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L-Values and R-Values

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:

// lValues_rValues.cpp
int main() {
   int i, j, *p;
   i = 7;   // OK variable name is an l-value.
   7 = i;   // C2106 constant is an r-value.
   j * 4 = 7;   // C2106 expression j * 4 yields an r-value.
   *p = i;   // OK a dereferenced pointer is an l-value.

   const int ci = 7;
   ci = 9;   // C3892 ci is a nonmodifiable l-value
   ((i < 3) ? i : j) = 7;   // OK conditional operator returns l-value.

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.