𝐑𝐯𝐚𝐥𝐮𝐞 𝐑𝐞𝐟𝐞𝐫𝐞𝐧𝐜𝐞𝐬 in C++

🎯Understanding 𝐑𝐯𝐚𝐥𝐮𝐞 𝐑𝐞𝐟𝐞𝐫𝐞𝐧𝐜𝐞𝐬 in C++

❓1️⃣What are 𝐫𝐯𝐚𝐥𝐮𝐞 𝐫𝐞𝐟𝐞𝐫𝐞𝐧𝐜𝐞𝐬 in C++?
They were introduced in C++11 to support 𝐦𝐨𝐯𝐞 𝐬𝐞𝐦𝐚𝐧𝐭𝐢𝐜𝐬. Fundamentally, an rvalue reference is a 𝒓𝒆𝒇𝒆𝒓𝒆𝒏𝒄𝒆, meaning it is an alias (not an object itself) to an existing object and must be initialized at the point of declaration. It is denoted with two ampersands (&&) to distinguish it from an ordinary (lvalue) reference, which is denoted with a single ampersand (&).

❓2️⃣Why is it called an 𝐫𝐯𝐚𝐥𝐮𝐞 reference?
Because an rvalue reference specifically binds to expressions with 𝐫𝐯𝐚𝐥𝐮𝐞 𝐜𝐚𝐭𝐞𝐠𝐨𝐫𝐢𝐞𝐬.

❓3️⃣What is a 𝐯𝐚𝐥𝐮𝐞 𝐜𝐚𝐭𝐞𝐠𝐨𝐫𝐲?
In C++, every expression has two main characteristics:
➖ 𝐓𝐲𝐩𝐞
➖ 𝐕𝐚𝐥𝐮𝐞 𝐜𝐚𝐭𝐞𝐠𝐨𝐫𝐲
Besides the type, the value category is essential in determining what you can do with an expression. In fact, value categories play an important role when binding references to objects in C++. For example, in C++98/C++03 you can assign or pass an rvalue (a temporary object without a name) to a const lvalue reference, but not to a non-const lvalue reference.

❗𝐍𝐎𝐓𝐄: the type of an expression is independent of whether the expression is an lvalue or an rvalue.

Consider this:

void func(std::string&& msg); // Forward declaration

➖ 𝐦𝐬𝐠's type is an rvalue reference (std::string&&)
➖ 𝐦𝐬𝐠's value category is an lvalue (it's a named object, and you can take its address).

👉 In fact, given a type 𝐓, you can have lvalues as well as rvalues of type 𝐓.

❓4️⃣What is an 𝐫𝐯𝐚𝐥𝐮𝐞 as a value category of an expression?
Expressions classified under the 𝘳𝘷𝘢𝘭𝘶𝘦 category are essential for enabling 𝐦𝐨𝐯𝐞 𝐬𝐞𝐦𝐚𝐧𝐭𝐢𝐜𝐬 in C++ and they are subdivided into:
➖𝐏𝐫𝐯𝐚𝐥𝐮𝐞 (pure readable value): These are either non-string literals or temporaries such as the result of an expression like 5 + 3 or the return type of a value-returning function.

➖𝐗𝐯𝐚𝐥𝐮𝐞 (eXpiring value): The result of marking an object with std::move(), indicating they are no longer needed in their current scope and are potential candidates for move semantics.

❓5️⃣Is an rvalue reference the only reference type that binds to temporaries? No, const lvalue references can also bind to temporaries.

Here's how they are categorized based on their binding properties:
➖ 𝐈𝐦𝐦𝐮𝐭𝐚𝐛𝐥𝐞 (read-only): Const lvalue references (const T&) allow temporary objects to bind 𝘣𝘶𝘵 prevent modification.
➖ 𝐌𝐮𝐭𝐚𝐛𝐥𝐞: Rvalue references (T&&) not only bind to temporaries but also allow modifications of these temporary objects, potentially 𝘦𝘯𝘢𝘣𝘭𝘪𝘯𝘨 move semantics.

❓6️⃣How do rvalue references 𝐚𝐟𝐟𝐞𝐜𝐭 the lifetime of temporaries?
Both rvalue and const lvalue references extend the lifetime of the temporaries they bind to. This ensures the temporary object remains valid for the lifetime of the reference, rather than being destroyed at the end of the statement.

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