Posted on Jun 23, 2021

Understanding lvalues and rvalues in C++

If you have worked with C++, you might've come across errors like "Expression must be a modifiable lvalue". This post will give you a clear understanding on what those terms mean.

In simple terms, lvalue is something that has a memory address and rvalue doesn't.

An easy example.

int a = 10;

Here, 10 is rvalue and a is lvalue. Simply because 10 does not have any location. It is just a temporary constant which the compiler creates to assign it to a;

Key points to note

An rvalue cannot be assigned any value

int a; 10 = a; // NOT VALID!

An lvalue can be assigned to another lvalue.

int a = 10; int b = a; // VALID

A function that returns a rvalue is also a rvalue

int getValue() { return 10; } int main() { getValue() = 20; // ERROR }

A function that returns a lvalue is also a lvalue

int& getValue() { static int a = 10; return a; } int main() { getValue() = 20; // NO ERRORS. }

lvalue and rvalue references

This is mainly important when dealing with functions.

A lvalue can be taken as a parameter using & operator

void setValue(int& a) { // Do something }

A rvalue can be taken as a parameter using && operator

void setValue (int&& a) { // Do something }

A const lvalue reference can be assigned to an rvalue

int& a = 10; // NOT VALID const int& b = 10; // VALID

This is the reason why int C++ a function that accepts a string reference should always be assigned with const.

// accepts only lvalue void showFullName_1(string& fullName) { cout<<fullName<<endl; } // accepts both lvalue and rvalue void showFullName_2(const string& fullName) { cout << fullName <<endl; } // accepts only rvalue void showFullName_3(string&& fullName) { cout << fullName <<endl; } int main() { string firstName = "john"; string lastName = "doe"; string fullName = firstName + lastName; //assigning to an lvalue showFullName_1(fullName); //works showFullName_1(firstName + lastName); // doesn't work because firstName + lastName is a rvalue showFullName_2(fullName); // works showFullName_2(firstName + lastName); // works showFullName_3(fullName); // doesn't work showFullName_3(firstName + lastName); // works }