 49533807
2 ctrFun(M) 1202 1301   1346 1415 49842042
R> 


Dirk


-- 
"Outside of a dog, a book is a man's best friend. Inside of a dog, it is too
dark to read." -- Groucho Marx