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Ares Lagae

9 May 2007 9 May '07

11:04 a.m.

I have a list of fundamental types (known at compile time). For each of these types I want to declare a templated member in a class. So, instead of class foo { private: int int_variable; double double_variable; ... /* rest of types */ }; I want something like class foo { private: variable_list variable; }; and then access "foo::variable" with a template-like syntax. E.g. "variable.get<int>()". Does anyone have any idea how I can accomplish this? -- Ares Lagae Computer Graphics Research Group, Katholieke Universiteit Leuven http://www.cs.kuleuven.be/~ares/

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David Klein

9 May 9 May

3:05 p.m.

Ares Lagae wrote:

Hi, you could try: Boost.Fusion (only in CVS) or Boost.Tuple (http://boost.org/libs/tuple/doc/tuple_users_guide.html). -- HTH, dave

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Ares Lagae

4:12 p.m.

David Klein wrote:

Boost.Tuple (http://boost.org/libs/tuple/doc/tuple_users_guide.html).

Boost.Tuple is close to what I want. However, I want to access based on type rather than index. E.g. I when I have a typelist typedef boost::mpl::vector I want to generate a kind of tuple from the typelist that contains variables that can be accessed using tuple.get<int>() and tuple.get<float>(). (Note that I do not want a union.) I also need the ability to, given two typelists typedef boost::mpl::vector types; typedef boost::mpl::vector types; want to declare member variables with type e.g. "std::pair", "std::pair", "std::pair", "std::pair".

Boost.Fusion (only in CVS)

I will have a look at the Fusion library. -- Ares Lagae Computer Graphics Research Group, Katholieke Universiteit Leuven http://www.cs.kuleuven.be/~ares/

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Jeffrey Holle

4:53 p.m.

Boost::variant if you want your variable to hold just one of the types, and hopefully you know what type is holds. You have to use a variant visitor to access it if you don't. Attempts to use get<int> when the type isn't castable to it will throw an exception. Ares Lagae wrote:

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Ares Lagae

10 May 10 May

8:12 a.m.

Jeffrey Holle wrote:

Union-like containers are not suited, because I want a variable for _each_ of the types. See my other post about boost::fusion::map. -- Ares Lagae Computer Graphics Research Group, Katholieke Universiteit Leuven http://www.cs.kuleuven.be/~ares/

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Joaquín Mª López Muñoz

8:49 a.m.

Ares Lagae ha escrito:

I guess fusion can be used to implement what you want, but MPL probably also suffices. Please see the attached file, is this kind of what you're after? Joaquín M López Muñoz Telefónica, Investigación y Desarrollo

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Ares Lagae

10:06 a.m.

Joaquín, Mª López Muñoz wrote:

Joaquín, Thank you very much! This is (almost) exactly what I want. I have a couple of questions though. 1/ Consider typedef variable_list_seq > vl_type; Instead of having, for each type in the typelist, a variable of that type, I need a variable depending on that type. This can easily be accomplished by changing variable_list_member to hold my_type<T> rather than T. I was wondering, however, if variable_list_member can be eliminated, and my_type passed as a template argument. E.g. suppose I write a metafunction template <typename T> struct my_type { typedef /* some type */ type; } to somehow declare typedef variable_list_seq, my_type> vl_type; to obtain a variable list with "keys" int, char, double and variables of type my_type<int>::type, .... 2/ would it be possible to generated variables my_type for all possible combinations of T1 and T2, where T1 and T2 are types in two typelists? Something like typedef product_variable_list_seq, boost::mpl::vector<...>, my_type> vl_type; where my_type is a template struct my_type { typedef /* some type */ type; } Best regards, -- Ares Lagae Computer Graphics Research Group, Katholieke Universiteit Leuven http://www.cs.kuleuven.be/~ares/

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Joaquín Mª López Muñoz

1:51 p.m.

Ares Lagae ha escrito: [...]

I was wondering, however, if variable_list_member can be eliminated, and my_type passed as a template argument. E.g. suppose I write a metafunction

template <typename T> struct my_type { typedef /* some type */ type; }

to somehow declare

typedef variable_list_seq, my_type> vl_type;

to obtain a variable list with "keys" int, char, double and variables of type my_type<int>::type, ....

Yep, this can be done adding an optional template parameter to variable_list_seq, as shown in the example attached. Note that variable_list_member is not elminated, but merely supplemented with your my_type metafunction so as to be able to customize member types.

2/ would it be possible to generated variables my_type for all possible combinations of T1 and T2, where T1 and T2 are types in two typelists?

Something like

typedef product_variable_list_seq, boost::mpl::vector<...>, my_type> vl_type;

where my_type is a

template struct my_type { typedef /* some type */ type; }

Again, starting from the simpler variable_list_seq one can write a new product_variable_list_seq working on the cartesian product of two MPL sequences: template<typename T> struct pair_with:boost::mpl::pair{}; template struct sequence_product: boost::mpl::fold< Sequence1, boost::mpl::vector0<>, boost::mpl::joint_view< boost::mpl::_1, boost::mpl::transform< Sequence2, pair_withboost::mpl::_2 > >

{}; The file attached also exercises a possible implementation. Things are a little more complex now, I strongly suspect this kind of stuff can be more easily done with fusion, but alas I haven't had time to learn about that lib. Joaquín M López Muñoz Telefónica, Investigación y Desarrollo

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Ares Lagae

2:38 p.m.

Thanks, This is how I thought it could be done, but I don't have enough experience with the Boost::MPL library yet to actually code it and get it compiled without errors :-) I have the template metaprogramming book at home. That should help me to understand the code in detail. Best regards, -- Ares Lagae Computer Graphics Research Group, Katholieke Universiteit Leuven http://www.cs.kuleuven.be/~ares/

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Ares Lagae

8:11 a.m.

I thing the first problem can be solved using Boost.Fusion: // the names of the types in the application typedef char int8; typedef short int16; typedef int int32; typedef unsigned char uint8; typedef unsigned short uint16; typedef unsigned int uint32; typedef float float32; typedef double float64; // an mpl vector containing all the types typedef boost::mpl::vector types; // a typedef of a fusion map with a variable for each type typedef boost::fusion::map< boost::fusion::pair boost::fusion::pair boost::fusion::pair boost::fusion::pair boost::fusion::pair boost::fusion::pair boost::fusion::pair boost::fusion::pair

map_type;

// the variables map_type variables; // use a variable at<int8>(variables) However, there is a lot of redundancy in the boost::fusion::map typedef. Is there any way to obtain the boost::fusion::map typedef automatically from the boost::mpl::vector? I have the following related problem. // an mpl vector containing all the size types typedef boost::mpl::vector size_types; // an mpl vector containing all the scalar types typedef boost::mpl::vector scalar_types; now i need a variable for each combination of size_types and scalar types. Something like typedef boost::fusion::map< boost::fusion::pair > boost::fusion::pair > boost::fusion::pair > boost::fusion::pair > boost::fusion::pair > boost::fusion::pair > boost::fusion::pair > boost::fusion::pair > boost::fusion::pair > boost::fusion::pair > boost::fusion::pair > boost::fusion::pair > boost::fusion::pair > boost::fusion::pair > boost::fusion::pair

boost::fusion::pair boost::fusion::pair > boost::fusion::pair > boost::fusion::pair > boost::fusion::pair > boost::fusion::pair > boost::fusion::pair > boost::fusion::pair

boost::fusion::pair

map_type;

Again, I want to obtain the boost::fusion::map typedef automatically from the two boost::mpl::vectors. -- Ares Lagae Computer Graphics Research Group, Katholieke Universiteit Leuven http://www.cs.kuleuven.be/~ares/

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participants (4)

Ares Lagae
David Klein
Jeffrey Holle
Joaquín Mª López Muñoz