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Looking for more advanced type_traits-like functionality

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Agoston Bejo

6 Jun 2004 6 Jun '04

2:41 p.m.

Hi, I have took a look at some parts of the boost library, but I couldn't find what I want, and that is something like this: How can I determine if a type T has such and such member? It would look like something like this: has_member::result (true if T::paint() exists) has_member::result (true if T::m_size exists) The closest to this I could find is boost::type_traits but that does not provide this functionality. Thx, Gus

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Wolfgang Meyer

6 Jun 6 Jun

11:28 p.m.

that is something like this: How can I determine if a type T has such and such member? It

would look like something like this:

has_member::result (true if T::paint() exists) has_member::result (true if T::m_size exists)

AFAIK, checking for arbitrary members at compile time is not possible in a portable way. However, it seems that you can check for operators: http://lists.boost.org/MailArchives/boost/msg29007.php Regards, Wolfgang Meyer

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David Abrahams

7 Jun 7 Jun

3:06 p.m.

"Wolfgang Meyer" writes:

...
that is something like this: How can I determine if a type T has such and such member? It

would look like something like this:

has_member::result (true if T::paint() exists) has_member::result (true if T::m_size exists)

AFAIK, checking for arbitrary members at compile time is not possible in a portable way.

However, it seems that you can check for operators:

http://lists.boost.org/MailArchives/boost/msg29007.php

Much better techniques than the ones shown there are now known. -- Dave Abrahams Boost Consulting http://www.boost-consulting.com

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Agoston Bejo

3:29 p.m.

New subject: Looking for more advanced type_traits-likefunctionality

"David Abrahams" wrote in message news:ur7srmmpm.fsf@boost-consulting.com...

"Wolfgang Meyer" writes:

...
...
that is something like this: How can I determine if a type T has such and such member? It

would look like something like this:

has_member::result (true if T::paint() exists) has_member::result (true if T::m_size exists)

AFAIK, checking for arbitrary members at compile time is not possible in a portable way.

However, it seems that you can check for operators:

http://lists.boost.org/MailArchives/boost/msg29007.php

Much better techniques than the ones shown there are now known.

-- Dave Abrahams Boost Consulting http://www.boost-consulting.com

And where are they available, or at least can be read of? Thx, Gus

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David Abrahams

8:05 p.m.

New subject: Looking for more advanced type_traits-likefunctionality

"Agoston Bejo" writes:

"David Abrahams" wrote in message news:ur7srmmpm.fsf@boost-consulting.com...

...
"Wolfgang Meyer" writes:

...
...
that is something like this: How can I determine if a type T has such and such member? It

would look like something like this:

has_member::result (true if T::paint() exists) has_member::result (true if T::m_size exists)

AFAIK, checking for arbitrary members at compile time is not possible in a portable way.

However, it seems that you can check for operators:

http://lists.boost.org/MailArchives/boost/msg29007.php

Much better techniques than the ones shown there are now known.

-- Dave Abrahams Boost Consulting http://www.boost-consulting.com

And where are they available, or at least can be read of?

You might look at http://www.boost-consulting.com/boost/boost/detail/is_incrementable.hpp, though it could be improved by using the comma operator to deal with the possibility of void returns from a user defined operator++. -- Dave Abrahams Boost Consulting http://www.boost-consulting.com

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John Maddock

8 Jun 8 Jun

9:56 a.m.

New subject: Looking for more advancedtype_traits-likefunctionality

And where are they available, or at least can be read of?

Yes, did you look at http://lists.boost.org/MailArchives/boost/msg52178.php as I suggested? John.

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John Maddock

7 Jun 7 Jun

11:26 a.m.

New subject: Looking for more advanced type_traits-likefunctionality

I have took a look at some parts of the boost library, but I couldn't find what I want, and

that is something like this: How can I determine if a type T has such and such member? It

would look like something like this:

has_member::result (true if T::paint() exists) has_member::result (true if T::m_size exists)

The closest to this I could find is boost::type_traits but that does not provide this

functionality.

There are two problems: 1) You have to write a separate traits class for every member that you want to detect, there is no real way to boilerplate this. 2) The techniques used are only supported on a minority of current compilers. However, take a look at http://lists.boost.org/MailArchives/boost/msg52178.php for a working example. Also if you want to detect member types (as opposed to member functions) then there is an almost-boilerplate example in boost/mpl/_aux/has_xxx.hpp. John.

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participants (4)

Agoston Bejo
David Abrahams
John Maddock
Wolfgang Meyer