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I recently started learning about rvalues,lvalues, move semantics and I can't understands some conceps like rvalue refs .. why can I assign a value to a rvalue ref like : int&& x = 10;.. doesn't that mean that an rvalue ref is an lvalue(I tought that a rvalue ref is a rvalue)? If that is true ... what's the point of having rvalue references? If that is false then what is the difference between an rvalue ref and an lvalue ?
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Since rv is lvalue, it can bind to lvalue without any problem. Note that rvalue-ness is a property of an expression, not a variable. In the above example, an rvalue is created out of 10 and binds to rv , which as I said, is lvalue.
An lvalue reference can bind to an lvalue, but not to an rvalue.
rvalue references have two properties that are useful: rvalue references extend the lifespan of the temporary object to which they are assigned. Non-const rvalue references allow you to modify the rvalue.
Typically rvalues are temporary objects that exist on the stack as the result of a function call or other operation. Returning a value from a function will turn that value into an rvalue. Once you call return on an object, the name of the object does not exist anymore (it goes out of scope), so it becomes an rvalue.
why can I assign a value to a rvalue ref like : int&& x = 10
That's not an assignment. That's initialisation. Here, a temporary object is materialised from the integer constant and the reference is bound to that temporary. Same thing can be done with lvalue references to const: const int& x = 10. It cannot be done with lvalue references to non-const since they cannot be bound to rvalues.
doesn't that mean that an rvalue ref is an lvalue
Only expressions have values. int&& x = 10; is a declaration and not an expression. Thus the declaration doesn't have a value in the first place.
The declaration contains an expression 10 which is a prvalue.
Types are not expressions either, so "rvalue reference is an lvalue" is meaningless.
Variables don't have value categories either. However id-expressions - which are just expressions that name a variable - do have a value category. They are lvalues regardless of the type of the named variable - yes, even if the type is rvalue reference.
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