How can we justify that initializer list is better in performance
than assignment list in constructor of C++ ??
than assignment list in constructor of C++ ??
-
-
Re: initializer Vs assignment list in constructor
On Tue, 15 Jan 2008 22:51:15 -0800 (PST), Pallav singh
<singh.pallav@g mail.comwrote in comp.lang.c++:
Why would we want to justify it? The C++ standard does not specify orHow can we justify that initializer list is better in performance
than assignment list in constructor of C++ ??
require such a thing.
An initializer list is the preferred way to initialize the members in
a constructor. It is the only possible way to initialize reference
members and constant members. So just use initializer lists.
--
Jack Klein
Home: http://JK-Technology.Com
FAQs for
comp.lang.c http://c-faq.com/
comp.lang.c++ http://www.parashift.com/c++-faq-lite/
alt.comp.lang.l earn.c-c++
-
Re: initializer Vs assignment list in constructor
On Jan 16, 8:09 am, Jack Klein <jackkl...@spam cop.netwrote:On Tue, 15 Jan 2008 22:51:15 -0800 (PST), Pallav singh
<singh.pal...@g mail.comwrote in comp.lang.c++:How can we justify that initializer list is better in
performance than assignment list in constructor of C++ ??Why would we want to justify it? The C++ standard does not
specify or require such a thing.You might add that by using an initializer list, you reduce theAn initializer list is the preferred way to initialize the
members in a constructor. It is the only possible way to
initialize reference members and constant members. So just
use initializer lists.
chances of accidentally using the variable before it has been
initialized. It's part of the larger philosophy of never
defining a variable without initializing it.
--
James Kanze (GABI Software) email:james.kan ze@gmail.com
Conseils en informatique orientée objet/
Beratung in objektorientier ter Datenverarbeitu ng
9 place Sémard, 78210 St.-Cyr-l'École, France, +33 (0)1 30 23 00 34
Comment
-
Re: initializer Vs assignment list in constructor
On 2008-01-16 08:09, Jack Klein wrote:No it does not, but it does guarantee some other things that makes itOn Tue, 15 Jan 2008 22:51:15 -0800 (PST), Pallav singh
<singh.pallav@g mail.comwrote in comp.lang.c++:
>>>How can we justify that initializer list is better in performance
>than assignment list in constructor of C++ ??
Why would we want to justify it? The C++ standard does not specify or
require such a thing.
very probably that using the initialiser-list if slightly more
efficient. Consider a class like this:
class Foo
{
Bar bar;
Baz baz;
public:
Foo(int a,int b);
}
And assume that both Bar and Baz each have constructors taking an
integer, a default constructor, and a assignment operator.
Before the body of the constructor starts to execute all members of the
class have to be initialised (so that they are complete objects when the
body of the constructor executes). If you want bar and baz to be objects
created with a and b passed to their constructors and do not use an
initialisation-list you would have to do something like this:
Foo::Foo(int a, int b)
{
bar = Bar(a);
baz = Baz(b);
}
Or alternatively use bar.set(a) or something like that. This means that
in a worst case scenario you have to run the constructors of Bar and Baz
twice just to create one Foo object. If you use an initialisation-list
instead bar and baz will be constructed with the correct parameters
before the body of Foo's constructor is executed.
You can never get better performance by not using initialisation-lists,
but you can sometimes get the same performance.
--
Erik Wikström
Comment
-
Re: initializer Vs assignment list in constructor
On Jan 16, 6:58 pm, Erik Wikström <Erik-wikst...@telia. comwrote:On 2008-01-16 08:09, Jack Klein wrote:On Tue, 15 Jan 2008 22:51:15 -0800 (PST), Pallav singh
<singh.pal...@g mail.comwrote in comp.lang.c++:How can we justify that initializer list is better in
performance than assignment list in constructor of C++ ??Why would we want to justify it? The C++ standard does not
specify or require such a thing.Or not. The cases where there is a measurable difference areNo it does not, but it does guarantee some other things that
makes it very probably that using the initialiser-list if
slightly more efficient.
probably fairly rare.
[...]I wouldn't say never---if I tried, I'm sure I could write someYou can never get better performance by not using
initialisation-lists, but you can sometimes get the same
performance.
perverse code where default construction followed by assignment
was faster than direct construction. Most of the time, however,
there's just no difference.
Performance isn't the motivation for initializer lists, however.
The motivation is security---not having uninitialized variables
floating around. (This isn't quite true, as the initialization
expressions can reference other member variables---including
those not yet initialized. But using initialization lists does
reduce the risk enormously.)
--
James Kanze (GABI Software) email:james.kan ze@gmail.com
Conseils en informatique orientée objet/
Beratung in objektorientier ter Datenverarbeitu ng
9 place Sémard, 78210 St.-Cyr-l'École, France, +33 (0)1 30 23 00 34
Comment
-
Re: initializer Vs assignment list in constructor
How can we justify that initializer list is better in performancethan assignment list in constructor of C++ ??
--
Marc
Comment
Comment