Const reference to a temporary object - Why?

[Dave Rahardja]

Hi all,

Although the following definition is legal:

const int& i = 5;

....and that the lifetime of the temporary variable to which i refers is
identical to i itself, why would anyone want to do this instead of a simple

const int i = 5;

....?

I can see how binding a const reference to a temporary object is necessary
(such as when passing an rvalue to a function expecting a const reference),
but the above usage perplexes me.

-dr

[Victor Bazarov]

Dave Rahardja wrote:

Although the following definition is legal:

const int& i = 5;

...and that the lifetime of the temporary variable to which i refers
is identical to i itself, why would anyone want to do this instead of
a simple

const int i = 5;

...?
You should ask somebody who did/does that.
I can see how binding a const reference to a temporary object is
necessary (such as when passing an rvalue to a function expecting a
const reference), but the above usage perplexes me.

There is nothing perplexing in that code by itself. It _would_ be
seriously perplexing in a production codebase. I have never seen
anything like that in production code.

V

[Andrej Hristoliubov]

> I have never seen anything like that in production code.

V

Dave -

Do listen to Victor, I neither had seen this in production not
Victor's code. And I know Victor for quite some time now (Vitya is it
25 or 30 years in total is the duration of our acquaintance and
bestfriendship? ).

[Pete C]

> Dave -

Do listen to Victor, I neither had seen this in production not
Victor's code. And I know Victor for quite some time now (Vitya is it
25 or 30 years in total is the duration of our acquaintance and
bestfriendship? ).

That is great advice. If anyone with your level of expertise in c++ has
experience in IRC, MIME and POP3 protocols I am happy to accept Job
Application for position in the US. Applicants must have skills of a
management of a team of the programmers.

[Dave Rahardja]

On Fri, 28 Oct 2005 23:24:40 -0400, "Victor Bazarov" <v.********@com Acast.net>
wrote:

Dave Rahardja wrote:

Although the following definition is legal:

const int& i = 5;

...and that the lifetime of the temporary variable to which i refers
is identical to i itself, why would anyone want to do this instead of
a simple

const int i = 5;

...?

You should ask somebody who did/does that.

I did, and his reasons (optimization) turned out to be misguided.

I can see how binding a const reference to a temporary object is
necessary (such as when passing an rvalue to a function expecting a
const reference), but the above usage perplexes me.

There is nothing perplexing in that code by itself. It _would_ be
seriously perplexing in a production codebase. I have never seen
anything like that in production code.

Neither have I, and I prevented one instance from creeping into our codebase
(by talking my colleague out of using it for (false) optimization).

What's perplexing is that such behavior as the lifetime of a temporary bound
to a const reference is defined at all. Stroustrup even gave an example of it
in C++PL (Section 5.5, pg 98 in 3rd ed). Seeing that the C++ language doesn't
seem to support constructs that are not either useful or maintain backward
compatibility, I'm wondering why such behavior is even defined at all.

I'm just trying to figure out the motivation behind the behavior, that's all.

-dr

[Jonathan Mcdougall]

Dave Rahardja wrote:

On Fri, 28 Oct 2005 23:24:40 -0400, "Victor Bazarov" <v.********@com Acast.net>
wrote:

Dave Rahardja wrote:

Although the following definition is legal:

const int& i = 5;

...and that the lifetime of the temporary variable to which i refers
is identical to i itself, why would anyone want to do this instead of
a simple

const int i = 5;

...?

You should ask somebody who did/does that.

I did, and his reasons (optimization) turned out to be misguided.

I can see how binding a const reference to a temporary object is
necessary (such as when passing an rvalue to a function expecting a
const reference), but the above usage perplexes me.

There is nothing perplexing in that code by itself. It _would_ be
seriously perplexing in a production codebase. I have never seen
anything like that in production code.

Neither have I, and I prevented one instance from creeping into our codebase
(by talking my colleague out of using it for (false) optimization).

What's perplexing is that such behavior as the lifetime of a temporary bound
to a const reference is defined at all. Stroustrup even gave an example of it
in C++PL (Section 5.5, pg 98 in 3rd ed). Seeing that the C++ language doesn't
seem to support constructs that are not either useful or maintain backward
compatibility, I'm wondering why such behavior is even defined at all.

I'm just trying to figure out the motivation behind the behavior, that's all.

Consistency.
Jonathan

[Victor Bazarov]

Dave Rahardja wrote:

On Fri, 28 Oct 2005 23:24:40 -0400, "Victor Bazarov"
<v.********@com Acast.net> wrote:

Dave Rahardja wrote:

Although the following definition is legal:

const int& i = 5;

...and that the lifetime of the temporary variable to which i refers
is identical to i itself, why would anyone want to do this instead
of a simple

const int i = 5;

...?

[..]

I'm just trying to figure out the motivation behind the behavior,
that's all.

I don't understand what special motivation except consistency you
might need. If you think that initialising a const reference by
binding to a temporary (and thus prolonging the life of the object)
is OK when passing to a function or as a member of another object,
then why not stand-alone?

V

[Alf P. Steinbach]

* Victor Bazarov:

Dave Rahardja wrote:

On Fri, 28 Oct 2005 23:24:40 -0400, "Victor Bazarov"
<v.********@com Acast.net> wrote:

Dave Rahardja wrote:
Although the following definition is legal:

const int& i = 5;

...and that the lifetime of the temporary variable to which i refers
is identical to i itself, why would anyone want to do this instead
of a simple

const int i = 5;

...?
[..]

I'm just trying to figure out the motivation behind the behavior,
that's all.

I don't understand what special motivation except consistency you
might need. If you think that initialising a const reference by
binding to a temporary (and thus prolonging the life of the object)
is OK when passing to a function or as a member of another object,
then why not stand-alone?

The point is that for a function call, or in an initializer list, and
especially in a function return, the temporary's life is not prolonged.
C++ adds _special_ support for local references to const. And then one
can view binding in function calls / init lists as just a consequence,
but why allow binding rvalues to local references in the first place?
The rules would be just as simple if only binding to arguments was
allowed for rvalues. I think the rules would be _simpler_, and also C++
compilation, and it would, I think, help to detect some obscure bugs.

I've tried to think of optimization scenarios, e.g. an 'extern "C"'
function returning a huge C struct, but no, the feature doesn't seem to
be able to help the compiler.

Perhaps the explanation is the same as (this is what's been stated by
folks Who Should Know) for the non-support of elision of copy
construction for arguments: some committee member(s) had or knew of
code, perhaps some company's zillion line application or perhaps a
popular compiler, that depended on having the language the way it's now
standardized.

--
A: Because it messes up the order in which people normally read text.
Q: Why is it such a bad thing?
A: Top-posting.
Q: What is the most annoying thing on usenet and in e-mail?

[Dave Rahardja]

On Sun, 30 Oct 2005 02:12:52 GMT, al***@start.no (Alf P. Steinbach) wrote:

The point is that for a function call, or in an initializer list, and
especially in a function return, the temporary's life is not prolonged.
C++ adds _special_ support for local references to const. And then one
can view binding in function calls / init lists as just a consequence,
but why allow binding rvalues to local references in the first place?
The rules would be just as simple if only binding to arguments was
allowed for rvalues. I think the rules would be _simpler_, and also C++
compilation, and it would, I think, help to detect some obscure bugs.

Right. The special extension of the temporary's lifetime is what got me
scratching my head.

Oh well, it may be one of those things that "just are".

-dr

[bjarne]

The rules for references are simply the most general and uniform I
could find. In the cases of arguments and local references, the
temporary lives as long as the reference to which it is bound. One
obvious use is as a shorthand for a complicated expression in a
deeplynested loop. For example:

for (int i = 0; i<xmax; ++i)
for (int j = 0; j< ymax; ++j) {
double& r = a[i][j];
for (int k = 0; k < zmax; ++k) {
// do something with a[i][j] and a[i][j][k]
}
}

This can improve readability as well as run-time performance.
-- Bjarne Stroustrup; http://www.research.att.com/~bs