Expressions in C++ can evaluate to “l-values” or “r-values.” L-values are expressions that evaluate to a type other than void and that designate a variable.
L-values appear on the left side of an assignment statement (hence the “l” in l-value). Variables that would normally be l-values can be made nonmodifiable by using the const keyword; these cannot appear on the left of an assignment statement. Reference types are always l-values.
The term r-value is sometimes used to describe the value of an expression and to distinguish it from an l-value. All l-values are r-values but not all r-values are l-values.
Some examples of correct and incorrect usages are:
i = 7; // Correct. A variable name, i, is an l-value.
7 = i; // Error. A constant, 7, is an r-value.
j * 4 = 7; // Error. The expression j * 4 yields an r-value.
*p = i; // Correct. A dereferenced pointer is an l-value.
const int ci = 7; // Declare a const variable.
ci = 9; // ci is a nonmodifiable l-value, so the
// assignment causes an error message to
// be generated.
((i < 3) ? i : j) = 7; // Correct. Conditional operator (? :)
// returns an l-value.
Note The examples in this section illustrate correct and incorrect usage when operators are not overloaded. By overloading operators, you can make an expression such as j * 4 an l-value.