Add a check to hessian sparsity? about pagmo2 HOT 9 CLOSED

A sparsity pattern is a vector of pairs. So when one inits a sparsity pattern with the syntax {{}}, what happens is that a vector of one value-initialized pair is created: [[0,0]] (because {} is value initialisation, and value initialisation for C++ integrals is equivalent to zero initialisation).

I don't see how we could detect such an occurrence (unless somehow repeated [[0,0]] patterns are invalid? Can't ever remember the details of the sparsity stuff...). Seems to me an unfortunate consequence of equating C++11's initialisation rules with vector-like initialisation (a-la python lists).

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One possible way to prevent that syntax from working would be to use, instead of std::pair, a custom coded class which prohibits that type of initialization (but otherwise behaves exactly like std::pair).

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Could we not check in here upon construction of a problem? https://github.com/esa/pagmo2/blob/master/include/pagmo/problem.hpp#L1215 ?

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Ah ok, now I get your point a [0,0] is created

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The error is still well phrased when raised, but I was thinking to the possibility of detecting it in check_hessians_sparsity(hs);
Now I understand why its impossible without changing drastically the representation of the sparisty ...

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Here's a demo of what I mean:

https://godbolt.org/g/suXAtx

(try changing sparsity_pattern1 to sparsity_pattern2 in the main function)

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Yeps. Nice solution, but I am not sure we want to override the std::pair .... there may be hidden consequences? Like that we have to implement a more complex my_pair class? Also we would need to have the python exp for my_pair ... Is it worth?

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Personally I don't think it's worth it. It's an unfortunate syntactic quirk, but I'd rather warn about it in the tutorial or something like that.

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Closing this for the time being, as we established it's not a bug and nobody wants to embark in re-implementing std::pair with the desired behaviour.

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