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

}