Vicente J. Botet Escriba wrote:

Le 07/04/12 09:13, Robert Ramey a écrit :

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Documentation for mpl::less<T> has the following section:

typedef less r; // a

Return type:Integral Constant. Semantics:Equivalent to struct r : less::type {}; //b

if //a is used then the following yields bool_<true>

is_same< less, r>

if // b is used then the following yields bool_<false>

is_same< less, r>

So in what sense are //a and // b equivalent?

Hi, they are not equivalents. The equivalence relation appears via the

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type access.

is_same< less::type, r::type>

should be true_type in both cases.

This is because integral_constant defines a nested typedef 'type' as itself.

Hmmm - this helps understand why I've always been confused about when to use "less::type" vs "less". 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)

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typedef less r; // a

Return type:Integral Constant. Semantics:Equivalent to struct r : less::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" ?

HTH, Vicente

Steven Watanabe wrote:

AMDG

On 04/07/2012 10:47 AM, Robert Ramey wrote:

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Vicente J. Botet Escriba wrote:

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Le 07/04/12 09:13, Robert Ramey a écrit :

...

Documentation for mpl::less<T> has the following section:

typedef less r; // a

Return type:Integral Constant. Semantics:Equivalent to struct r : less::type {}; //b

if //a is used then the following yields bool_<true>

is_same< less, r>

if // b is used then the following yields bool_<false>

is_same< less, r>

So in what sense are //a and // b equivalent?

<snip> So you agree that the statement above (which appears in the documentation)

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...

typedef less r; // a

Return type:Integral Constant. Semantics:Equivalent to struct r : less::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" ?

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, 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" really is. This has always been confusing to me - and still is. I totally get what "less::type" is supposed to mean but the meaning and intended usage of "less" is still lost on me.

In Christ, Steven Watanabe

Steven Watanabe wrote:

AMDG

On 04/07/2012 12:05 PM, Robert Ramey wrote:

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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" really is.

Yes. I agree that the documentation needs some work here.

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This has always been confusing to me - and still is. I totally get what "less::type" is supposed to mean but the meaning and intended usage of "less" is still lost on me.

Okay, I'll try to explain more clearly. mpl::less::type is the result of invoking the metafunction. This is the way all metafunctions work in MPL so no surprises here.

Now, mpl::less 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 instead of typename mpl::less::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