lvalue vs rvalue

 

1. What is an lvalue?

An lvalue is an object that:

• Has an identifiable memory location.
• Has a name (in most cases).
• Can appear on the left-hand side of an assignment.

Example:

int x = 10;

Here:

• x is an lvalue.

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2. What is an rvalue?

An rvalue is a temporary value that:

• Usually does not have a persistent identity.
• Is typically not assigned to directly.
• Is often created as the result of an expression.

Examples:

20
x + y
std::string("Hello")

These are rvalues.

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3. Why doesn't this compile?

int x = 10;
int&& r = x;   // Error

Because:

• x is an lvalue.
• An rvalue reference (&&) can only bind to an rvalue.

If you want to bind x to an rvalue reference, you must explicitly say you're willing to treat it as an rvalue:

int&& r = std::move(x);

std::move(x) converts x into an rvalue.

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4. Why are rvalue references useful?

They enable move semantics.

Instead of copying expensive resources (like dynamically allocated memory), C++ can transfer ownership of those resources, improving performance.

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Interview Tip

A senior interviewer may ask:

Is std::move() actually moving the object?

The best answer is:

No. std::move() simply casts an lvalue to an rvalue reference. The actual move happens when a move constructor or move assignment operator is invoked.