filledFrom using lambda function #2037
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Simplifies a common pattern
```
Field3D result{emptyFrom(field)};
BOUT_FOR(i, result) {
result[i] = ...;
}
```
and allows it to be used as an expression
```
auto result = filledFrom(field, [&](const auto& i) {
return ...;
});
```
Compiling with OpenMP fails due to asserts with an exit from loops
| /// An optional third argument is the region string | ||
| template < | ||
| typename T, typename Function, | ||
| typename = decltype(std::declval<Function&>()(std::declval<typename T::ind_type&>()))> |
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Ahh, C++! 😄 The only bit I'm not sure of is the reference in declval<Function&> -- what am I missing?
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I'm also a bit unclear on this magic incantation. It's adapted from https://stackoverflow.com/questions/50859115/enable-if-template-param-is-lambda-with-particular-signature
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Oh, I see, it's to guard against strange types! If F had a call operator like void operator(ind_type) &&, which is rvalue qualified, it can only be called on a temporary. But we definitely have an lvalue here, that is, a named variable. F& is a mutable reference, so it must be an lvalue, so it ensures that this will definitely be the right operator() overload.
Phew!
This might be a FIXME for when we get to move to C++17 and it can be std::is_invocable
Simplifies a common pattern
and allows it to be used as an expression:
Examples of use: https://github.com/bendudson/hermes-3/blob/master/src/collisions.cxx#L106