[mpl] can't transform a map

Yuval Ronen

31 May 2007 31 May '07

9:31 p.m.

I have the following code: #include <utility> #include #include template <typename MplPair> struct make_ptr_pair1 { typedef std::pair type; }; template struct make_ptr_pair2 { typedef std::pair type; }; int main(int argc, char *argv[]) { namespace mpl = boost::mpl; typedef mpl::map< mpl::pair > my_map; //typedef mpl::transform< my_map, make_ptr_pair1mpl::_1 >::type ptr_pair_seq1; //typedef mpl::transform< my_map, make_ptr_pair2

...

::type ptr_pair_seq2; return 0; }

If I uncomment any of the two mpl::transform lines in main(), the code doesn't compile (VC8). The point is to get a sequence of std::pairs out of the content of the mpl::map. Is there anything I'm doing wrong? Can mpl::transform be used for mpl::map? Maybe I can do it with fusion, but the above code seems to be something that's supposed to be working, isn't it? Yuval

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Aleksey Gurtovoy

5 Jun 5 Jun

9:42 a.m.

Yuval Ronen writes:

...

I have the following code:

#include <utility> #include #include

template <typename MplPair> struct make_ptr_pair1 { typedef std::pair type; ^^^ mpl

...

};

template struct make_ptr_pair2 { typedef std::pair type; };

int main(int argc, char *argv[]) { namespace mpl = boost::mpl; typedef mpl::map< mpl::pair > my_map; //typedef mpl::transform< my_map, make_ptr_pair1mpl::_1 >::type ptr_pair_seq1; //typedef mpl::transform< my_map, make_ptr_pair2 >::type ptr_pair_seq2; return 0; }

If I uncomment any of the two mpl::transform lines in main(), the code doesn't compile (VC8). The point is to get a sequence of std::pairs out of the content of the mpl::map. Is there anything I'm doing wrong? Can mpl::transform be used for mpl::map?

It can be, but you need to specify the inserter manually, because the default (front_inserter, as per http://boost.org/libs/mpl/doc/refmanual/reversible-algorithm.html) is not suitable for accociated sequences: ... #include #include ... int main(int argc, char *argv[]) { typedef mpl::map< mpl::pair > my_map; typedef mpl::inserter< mpl::map0<>, mpl::insertmpl::_1,mpl::_2 > map_inserter; typedef mpl::transform< my_map, make_ptr_pair1mpl::_1, map_inserter >::type ptr_pair_seq1; // ^^^^^^^^^^^^^^ return 0; }

...

Maybe I can do it with fusion, but the above code seems to be something that's supposed to be working, isn't it?

Yes, ideally, your original code should just work. It's a design bug. I've added the issue to the library's TODO list (http://www.crystalclearsoftware.com/cgi-bin/boost_wiki/wiki.pl?MPL_TODO_List). Thanks for the report, -- Aleksey Gurtovoy MetaCommunications Engineering

Yuval Ronen

6 Jun 6 Jun

6:55 a.m.

Aleksey Gurtovoy wrote:

...

Yuval Ronen writes:

...
I have the following code:

#include <utility> #include #include

template <typename MplPair> struct make_ptr_pair1 { typedef std::pair type; ^^^ mpl

I'm really puzzled about this. Why can't I have std::pair here? The whole purpose of the transform operation is to create a type-sequence of std::pairs. This sequence can be used, for example, with make_variant_over to create a variant of std::pairs. Can't I do that?

...

...
};

template struct make_ptr_pair2 { typedef std::pair type; };

int main(int argc, char *argv[]) { namespace mpl = boost::mpl; typedef mpl::map< mpl::pair > my_map; //typedef mpl::transform< my_map, make_ptr_pair1mpl::_1 >::type ptr_pair_seq1; //typedef mpl::transform< my_map, make_ptr_pair2 >::type ptr_pair_seq2; return 0; }

If I uncomment any of the two mpl::transform lines in main(), the code doesn't compile (VC8). The point is to get a sequence of std::pairs out of the content of the mpl::map. Is there anything I'm doing wrong? Can mpl::transform be used for mpl::map?

It can be, but you need to specify the inserter manually, because the default (front_inserter, as per http://boost.org/libs/mpl/doc/refmanual/reversible-algorithm.html) is not suitable for accociated sequences:

... #include #include

...

int main(int argc, char *argv[]) { typedef mpl::map< mpl::pair > my_map; typedef mpl::inserter< mpl::map0<>, mpl::insertmpl::_1,mpl::_2 > map_inserter; typedef mpl::transform< my_map, make_ptr_pair1mpl::_1, map_inserter >::type ptr_pair_seq1; // ^^^^^^^^^^^^^^ return 0; }

...
Maybe I can do it with fusion, but the above code seems to be something that's supposed to be working, isn't it?

Yes, ideally, your original code should just work. It's a design bug. I've added the issue to the library's TODO list (http://www.crystalclearsoftware.com/cgi-bin/boost_wiki/wiki.pl?MPL_TODO_List).

Thanks for the report,

Thanks you!

Yuval Ronen

7:54 a.m.

Yuval Ronen wrote:

...

Aleksey Gurtovoy wrote:

...
Yuval Ronen writes:

...
I have the following code:

#include <utility> #include #include

template <typename MplPair> struct make_ptr_pair1 { typedef std::pair type; ^^^ mpl

I'm really puzzled about this. Why can't I have std::pair here? The whole purpose of the transform operation is to create a type-sequence of std::pairs. This sequence can be used, for example, with make_variant_over to create a variant of std::pairs. Can't I do that?

Ah, I think I got it. When the input to mpl::transform is a mpl::map, then it tries to generate a mpl::map as output, and therefore the need for mpl::pair. However, I don't need mpl::map as output, any Forward Sequence will suffice (as this is what make_variant_over requires). So is mpl::fold the answer here?

Aleksey Gurtovoy

12 Jun 12 Jun

9:31 a.m.

Yuval Ronen writes:

...

Yuval Ronen wrote:

...
Aleksey Gurtovoy wrote:

...
Yuval Ronen writes:

...
I have the following code:

#include <utility> #include #include

template <typename MplPair> struct make_ptr_pair1 { typedef std::pair type; ^^^ mpl

I'm really puzzled about this. Why can't I have std::pair here? The whole purpose of the transform operation is to create a type-sequence of std::pairs. This sequence can be used, for example, with make_variant_over to create a variant of std::pairs. Can't I do that?

Ah, I think I got it. When the input to mpl::transform is a mpl::map, then it tries to generate a mpl::map as output, and therefore the need for mpl::pair.

Correct.

...

However, I don't need mpl::map as output, any Forward Sequence will suffice (as this is what make_variant_over requires). So is mpl::fold the answer here?

No, you can still use 'transform', like this: typedef mpl::front_inserter< mpl::vector0<> > inserter; typedef mpl::transform< my_map, make_ptr_pair1mpl::_1, inserter >::type ptr_pair_seq1; Sorry for the late reply, -- Aleksey Gurtovoy MetaCommunications Engineering

Yuval Ronen

9:18 p.m.

Aleksey Gurtovoy wrote:

...

Yuval Ronen writes:

...
However, I don't need mpl::map as output, any Forward Sequence will suffice (as this is what make_variant_over requires). So is mpl::fold the answer here?

No, you can still use 'transform', like this:

typedef mpl::front_inserter< mpl::vector0<> > inserter; typedef mpl::transform< my_map, make_ptr_pair1mpl::_1, inserter >::type ptr_pair_seq1;

Great, thanks.

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