C++ Academy · Lesson

Lvalue vs Rvalue The Distinction

Reason about value categories: lvalues, prvalues and xvalues.

Lesson 1 of 413 steps

Lvalue vs Rvalue The Distinction is a free C++ Academy lesson on CoddyKit. This is lesson 1 of 4. Any 3 lessons of this learning path are free to read in full — after that, CoddyKit PRO unlocks every lesson, plus hands-on practice with a built-in code editor and a 24/7 AI tutor. It is part of the C++ Academy learning path, and your progress syncs across the web and the CoddyKit app. The C++ Academy course includes 4 lessons in total.

Two Worlds: Lvalue and Rvalue

Every expression in C++ is either an lvalue (a thing with a name and an address) or an rvalue (a temporary that has no name).

Lvalues

An lvalue refers to a persistent object — you can take its address with &.

int x = 42;           // x is an lvalue
int* p = &x;          // taking the address of an lvalue

int& r = x;           // r is an lvalue reference

Rvalues

An rvalue is a temporary — literals, function return values, expressions like a + b. Cannot take its address.

int a = 5, b = 3;
int c = a + b;        // (a + b) is an rvalue
// int* p = &(a + b); // ERROR: cannot take address of an rvalue

Why the Distinction Matters

An rvalue is about to be destroyed. We can safely steal its resources rather than copy them — that is the basis of move semantics.

Three Categories Since C++11

Modern C++ refines the model:

lvalue — has a name and identity
prvalue (pure rvalue) — pure temporary (a literal, return-by-value)
xvalue (expiring) — about to die (the result of std::move)

lvalue vs rvalue References

An lvalue reference uses &; an rvalue reference uses &&.

int x = 10;
int&  lr = x;        // lvalue reference
int&& rr = 42;       // rvalue reference, binds to literal

Binding Rules

Type compatibility:

lvalue reference (T&) binds to lvalues
const lvalue reference (const T&) binds to both
rvalue reference (T&&) binds only to rvalues (and xvalues)

Checking with decltype

You can ask the compiler what category an expression is, by using decltype on it.

int x = 10;
static_assert(std::is_lvalue_reference<decltype((x))>::value);
// the extra parentheses make decltype yield the lvalue ref

Why C++11 Added Rvalue References

To enable move semantics: a constructor that takes T&& can take ownership of a temporary s internal data, avoiding a deep copy.

Common Mistake: Named rvalue ref is an lvalue

An rvalue reference variable, once declared, has a name — making it an lvalue. To pass it on as an rvalue, use std::move.

void f(std::string&& s) {
    // s here is an lvalue!
    process(std::move(s));   // now it is an rvalue again
}

Perfect Forwarding Preview

Templates that take T&& deduce either an lvalue or rvalue reference. Combined with std::forward, they can preserve the value category — covered in detail later.

Quick Check

Which of these expressions is an rvalue?

Answer it in the interactive lesson

Recap

An lvalue has a name and address; an rvalue is a temporary. T& binds to lvalues, T&& binds to rvalues. This distinction enables move semantics — avoiding unnecessary copies of temporaries.

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Frequently Asked Questions

Is the “Lvalue vs Rvalue The Distinction” lesson free?

Yes — any 3 lessons of the C++ Academy learning path, including “Lvalue vs Rvalue The Distinction”, are free to read in full here on the web. After that, CoddyKit PRO unlocks every lesson, plus interactive practice with a built-in code editor and a 24/7 AI tutor. The C++ Academy course includes 4 lessons in total.

What will I learn in “Lvalue vs Rvalue The Distinction”?

Reason about value categories: lvalues, prvalues and xvalues. You practise C++ Academy with hands-on code you run directly in the browser, and a 24/7 AI tutor answers your questions as you work through the lesson.

Do I need any experience to start C++ Academy?

No prior experience is required. C++ Academy on CoddyKit is structured for beginners through advanced learners, so you can start here or from the beginning and move at your own pace. This is lesson 1 of 4.

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