From:

=?ISO-8859-1?Q?Daniel_Kr=FCgler?= <daniel.kruegler@googlemail.com>

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comp.lang.c++.moderated

Date:

Thu, 16 Dec 2010 05:20:11 CST

Message-ID:

<ieafpi$ivd$1@news.eternal-september.org>

On 13 Dec, 20:07, Daniel Kru"gler<daniel.krueg...@googlemail.com>

wrote:

Are you sure?

wrote:

Am 13.12.2010 12:33, schrieb Nikolay Ivchenkov:

At this point we need to halt, because [swappable.requirements] p. 2 . b. 1

already requires that the "expressions swap(t, u) and swap(u, t) are valid when

evaluated in the context described below". This is a basic requirement that you

will find in all requirement tables/descriptions. Anything that violates the

basic requirements cannot be considered to satisfy the requirement set as a

whole thing.

It cannot be, because the expression swap(arr[0], arr[1]) "is not a valid when

evaluated".

No, see above.

I don't understand what "swappable expression" should mean. Let's

consider the following example:

#include<algorithm>

#include<iostream>

#include<iterator>

int main()

{

int *arr[2] = {};

{

int i;

arr[1] =&i;

}

std::reverse(std::begin(arr), std::end(arr));

std::cout<< arr[1]<< std::endl;

}

When std::reverse is called, arr[0] holds the null pointer value and

arr[1] holds an invalid pointer value (Note: effect of using an

invalid pointer value is undefined - see N3225 - 3.7.4.2/4).

consider the following example:

#include<algorithm>

#include<iostream>

#include<iterator>

int main()

{

int *arr[2] = {};

{

int i;

arr[1] =&i;

}

std::reverse(std::begin(arr), std::end(arr));

std::cout<< arr[1]<< std::endl;

}

When std::reverse is called, arr[0] holds the null pointer value and

arr[1] holds an invalid pointer value (Note: effect of using an

invalid pointer value is undefined - see N3225 - 3.7.4.2/4).

At this point we need to halt, because [swappable.requirements] p. 2 . b. 1

already requires that the "expressions swap(t, u) and swap(u, t) are valid when

evaluated in the context described below". This is a basic requirement that you

will find in all requirement tables/descriptions. Anything that violates the

basic requirements cannot be considered to satisfy the requirement set as a

whole thing.

Now let's

consider the following questions:

1) Is arr[0] swappable with arr[1]?

consider the following questions:

1) Is arr[0] swappable with arr[1]?

It cannot be, because the expression swap(arr[0], arr[1]) "is not a valid when

evaluated".

2) Is arr[0] swappable?

No, see above.

Are you sure?

Yes ;-)

What can you say about the evaluation of

std::reverse(arr + 0, arr + 1)? And what can you say about the

evaluation of std::reverse(a + 0, a + 2) in the example below?

std::reverse(arr + 0, arr + 1)? And what can you say about the

evaluation of std::reverse(a + 0, a + 2) in the example below?

These are both questions different from the original one. The original

example is one, where std::swap(arr[0], arr[1]) *is* evaluated. In this

context the expression is clearly no valid expression.

The library requirement sets have at least two different purposes:

They require that an expression is valid, even if it is not evaluated -

informally I denote that as a "static" requirement (or the

"well-formed"-criterion).

In this case we cannot argue about effects and it is perfectly OK to say

that the expression std::swap(arr[0], arr[1]) is valid. This is

important, because that means that the standard does not impose overly

strict requirements on implementations to prevent those expressions

occurring at all.

The second important requirement of the requirement sets is a "dynamic"

(also my informal notation) or runtime requirement: If the corresponding

expressions are required *or* allowed to be evaluated for given

arguments for a correspondingly constrained function, the expression

must also be well-defined. This requirement is not satisfied in your

original example, but - since there is no actually allowed evaluation of

similarly critical (that would cause undefined behaviour) expressions in

your second example std::reverse(a + 0, a + 2), were there is no

violation of the client contract.

Let me emphasize that this is nothing special about the swappable

requirements. It applies to all other requirements set if they are

imposed on algorithms. I already mentioned the example of an assignment

that is not evaluated in my previous reply.

Returning to you "a-example": The evaluation of

std::reverse(a + 0, a + 2)

is well-formed and well-defined, because the correspondingly evaluated

swap expression satisfies both the static and the dynamic requirements

(well-formed and well-defined). I see no magic here.

What exactly

A type X satisfying any of the iterator requirements (24.2) is

ValueSwappable if, for any dereferenceable object x of type X, *x is

swappable.

is supposed to mean:

1) A type X satisfying any of the iterator requirements (24.2) is

ValueSwappable if _there can exist_ a dereferenceable object x of type

X _such that_ *x _would be_ swappable.

A type X satisfying any of the iterator requirements (24.2) is

ValueSwappable if, for any dereferenceable object x of type X, *x is

swappable.

is supposed to mean:

1) A type X satisfying any of the iterator requirements (24.2) is

ValueSwappable if _there can exist_ a dereferenceable object x of type

X _such that_ *x _would be_ swappable.

Looks too strict to me. There could be a type satisfying the iterator

requirements where no object of this type could be dereferencable.

Typically examples where these types are valid are degenerate (empty)

ranges. I see no evidence that the standard imposes this strictness for

any other requirement set or language concept.

or

2) A type X satisfying any of the iterator requirements (24.2) is

ValueSwappable if for _every_ dereferenceable object x of type X, *x

is swappable.

2) A type X satisfying any of the iterator requirements (24.2) is

ValueSwappable if for _every_ dereferenceable object x of type X, *x

is swappable.

This is surely incorrect. The preconditions of both dereferencing and

swap operation must be satisfied in a particular evaluation context.

3) something else?

I already tried to explain it. Here a last attempt to present it

differently: The answer is that there is a static and a dynamic

requirement. If the dynamic requirement is never allowed to be evaluated

as part of some particular algorithm there is no reason why a type does

not satisfy these requirements if the static requirements are satisfied

because there is no way to prove the contrary. This also nicely shows,

that we cannot only speak of type properties but also of runtime

behaviour. This is the reason why referring to expressions (and not only

on types) is important. This also shows that the swappable requirements

are not different from other expression-based requirement sets.

I don't see any _correct_ rationale for presence of such definitions.

Sorry for that, but that judgment seems to go beyond a normal criticism

on Swappable requirements. This seems to be an overall and general

criticism on expression-based requirement sets. But I believe that we

have to live with them for C++0x.

In general, for a given expression t, there are 2 subsets of

expressions with the same type and value category (lvalue/rvalue):

1) subset of expressions which are swappable with t and

2) subset of expressions which are not swappable with t.

Both subsets may be non-empty. For example, in the sample above a[0]

is swappable with a[1], but a[0] is not swappable with a[2].

expressions with the same type and value category (lvalue/rvalue):

1) subset of expressions which are swappable with t and

2) subset of expressions which are not swappable with t.

Both subsets may be non-empty. For example, in the sample above a[0]

is swappable with a[1], but a[0] is not swappable with a[2].

This interpretation exceeds what the standard says. There is no

*specific* requirement that a[0] is swappable. What you are presenting

here is your interpretation of what the standard says. I argue this

interpretation goes too far based on the general way of how the

requirements on algorithms are imposed. Otherwise every algorithm would

need to describe dynamic situations where these requirements are not

necessary. Just one example: In [alg.is_permutation] the requirement exists:

"The comparison function shall be an equivalence relation."

The intention clearly is that of an dynamic requirement. We can easily

construct examples where for some argument values - that are out of the

domain of a specific comparison function - this requirement would not

hold but still this comparison function can be used as valid argument of

such a constrained function, *unless* the dereferenced iterator values

would cause to enter this out-of-domain situation.

If, for a library component, the preconditions are defined in terms

of simply

swappable (not "swappable with") expressions or ValueSwappable types,

we have either:

1) unreasonable violation of preconditions - when the 2-nd subset is

required to be empty (e.g., in std::reverse(a + 0, a + 2)), or

2) undefined behavior of the library component having its

preconditions satisfied - when the 2-nd subset is not required to be

empty (e.g. in std::reverse(a + 0, a + 3)).

we have either:

1) unreasonable violation of preconditions - when the 2-nd subset is

required to be empty (e.g., in std::reverse(a + 0, a + 2)), or

2) undefined behavior of the library component having its

preconditions satisfied - when the 2-nd subset is not required to be

empty (e.g. in std::reverse(a + 0, a + 3)).

I believe I already answered on this above.

The definition of "swappable with" consists of two parts:

1) description of well-formed expressions and

2) description of required effects.

1) description of well-formed expressions and

2) description of required effects.

... in a context where the evaluation is allowed or required, yes.

A definition for ValueSwappable types can be introduced only in terms

of well-formed expressions.

of well-formed expressions.

Nope. It has also effect requirements, when the corresponding algorithm

requires a corresponding swap call with specific arguments.

Required effects are meaningful only with

regard to concrete pair of expressions,

regard to concrete pair of expressions,

Correct, similar for any other expression-based requirements that is

binary. For n-ary expression based operations, these are relevant for

the actually invoked or allowed to be invoked operation. Again: There is

nothing special in regard to swappable requirements here.

so required effects shall not be a part of a type classification.

This is a false conclusion, sorry. They are *very* important, because

otherwise a given algorithm that evaluates the corresponding expression

where it is allowed to do that, cannot make reasonable assumptions on

the effects and without such assumption an algorithm does not make

sense. The CopyConstructible requirements are not very much helpful

without effects requirements as well.

The same conclusions can be

applied to abstract lvalues or rvalues of some types.

void f(int **i, int **j)

{

std::swap(*i, *j);

}

Even if we know that i and j are dereferenceable, we can't say whether

*i is swappable with *j without additional knowledge about passed

arguments.

applied to abstract lvalues or rvalues of some types.

void f(int **i, int **j)

{

std::swap(*i, *j);

}

Even if we know that i and j are dereferenceable, we can't say whether

*i is swappable with *j without additional knowledge about passed

arguments.

This is true for many if not most standard algorithms, believe it or not.

It is already used at several places, e.g. [nullablepointer.requirements] p. 1 b. 2:

"lvalues of type P are swappable (20.2.2),"

"lvalues of type P are swappable (20.2.2),"

It seems, the problem is systematic:

I see it differently as a systematic misunderstanding ;-) [I could not

resist, but I'm kidding of-course]

A type P meets the requirements of NullablePointer if:

[...]

the expressions shown in Table 41 are valid and have the indicated

semantics

I believe that such method of description is far from being perfect.

[...]

the expressions shown in Table 41 are valid and have the indicated

semantics

I believe that such method of description is far from being perfect.

Good point, the standard is not perfect. I don't deny that.

etc. This short-cut says exactly the same as if I would write that

"With lv1 and lv2 being lvalues of type T, lv1 shall be swappable with lv2" for

lvalues or "With rv1 and rv2 being rvalues of type T, rv1 shall be swappable

with rv2".

"With lv1 and lv2 being lvalues of type T, lv1 shall be swappable with lv2" for

lvalues or "With rv1 and rv2 being rvalues of type T, rv1 shall be swappable

with rv2".

Both options are incorrect if the semantics shall be under

consideration.

It is not incorrect as explained before.

We don't want to repeat these lengthy sentences

Happily, some descriptions have correct form:

std::swap for arrays:

Requires: a[i] shall be swappable with (20.2.2) b[i] for all i in

the range [0,N).

I'm happy to see that there are at least some sentences in the standard

that are satisfactory for you ;-) [Kidding again]

The requirement that

the arguments must be valid, is already part of the basic requirements. It is

also part of the basic library requirements, as mentioned in 17.6.3.9 Function

arguments [res.on.arguments], in particular for p. 1 b 1 (for library functions)

and [res.on.functions] p. 2 b. 3.

the arguments must be valid, is already part of the basic requirements. It is

also part of the basic library requirements, as mentioned in 17.6.3.9 Function

arguments [res.on.arguments], in particular for p. 1 b 1 (for library functions)

and [res.on.functions] p. 2 b. 3.

How this is related to my examples?

I think I explained it above.

3) change "Requres" paragraph for std::reverse as described below:

Requires: For each non-negative integer i< distance(first, last)/2,

*next(first, i) shall be swappable with *prev(prev(last, i)).

Requires: For each non-negative integer i< distance(first, last)/2,

*next(first, i) shall be swappable with *prev(prev(last, i)).

The short-cuts where introduced to prevent this. They have the same meaning.

I strongly disagree with that. My wording clearly states that for

every pair of iterators {x, y}, to which std::iter_swap(x, y) is

applied, *x shall be swappable with *y. It imposes no requirements on

any values outside the specified range (to which std::reverse

applies).

I did not deny that. Please be careful when interpreting my response. We

must consider the requirement of an algorithm as a whole. If you start

dissecting it, this can be a different example.

In particular, the call std::reverse(a + 0, a + 2) in the

example above is well-defined in spite of the fact that a[0] is not

swappable with a[2].

example above is well-defined in spite of the fact that a[0] is not

swappable with a[2].

Sure, I don't disagree with that. And I don't think that the wording

which is used by the standard contradicts with that.

In contrast, use of generalized forms "swappable

expression" and "ValueSwappable type" does not allow to consider

concrete subset of expressions.

expression" and "ValueSwappable type" does not allow to consider

concrete subset of expressions.

I disagree with that interpretation and I think I have given enough

evidence for that.

This is clearly incorrect style of description. It would be really

funny to see such specification in an International Standard.

funny to see such specification in an International Standard.

You are free to laugh about it ;-)

HTH & Greetings from Bremen,

Daniel Kr?gler

--

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