> I thought "move doesn't move" was a fairly common C++ mantra at this point.
It is. The fact that std::move is just a cast and that move constructors are expected to transfer resources are basic intro to C++ topics, covered in intro to constructors.
It's far too late to put the genie back in the bottle, but I am morbidly curious as to why the standards committee didn't choose an approach that made moves destructive.
What would you want to happen when an object that's on the stack is moved? Do you want its destructor to run, or not? If not, how exactly do you want that to no longer occur? And what do you want to happen if the stack object is moved in multiple places? How willing are you to pay a performance or UB penalty for these?
It solves some rare edge cases where the destruction of the moved-from object must be deferred -- the memory is still live even if the object is semantically dead. Non-destructive moves separate those concerns.
There is a related concept of "relocatable" objects in C++ where the move is semantically destructive but the destructor is never called for the moved-from object.
C++ tries to accommodate a lot of rare cases that you really only see in low-level systems code. There are many features in C++ that seem fairly useless to most people (e.g. std::launder) but are indispensable when you come across the specific problem they were intended to solve.
> I thought "move doesn't move" was a fairly common C++ mantra at this point.
It is. The fact that std::move is just a cast and that move constructors are expected to transfer resources are basic intro to C++ topics, covered in intro to constructors.
It's far too late to put the genie back in the bottle, but I am morbidly curious as to why the standards committee didn't choose an approach that made moves destructive.
What would you want to happen when an object that's on the stack is moved? Do you want its destructor to run, or not? If not, how exactly do you want that to no longer occur? And what do you want to happen if the stack object is moved in multiple places? How willing are you to pay a performance or UB penalty for these?
https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2002/n13...
"There is significant desire among C++ programmers for what we call destructive move semantics [...]"
"In the end, we simply gave up on this as too much pain for not enough gain."
1 reply →
It solves some rare edge cases where the destruction of the moved-from object must be deferred -- the memory is still live even if the object is semantically dead. Non-destructive moves separate those concerns.
There is a related concept of "relocatable" objects in C++ where the move is semantically destructive but the destructor is never called for the moved-from object.
C++ tries to accommodate a lot of rare cases that you really only see in low-level systems code. There are many features in C++ that seem fairly useless to most people (e.g. std::launder) but are indispensable when you come across the specific problem they were intended to solve.
1 reply →