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John Vincent
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 Posted: Thu May 19, 2005 2:11 am Post subject: problem with definition of pop_back() |
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Hi,
I have examined both the first and the second edition of the C++ standard,
and as far as I can see the pop_back() method is not properly defined. I
cannot find anything in the standard that specifies either that it removes
an item from a container, or which item it removes.
Is this an error, or did I miss something?
Thanks
/John Vincent.
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James Dennett
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| Posted: Thu May 19, 2005 8:35 pm Post subject: Re: problem with definition of pop_back() |
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John Vincent wrote:
| Quote: | Hi,
I have examined both the first and the second edition of the C++
standard, and as far as I can see the pop_back() method is not properly
defined. I cannot find anything in the standard that specifies either
that it removes an item from a container, or which item it removes.
Is this an error, or did I miss something?
|
Table 68 defines the operational semantics of pop_back; were
you looking for additional definitions, or is this sufficient?
-- James
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Seungbeom Kim
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| Posted: Thu May 19, 2005 8:35 pm Post subject: Re: problem with definition of pop_back() |
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John Vincent wrote:
| Quote: |
I have examined both the first and the second edition of the C++
standard, and as far as I can see the pop_back() method is not properly
defined. I cannot find anything in the standard that specifies either
that it removes an item from a container, or which item it removes.
Is this an error, or did I miss something?
|
The operational semantics of a.pop_back() is defined as
a.erase(--a.end()), and it is defined for vector, list, and deque.
See Table 68 ("Optional sequence operations") in 23.1.1.
--
Seungbeom Kim
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Bronek Kozicki
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| Posted: Thu May 19, 2005 8:35 pm Post subject: Re: problem with definition of pop_back() |
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"John Vincent" <jpv50 (AT) hotmail (DOT) com> wrote:
| Quote: | I have examined both the first and the second edition of the C++
standard, and as far as I can see the pop_back() method is not
properly defined.
|
in fact, it is not defined at all. This is by design - trying to do two
operations at once (copy last element to function result and then remove
it from top of stack) is bad design (especially from exception safety
POV). This has been discussed in literature, e.g. Herb Sutter
"Exceptional C++"
B.
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Alberto Barbati
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| Posted: Thu May 19, 2005 8:35 pm Post subject: Re: problem with definition of pop_back() |
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John Vincent wrote:
| Quote: | Hi,
I have examined both the first and the second edition of the C++
standard, and as far as I can see the pop_back() method is not properly
defined. I cannot find anything in the standard that specifies either
that it removes an item from a container, or which item it removes.
Is this an error, or did I miss something?
|
Have a look to table 68, in 23.1.1.
Alberto
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Clark S. Cox III
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| Posted: Thu May 19, 2005 8:36 pm Post subject: Re: problem with definition of pop_back() |
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On 2005-05-18 22:11:54 -0400, [email]jpv50 (AT) hotmail (DOT) com[/email] ("John Vincent") said:
| Quote: | Hi,
I have examined both the first and the second edition of the C++
standard, and as far as I can see the pop_back() method is not properly
defined. I cannot find anything in the standard that specifies either
that it removes an item from a container, or which item it removes.
Is this an error, or did I miss something?
From 23.1.1 [lib.sequence.reqmts]
-------------- |
12 The operations in Table 68 are provided only for the containers for
which they take constant time:
--------------
And, from said table:
--------------
expression | return type | operational semantics | container
.. | ... | ... | ...
a.pop_back() | void | a.erase(--a.end()) | vector, list, deque
--------------
--
Clark S. Cox, III
[email]clarkcox3 (AT) gmail (DOT) com[/email]
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kuyper@wizard.net
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| Posted: Thu May 19, 2005 9:35 pm Post subject: Re: problem with definition of pop_back() |
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"John Vincent" wrote:
| Quote: | Hi,
I have examined both the first and the second edition of the C++
standard,
and as far as I can see the pop_back() method is not properly
defined. I
cannot find anything in the standard that specifies either that it
removes
an item from a container, or which item it removes.
Is this an error, or did I miss something?
|
You are correct that the definitions of list<>::pop_back(),
deque::pop_back() and vector<>::pop_back() do not specify those things.
However, what you missed is the implications of the fact that 23.2.1p2,
23.2.2p2 and 23.2.4p2 all specify that those containers satisfy "most
of the optional sequence requirements (23.1.1)".
If you look at 23.1.1p12 you'll find Table 68, which describes the
optional sequence requirements. The requirements are optional, because
the operations they specify are mandotory only when they have constant
complexity. It says that for pop_back() this is the case for list,
deque, and vector. It specifies that a.pop_back() is equivalent to
a.erase(--a.end()).
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David Abrahams
Guest
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| Posted: Fri May 20, 2005 2:49 am Post subject: Re: problem with definition of pop_back() |
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[email]brok (AT) rubikon (DOT) pl[/email] ("Bronek Kozicki") writes:
| Quote: | "John Vincent" <jpv50 (AT) hotmail (DOT) com> wrote:
I have examined both the first and the second edition of the C++
standard, and as far as I can see the pop_back() method is not
properly defined.
in fact, it is not defined at all. This is by design - trying to do
two operations at once (copy last element to function result and
then remove it from top of stack) is bad design (especially from
exception safety POV). This has been discussed in literature,
e.g. Herb Sutter "Exceptional C++"
|
1. Of course pop_back is defined -- in fact, it's defined
for all the containers in the standard.
2. Almost every useful function is a composite of multiple
sub-operations. Labelling that "bad design" is not only dogmatic,
it's silly.
3. The issue you're thinking of was raised by Tom Cargill's Article:
"Exception Handling: A False Sense of Security"
(http://tinyurl.com/9tnnk) or
(http://www.awprofessional.com/content/images/020163371x/supplements/Exception_Handling_Article.html)
It was improperly labelled a safety problem. The interface as
described is simply unable to provide more than the basic
guarantee. There's nothing wrong with that in principle, though
it's true that in this case it makes sense to also supply a
separate top() function. The assumption that every operation must
give the strong guarantee in order to be exception safe is
widespread, but incorrect. In general, the strong guarantee is
neither sufficient, nor is it always neccessary, for exception
safety.
--
Dave Abrahams
Boost Consulting
www.boost-consulting.com
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Howard Hinnant
Guest
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| Posted: Fri May 20, 2005 4:10 am Post subject: Re: problem with definition of pop_back() |
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In article <BAY101-F25F93A3613EF628DD56370A6170 (AT) phx (DOT) gbl>,
[email]jpv50 (AT) hotmail (DOT) com[/email] ("John Vincent") wrote:
| Quote: | Hi,
I have examined both the first and the second edition of the C++ standard,
and as far as I can see the pop_back() method is not properly defined. I
cannot find anything in the standard that specifies either that it removes
an item from a container, or which item it removes.
Is this an error, or did I miss something?
|
The table in 23.1.1p12 says that the operational semantics of
a.pop_back() is a.erase(--a.end()).
-Howard
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Pete Becker
Guest
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| Posted: Fri May 20, 2005 4:10 am Post subject: Re: problem with definition of pop_back() |
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John Vincent wrote:
| Quote: |
I have examined both the first and the second edition of the C++
standard, and as far as I can see the pop_back() method is not properly
defined. I cannot find anything in the standard that specifies either
that it removes an item from a container, or which item it removes.
Is this an error, or did I miss something?
|
The table entitled "Optional Sequence Operations" (I think it's 68 in
the official ISO versions, but it's 69 in the latest working draft)
gives the specification as { iterator tmp = a.end(); --tmp; a.erase(tmp); }
--
Pete Becker
Dinkumware, Ltd. (http://www.dinkumware.com)
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Bronek Kozicki
Guest
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| Posted: Fri May 20, 2005 4:10 am Post subject: Re: problem with definition of pop_back() |
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Bronek Kozicki wrote:
| Quote: | in fact, it is not defined at all. This is by design - trying to do two
operations at once (copy last element to function result and then remove
it from top of stack) is bad design (especially from exception safety
POV). This has been discussed in literature, e.g. Herb Sutter
"Exceptional C++"
|
oooops. I do not know how it happend, but when writing above I was sure
that you are asking about std::stack.
B.
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John Vincent
Guest
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| Posted: Fri May 20, 2005 7:22 pm Post subject: Re: problem with definition of pop_back() |
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Hi,
Thanks to everyone who pointed out Table 68. For some reason I'd missed it
?!?
/John Vincent.
| Quote: | From: Ray Lischner
To: John Vincent
Subject: Re: problem with definition of pop_back()
Date: Thu, 19 May 2005 17:15:51 -0400
On Wednesday 18 May 2005 10:11 pm, John Vincent wrote:
... as far as I can see the pop_back() method is not
properly defined.
Section 23.1.1, Table 68.
--
Ray Lischner, author of C++ in a Nutshell
http://www.tempest-sw.com/cpp
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Joe Gottman
Guest
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| Posted: Fri May 20, 2005 9:59 pm Post subject: Re: problem with definition of pop_back() |
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"David Abrahams" <dave (AT) boost-consulting (DOT) com> wrote
| Quote: | brok (AT) rubikon (DOT) pl ("Bronek Kozicki") writes:
"John Vincent" <jpv50 (AT) hotmail (DOT) com> wrote:
I have examined both the first and the second edition of the C++
standard, and as far as I can see the pop_back() method is not
properly defined.
in fact, it is not defined at all. This is by design - trying to do
two operations at once (copy last element to function result and
then remove it from top of stack) is bad design (especially from
exception safety POV). This has been discussed in literature,
e.g. Herb Sutter "Exceptional C++"
3. The issue you're thinking of was raised by Tom Cargill's Article:
"Exception Handling: A False Sense of Security"
(http://tinyurl.com/9tnnk) or
(http://www.awprofessional.com/content/images/020163371x/supplements/Exception_Handling_Article.html)
It was improperly labelled a safety problem. The interface as
described is simply unable to provide more than the basic
guarantee. There's nothing wrong with that in principle, though
it's true that in this case it makes sense to also supply a
separate top() function. The assumption that every operation must
give the strong guarantee in order to be exception safe is
widespread, but incorrect. In general, the strong guarantee is
neither sufficient, nor is it always neccessary, for exception
safety.
|
If the rvalue reference proposal is accepted, then pop_back() might be
able to use move() to return the top element by rvalue reference. This
would be much safer, though not 100% safe, as move constructors and move
assignment operators will probably be nothrow. Java stacks return their top
element with move semantics when they call pop(), and there is no risk of
them throwing.
Joe Gottman
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kuyper@wizard.net
Guest
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| Posted: Fri May 20, 2005 11:00 pm Post subject: Re: problem with definition of pop_back() |
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"Bronek Kozicki" wrote:
| Quote: | "John Vincent" <jpv50 (AT) hotmail (DOT) com> wrote:
I have examined both the first and the second edition of the C++
standard, and as far as I can see the pop_back() method is not
properly defined.
in fact, it is not defined at all. This is by design - trying to do
two
operations at once (copy last element to function result and then
remove
it from top of stack) is bad design (especially from exception safety
POV). This has been discussed in literature, e.g. Herb Sutter
"Exceptional C++"
|
The pop_back() member is specified in Table 68 as having no return
value. What are you talking about?
---
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James Dennett
Guest
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| Posted: Sat May 21, 2005 9:22 pm Post subject: Re: problem with definition of pop_back() |
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Joe Gottman wrote:
| Quote: | "David Abrahams" <dave (AT) boost-consulting (DOT) com> wrote in message
news:u3bsig3nz.fsf (AT) boost-consulting (DOT) com...
[email]brok (AT) rubikon (DOT) pl[/email] ("Bronek Kozicki") writes:
"John Vincent" <jpv50 (AT) hotmail (DOT) com> wrote:
I have examined both the first and the second edition of the C++
standard, and as far as I can see the pop_back() method is not
properly defined.
in fact, it is not defined at all. This is by design - trying to do
two operations at once (copy last element to function result and
then remove it from top of stack) is bad design (especially from
exception safety POV). This has been discussed in literature,
e.g. Herb Sutter "Exceptional C++"
3. The issue you're thinking of was raised by Tom Cargill's Article:
"Exception Handling: A False Sense of Security"
(http://tinyurl.com/9tnnk) or
(http://www.awprofessional.com/content/images/020163371x/supplements/Exception_Handling_Article.html)
It was improperly labelled a safety problem. The interface as
described is simply unable to provide more than the basic
guarantee. There's nothing wrong with that in principle, though
it's true that in this case it makes sense to also supply a
separate top() function. The assumption that every operation must
give the strong guarantee in order to be exception safe is
widespread, but incorrect. In general, the strong guarantee is
neither sufficient, nor is it always neccessary, for exception
safety.
If the rvalue reference proposal is accepted, then pop_back() might be
able to use move() to return the top element by rvalue reference. This
would be much safer, though not 100% safe, as move constructors and move
assignment operators will probably be nothrow. Java stacks return their top
element with move semantics when they call pop(), and there is no risk of
them throwing.
|
Note that Java stacks don't return things with move semantics;
the object is never part of the stack, but rather is allocated
separately on the heap, and when it is popped it doesn't move,
but rather a pointer (reference, in Java-speak) to it is returned
to the caller and the stack ceases to reference it. Copying a
pointer (reference) is a no-throw operation, so that's all fine;
you just pay the penalties of another level of indirection
compared to a value-based C++ stack. The complications in C++
come from allowing values to be stored inside the stack, rather
than having the stack just refer to objects elsewhere.
-- James
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