Stjepan Rajko wrote:
Hello,
I recently ran into a need to initialize a fusion vector with a single argument which is then used to construct all of the vector elements. For example, something that would allow:
fusion::vector<int, float, double> ones(initialize_all_with(),1);
// at_c<0>(ones) == 1 // at_c<1>(ones) == 1.0f // at_c<2>(ones) == 1.0;
I hacked fusion::vector so that the above works. Is this addition of interest to anyone else?
Definitely. I'm not sure about the interface though. It seems there could be a more generic "lazy-construction" scheme that can have a more general appeal. After thinking about it, we don't even have to do anything -- fusion already supports it, but we haven't taken advantage of it yet. As you know, *all* fusion containers can be initialized from arbitrary fusion sequences. For example: vector<int, double> v(std::pair<char, short>(1, 2)); schematically: C c(s); where C is a fusion container, s in a fusion sequence. Now... If we have a /lazy-sequence/ s, that generates the ones for you, then you can use that for s: vector<int, float, double> ones(all_ones); all_ones is a lazy-sequence. It generates the ones every time the iterator is dereferenced. A nice strategy is to write a function adaptor that creates a fusion sequence. Example: int one() {return 1} ... vector<int, float, double> ones(fusion::make_lazy_sequence(&one)); make_lazy_sequence can take in a function pointer or a function object -anything callable. It simply calls /one/ each time its iterator is dereferenced.
There is another (perhaps more elegant) solution in implementing a new fusion "array" Sequence (one where all element types are identical) which can construct all elements from a single constructor argument, and then using that sequence to do the above initialization. E.g.,
fusion::vector<int, float, double> ones(fusion::array<int, 3>(1));
... or (another solution) a make_array function that returns a fusion::vector:
fusion::vector<int, float, double> ones(fusion::make_array<3>(1));
It would be great if one of these solutions (or a different solution) were added to fusion. I can take a stab at putting something together, if any of them are acceptable. That is, unless this is already possible in some way that I can't see at the moment :-)
How about hacking on the /lazy_sequence/ idea? Thoughts? Regards, -- Joel de Guzman http://www.boostpro.com http://spirit.sf.net