Second attempt. Hopefully this is more succinct and clear.
I am going to ignore RVO almost entirely for this discussion. It makes it really confusing as to what should happen sans optimizations - this is just about move vs copy semantics.
To assist this a reference is going to be very helpful here on the sorts of value types in c++11.
When to move?
lvalue
These are never moved. They refer to variables or storage locations that are potentially being referred to elsewhere, and as such should not have their contents transferred to another instance.
prvalue
The above defines them as "expressions that do not have identity". Clearly nothing else can refer to a nameless value so these can be moved.
rvalue
The general case of "right-hand" value, and the only thing that's certain is they can be moved from. They may or may not have a named reference, but if they do it is the last such usage.
xvalue
These are sort of a mix of both - they have identity (are a reference) and they can be moved from. They need not have a named variable. The reason? They are eXpiring values, about to be destroyed. Consider them the 'final reference'. xvalues can only be generated from rvalues which is why/how std::move works in converting lvalues to xvalues (through the result of a function call).
glvalue
Another mutant type with its rvalue cousin, it can be either an xvalue or an lvalue - it has identity but it's unclear if this is the last reference to the variable / storage or not, hence it is unclear if it can or cannot be moved from.
Resolution Order
Where an overload exists that can accept either a const lvalue ref or rvalue ref, and an rvalue is passed, the rvalue is bound otherwise the lvalue version is used. (move for rvalues, copy otherwise).
Where it potentially happens
(assume all types are A where not mentioned)
It only occurs where an object is "initialized from an xvalue of the same type". xvalues bind to rvalues but are not as restricted as pure expressions. In other words, movable things are more than unnamed references, they can also be the 'last' reference to an object with respect to the compiler's awareness.
initialization
A a = std::move(b); // assign-move
A a( std::move(b) ); // construct-move
function argument passing
void f( A a );
f( std::move(b) );
function return
A f() {
// A a exists, will discuss shortly
return a;
}
Why it will not happen in f
Consider this variation on f:
void action1(A & a) {
// alter a somehow
}
void action2(A & a) {
// alter a somehow
}
A f(A && a) {
action1( a );
action2( a );
return a;
}
It is not illegal to treat a as an lvalue within f. Because it is an lvalue it must be a reference, whether explicit or not. Every plain-old variable is technically a reference to itself.
That's where we trip up. Because a is an lvalue for the purposes of f, we are in fact returning an lvalue.
To explicitly generate an rvalue, we must use std::move (or generate an A&& result some other way).
Why it will happen in g
With that under our belts, consider g
A g(A a) {
action1( a ); // as above
action2( a ); // as above
return a;
}
Yes, a is an lvalue for the purposes of action1 and action2. However, because all references to a only exist within g (it's a copy or moved-into copy), it can be considered an xvalue in the return.
But why not in f?
There is no specific magic to &&. Really, you should think of it as a reference first and foremost. The fact that we are demanding an rvalue reference in f as opposed to an lvalue reference with A& does not alter the fact that, being a reference, it must be an lvalue, because the storage location of a is external to f and that's as far as any compiler will be concerned.
The same does not apply in g, where it's clear that a's storage is temporary and exists only when g is called and at no other time. In this case it is clearly an xvalue and can be moved.
rvalue ref vs lvalue ref and safety of reference passing
Suppose we overload a function to accept both types of references. What would happen?
void v( A & lref );
void v( A && rref );
The only time void v( A&& ) will be used per the above ("Where it potentially happens"), otherwise void v( A& ). That is, an rvalue ref will always attempt to bind to an rvalue ref signature before an lvalue ref overload is attempted. An lvalue ref should not ever bind to the rvalue ref except in the case where it can be treated as an xvalue (guaranteed to be destroyed in the current scope whether we want it to or not).
It is tempting to say that in the rvalue case we know for sure that the object being passed is temporary. That is not the case. It is a signature intended for binding references to what appears to be a temporary object.
For analogy, it's like doing int * x = 23; - it may be wrong, but you could (eventually) force it to compile with bad results if you run it. The compiler can't say for sure if you're being serious about that or pulling its leg.
With respect to safety one must consider functions that do this (and why not to do this - if it still compiles at all):
A & make_A(void) {
A new_a;
return new_a;
}
While there is nothing ostensibly wrong with the language aspect - the types work and we will get a reference to somewhere back - because new_a's storage location is inside a function, the memory will be reclaimed / invalid when the function returns. Therefore anything that uses the result of this function will be dealing with freed memory.
Similarly, A f( A && a ) is intended to but is not limited to accepting prvalues or xvalues if we really want to force something else through. That's where std::move comes in, and let's us do just that.
The reason this is the case is because it differs from A f( A & a ) only with respect to which contexts it will be preferred, over the rvalue overload. In all other respects it is identical in how a is treated by the compiler.
The fact that we know that A&& is a signature reserved for moves is a moot point; it is used to determine which version of "reference to A -type parameter" we want to bind to, the sort where we should take ownership (rvalue) or the sort where we should not take ownership (lvalue) of the underlying data (that is, move it elsewhere and wipe the instance / reference we're given). In both cases, what we are working with is a reference to memory that is not controlled by f.
Whether we do or not is not something the compiler can tell; it falls into the 'common sense' area of programming, such as not to use memory locations that don't make sense to use but are otherwise valid memory locations.
What the compiler knows about A f( A && a ) is to not create new storage for a, since we're going to be given an address (reference) to work with. We can choose to leave the source address untouched, but the whole idea here is that by declaring A&& we're telling the compiler "hey! give me references to objects that are about to disappear so I might be able to do something with it before that happens". The key word here is might, and again also the fact that we can explicitly target this function signature incorrectly.
Consider if we had a version of A that, when move-constructing, did not erase the old instance's data, and for some reason we did this by design (let's say we had our own memory allocation functions and knew exactly how our memory model would keep data beyond the lifetime of objects).
The compiler cannot know this, because it would take code analysis to determine what happens to the objects when they're handled in rvalue bindings - it's a human judgement issue at that point. At best the compiler sees 'a reference, yay, no allocating extra memory here' and follows rules of reference passing.
It's safe to assume the compiler is thinking: "it's a reference, I don't need to deal with its memory lifetime inside f, it being a temporary will be removed after f is finished".
In that case, when a temporary is passed to f, the storage of that temporary will disappear as soon as we leave f, and then we're potentially in the same situation as A & make_A(void) - a very bad one.
An issue of semantics...
std::move
The very purpose of std::move is to create rvalue references. By and large what it does (if nothing else) is force the resulting value to bind to rvalues as opposed to lvalues. The reason for this is a return signature of A& prior to rvalue references being available, was ambiguous for things like operator overloads (and other uses surely).
Operators - an example
class A {
// ...
public:
A & operator= (A & rhs); // what is the lifetime of rhs? move or copy intended?
A & operator+ (A & rhs); // ditto
// ...
};
int main() {
A result = A() + A(); // wont compile!
}
Note that this will not accept temporary objects for either operator! Nor does it make sense to do this in the case of object copy operations - why do we need to modify an original object that we are copying, probably in order to have a copy we can modify later. This is the reason we have to declare const A & parameters for copy operators and any situation where a copy is to be taken of the reference, as a guarantee that we are not altering the original object.
Naturally this is an issue with moves, where we must modify the original object to avoid the new container's data being freed prematurely. (hence "move" operation).
To solve this mess along comes T&& declarations, which are a replacement to the above example code, and specifically target references to objects in the situations where the above won't compile. But, we wouldn't need to modify operator+ to be a move operation, and you'd be hard pressed to find a reason for doing so (though you could I think). Again, because of the assumption that addition should not modify the original object, only the left-operand object in the expression. So we can do this:
class A {
// ...
public:
A & operator= (const A & rhs); // copy-assign
A &a
