mpl documentation question
Documentation for mpl::less<T> has the following section: Expression semantics For any Integral Constants c1 and c2: typedef less<c1,c2>::type r; Return type:Integral Constant. Semantics:Equivalent to typedef bool_< (c1::value < c2::value) > r; typedef less<c1,c2> r; // a Return type:Integral Constant. Semantics:Equivalent to struct r : less<c1,c2>::type {}; //b if //a is used then the following yields bool_<true> is_same< less<c1, c2>, r> if // b is used then the following yields bool_<false> is_same< less<c1, c2>, r> So in what sense are //a and // b equivalent? Robert Ramey
Le 07/04/12 09:13, Robert Ramey a écrit :
Documentation for mpl::less<T> has the following section:
Expression semantics For any Integral Constants c1 and c2: typedef less<c1,c2>::type r;
Return type:Integral Constant. Semantics:Equivalent to typedef bool_< (c1::value< c2::value)> r;
typedef less<c1,c2> r; // a
Return type:Integral Constant. Semantics:Equivalent to struct r : less<c1,c2>::type {}; //b
if //a is used then the following yields bool_<true>
is_same< less<c1, c2>, r>
if // b is used then the following yields bool_<false>
is_same< less<c1, c2>, r>
So in what sense are //a and // b equivalent?
Hi, they are not equivalents. The equivalence relation appears via the ::type access. is_same< less<c1, c2>::type, r::type> should be true_type in both cases. This is because integral_constant<typename T, T V> defines a nested typedef 'type' as itself. HTH, Vicente
Vicente J. Botet Escriba wrote:
Le 07/04/12 09:13, Robert Ramey a écrit :
Documentation for mpl::less<T> has the following section:
typedef less<c1,c2> r; // a
Return type:Integral Constant. Semantics:Equivalent to struct r : less<c1,c2>::type {}; //b
if //a is used then the following yields bool_<true>
is_same< less<c1, c2>, r>
if // b is used then the following yields bool_<false>
is_same< less<c1, c2>, r>
So in what sense are //a and // b equivalent?
Hi, they are not equivalents. The equivalence relation appears via the
type access.
is_same< less<c1, c2>::type, r::type>
should be true_type in both cases.
This is because integral_constant<typename T, T V> defines a nested typedef 'type' as itself.
Hmmm - this helps understand why I've always been confused about when to use "less<c1,c2>::type" vs "less<c1,c2>". Note that it doesn't clarify my confusion itself - it just clarifies why I've always been confused - which of course is an entirely different thing. So you agree that the statement above (which appears in the documentation)
typedef less<c1,c2> r; // a
Return type:Integral Constant. Semantics:Equivalent to struct r : less<c1,c2>::type {}; //b
is incorrect and misleading? I'm thinking that it shouldn't even be in the documentation as written. This raises the question as to what the documentation should say about the type "less<c1, c2>" ?
HTH, Vicente
AMDG On 04/07/2012 10:47 AM, Robert Ramey wrote:
Vicente J. Botet Escriba wrote:
Le 07/04/12 09:13, Robert Ramey a écrit :
Documentation for mpl::less<T> has the following section:
typedef less<c1,c2> r; // a
Return type:Integral Constant. Semantics:Equivalent to struct r : less<c1,c2>::type {}; //b
if //a is used then the following yields bool_<true>
is_same< less<c1, c2>, r>
if // b is used then the following yields bool_<false>
is_same< less<c1, c2>, r>
So in what sense are //a and // b equivalent?
<snip> So you agree that the statement above (which appears in the documentation)
typedef less<c1,c2> r; // a
Return type:Integral Constant. Semantics:Equivalent to struct r : less<c1,c2>::type {}; //b
is incorrect and misleading?
I'm thinking that it shouldn't even be in the documentation as written. This raises the question as to what the documentation should say about the type "less<c1, c2>" ?
is_same doesn't work, but what you can use is mpl::equal_to. mpl::equal_to compares integral constants for equality without requiring the types to match exactly: equal_to<less<c1, c2>, r> -> mpl::true_ In Christ, Steven Watanabe
Steven Watanabe wrote:
AMDG
On 04/07/2012 10:47 AM, Robert Ramey wrote:
Vicente J. Botet Escriba wrote:
Le 07/04/12 09:13, Robert Ramey a écrit :
Documentation for mpl::less<T> has the following section:
typedef less<c1,c2> r; // a
Return type:Integral Constant. Semantics:Equivalent to struct r : less<c1,c2>::type {}; //b
if //a is used then the following yields bool_<true>
is_same< less<c1, c2>, r>
if // b is used then the following yields bool_<false>
is_same< less<c1, c2>, r>
So in what sense are //a and // b equivalent?
<snip> So you agree that the statement above (which appears in the documentation)
typedef less<c1,c2> r; // a
Return type:Integral Constant. Semantics:Equivalent to struct r : less<c1,c2>::type {}; //b
is incorrect and misleading?
I'm thinking that it shouldn't even be in the documentation as written. This raises the question as to what the documentation should say about the type "less<c1, c2>" ?
is_same doesn't work, but what you can use is mpl::equal_to. mpl::equal_to compares integral constants for equality without requiring the types to match exactly:
equal_to<less<c1, c2>, r> -> mpl::true_
That's interesting and useful. So I guess my problem is with the usage of the term "equivalent". I had interpreted this as "can be substituted for". So I'm now thinking that my original concern is justified - that the document doesn't accurately convey what "less<c1, c2>" really is. This has always been confusing to me - and still is. I totally get what "less<c1, c2>::type" is supposed to mean but the meaning and intended usage of "less<c1, c2>" is still lost on me.
In Christ, Steven Watanabe
AMDG On 04/07/2012 12:05 PM, Robert Ramey wrote:
That's interesting and useful. So I guess my problem is with the usage of the term "equivalent". I had interpreted this as "can be substituted for".
So I'm now thinking that my original concern is justified - that the document doesn't accurately convey what "less<c1, c2>" really is.
Yes. I agree that the documentation needs some work here.
This has always been confusing to me - and still is. I totally get what "less<c1, c2>::type" is supposed to mean but the meaning and intended usage of "less<c1, c2>" is still lost on me.
Okay, I'll try to explain more clearly. mpl::less<c1, c2>::type is the result of invoking the metafunction. This is the way all metafunctions work in MPL so no surprises here. Now, mpl::less<c1, c2> is itself an MPL Integral Constant. This works because MPL Integral Constant is a Concept rather than a specific template. In other words, if you only care about the *value* of the result, and not the *exact type*, then you can use mpl::less<c1, c2> instead of typename mpl::less<c1, c2>::type. Just think of this as a convenient short-cut for arithmetic metafunctions. In Christ, Steven Watanabe
Steven Watanabe wrote:
AMDG
On 04/07/2012 12:05 PM, Robert Ramey wrote:
That's interesting and useful. So I guess my problem is with the usage of the term "equivalent". I had interpreted this as "can be substituted for".
So I'm now thinking that my original concern is justified - that the document doesn't accurately convey what "less<c1, c2>" really is.
Yes. I agree that the documentation needs some work here.
This has always been confusing to me - and still is. I totally get what "less<c1, c2>::type" is supposed to mean but the meaning and intended usage of "less<c1, c2>" is still lost on me.
Okay, I'll try to explain more clearly. mpl::less<c1, c2>::type is the result of invoking the metafunction. This is the way all metafunctions work in MPL so no surprises here.
Now, mpl::less<c1, c2> is itself an MPL Integral Constant. This works because MPL Integral Constant is a Concept rather than a specific template. In other words, if you only care about the *value* of the result, and not the *exact type*, then you can use mpl::less<c1, c2> instead of typename mpl::less<c1, c2>::type. Just think of this as a convenient short-cut for arithmetic metafunctions.
In Christ, Steven Watanabe
great answer - next time you come up to Santa Barbara, give me a call, I'll buy you lunch. Robert Ramey
participants (3)
-
Robert Ramey -
Steven Watanabe -
Vicente J. Botet Escriba