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
}
Hope this gives you a basic understanding of what lvalues and rvalues are. If you found this helpful please share and comment!
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