Why am I getting this error?

[vivid ivy]

error: ‘constexpr’ needed for in-class initialization of static data member ‘const double Grid_3d::Table::_epsilon0’ of non-integral type [-fpermissive]
285 | static const double _epsilon0 = 8.85418781762e-12; // [epsilon0] = As/(Vm)

[meager light]

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[terse sparrow]

As I understand it, for const static members, you must only declare the variable in the class definition, and then actually define it out of the class (usually in a cpp file)

Haven't done this is a while, so I'm fuzzy on the details

[tropic agate]

Fairly close
Excluding some exceptions, any static member variable had to have an out-of-class definition/initializer
Then c++17 introduced inline and constexpr variables, so now with the right set of keywords you are allowed to have more in-class initialization

[tropic agate]

Because that's just literally not allowed
If you're asking for practical and historic reasons then yeah we can talk about that, but that aside you can just stick constexpr there

[meager light]

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