Question

for (Something something : setOfSomething)          // OK
for (Something const& something : setOfSomething)   // OK
for (Something& something : setOfSomething)         // ERROR

error: invalid initialization of reference of type 'Something&'
from expression of type 'const Something'

Since when does iterator return const Something? It should return either Something& or Something const&. And since range-based ‘for’ loop is interpreted like that, I have no plausible explanation for what’s going on.

Edit: I’m talking about unordered_set rather than set, sorry about this confusion.

Answer 1

You can’t mutate the members of a set because that could violate the set invariants. So the compiler restricts you to getting const references or copies back out.