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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;
int a;
10 = a; // NOT VALID!int a = 10;
int b = a; // VALIDint getValue() {
return 10;
}
int main() {
getValue() = 20; // ERROR
}int& getValue() {
static int a = 10;
return a;
}
int main() {
getValue() = 20; // NO ERRORS.
}This is mainly important when dealing with functions.
void setValue(int& a) {
// Do something
}void setValue (int&& a) {
// Do something
}int& a = 10; // NOT VALID
const int& b = 10; // VALIDThis 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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