lvalue，rvalue及rvalue reference
lvalue及rvalue
摘自Understanding the meaning of lvalues and rvalues in C++：
In C++ an lvalue is something that points to a specific memory location.
On the other hand, a rvalue is something that doesn't point anywhere.
In general, rvalues are temporary and short lived, while lvalues live a longer life since they exist as variables.
It's also fun to think of lvalues as containers and rvalues as things contained in the containers.
lvalue佔據了記憶體空間，壽命較長，可以將它想像為容器。
rvalue並未指向記憶體的任何地方，是暫時的，可以將它想像為容器裡的東西。
rvalue reference
rvalue是暫時的，無法被修改。
而C++0x新引入的rvalue reference則讓使用者有修改rvalue的能力。
以下範例摘自C++ rvalue references and move semantics for beginners：
std::string s1 = "Hello ";
std::string s2 = "world";
std::string&& s_rref = s1 + s2; // the result of s1 + s2 is an rvalue
s_rref += ", my friend"; // I can change the temporary string!
std::cout << s_rref << '\n'; // prints "Hello world, my friend"
s1 + s2的返回值本來是rvalue，是無法被更改的。但是這裡定義了一個std::string&&型別的變數，std::string後面的&&使得它可以指向s1 + s2這個暫存物件（指向右值，即右值引用，rvalue reference）。有了rvalue reference，我們就可以修改它所指向的暫存物件，因此 s_rref += ", my friend";才可以執行成功。
參考連結
