StaticVector and StaticMatrix std::initializer_list::size validation at compile time

[Matthias Moulin]

StaticVector and StaticMatrix have known sizes at compile-time. Currently, their constructors taking a std::initializer_list perform a size check at runtime instead of compile time which is quite error-prone at compile time. Ideally, the check needs to be performed at compile-time instead, which is trivially achievable in C++14 as std::initializer_list<T>::size is made constexpr.

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[Klaus Iglberger]

Hi Matthias!

Thanks a lot for pointing out this potential improvement. We indeed missed this detail. We’ll implement the necessary changes for both StaticVector and StaticMatrix. Thanks again,

Best regards,

Klaus!

[Klaus Iglberger]

Hi Matthias!

Although the idea is great, it unfortunately doesn't work. In a function taking a std::initializer_list you cannot evaluate the size() member function as constexpr. The best explanation I found is from Dietmar Kühl in this stackoverflow article.

Thanks again for the creating this issue,

Best regards,

Klaus!

[Matthias Moulin]

Thanks for the information explaining the underlying motivation of this odd C++ behavior.

An alternative that would not break backward compatibility for StaticVector (but will break backward compatibility for StaticMatrix), consists of using a variadic templated constructor of which the variable number of arguments can be trivially queried at run-time. The latter should be constraint to at least 2 arguments as the 0 and 1 argument cases are already covered, and should be guarded against potential clashes with the count/pointer constructor.

This would still allow the following code:

StaticVector< unsigned long long, 3u > v = { 1u, 2u, 3u };

but not:

StaticMatrix< int, 2u, 2u > m = { { 0, 1 }, { 2, 3 } };

Instead, some alternatives are possible:

StaticMatrix< int, 2u, 2u > m = { 0, 1, 2, 3 }; // Alternative 1
StaticMatrix< int, 2u, 2u > m = { StaticVector< int, 2u >{ 0, 1 }, StaticVector< int, 2u >{ 2, 3 } }; // Alternative 2
StaticMatrix< int, 2u, 2u > m = {{ {{ 0, 1 }}, {{ 2, 3 }} }}; // Alternative 2

(which are, unfortunately, not very appealing).

[Klaus Iglberger]

Hi Matthias!

This is the technique we use for the AlignedArray class template. However, as you state yourself, this unfortunately does not work for multi-dimensional data structures and the alternatives are definitely not appealing. Since a breaking change at this point is not possible, we unfortunately have to drop this idea.

Best regards,

Klaus!