What expressions create xvalues?

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I'm trying to understand the C++11 concepts.

The standard draft which I have says:

An xvalue (an “eXpiring” value) also refers to an object, usually near the end of its lifetime (so that its resources may be moved, for example). An xvalue is the result of certain kinds of expressions involving rvalue references (8.3.2). [ Example: The result of calling a function whose return type is an rvalue reference is an xvalue. —end example ]

OK, so what exactly are the "certain kinds of expressions" that produce xvalues? This part of the spec does not detail a list of these expressions.

I understand lvalue and prvalue (at least I think, I understand).

xvalue language-lawyer c++11 c++
4 Answers

As you mentioned,

An xvalue (an “eXpiring” value) also refers to an object, usually near the end of its lifetime (so that its resources may be moved, for example).

Focus: object, be moved, end lifetime

Here is an example:

void move_test(){
    std::string s = "I'm here!";
    std::string m  =  std::move(s);  // move from <s> to <m>
    // s is now in an undefined, but valid state; 
    std::cout << "s=" << s << "; &s=" << &s << std::endl;
}

By moving, we can convert a named value (an lvalue,here is s) to being an rvalue(that is std::move(s)). More specifically, since it can’t be a prvalue (it has a name! In other words, it has an identity), it ends up being an xvalue.

xvalue always serves C++ move semantics.

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