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Lvalue and Rvalue

Module by: Kenneth Leroy Busbee. E-mail the author

Summary: Concepts and examples of Lvalue and Rvalue within the C++ programming language.

Discussion

They refer to on the left and right side of the assignment operator. The Lvalue (pronounced: L value) concept refers to the requirement that the operand on the left side of the assignment operator is modifiable, usually a variable. Rvalue concept pulls or fetches the value of the expression or operand on the right side of the assignment operator. Some examples:

Example 1


int age;     // variable set up
    then later in the program
age = 39;

The value 39 is pulled or fetched (Rvalue) and stored into the variable named age (Lvalue); destroying the value previously stored in that variable.

Example 2


int age;              // variable set up
int voting_age = 18;  // variable set up with initialization
    then later in the program
age = voting_age;

If the expression has a variable or named constant on the right side of the assignment operator, it would pull or fetch the value stored in the variable or constant. The value 18 is pulled or fetched from the variable named voting_age and stored into the variable named age.

Example 3


age < 17;

If the expression is a test expression or Boolean expression, the concept is still an Rvalue one. The value in the identifier named age is pulled or fetched and used in the relational comparison of less than.

Example 4


const int JACK_BENNYS_AGE = 39;  // constant set up
    then later in the program
JACK_BENNYS_AGE = 65;

This is illegal because the identifier JACK_BENNYS_AGE does not have Lvalue properties. It is not a modifiable data object, because it is a constant.

Some uses of the Lvalue and Rvalue can be confusing.

Example 5


int oldest = 55;  // variable set up with initialization
    then later in the program
age = oldest++;

Postfix increment says to use my existing value then when you are done with the other operators; increment me. Thus, the first use of the oldest variable is an Rvalue context where the existing value of 55 is pulled or fetched and then assigned to the variable age; an Lvalue context. The second use of the oldest variable is an Lvalue context where in the value of oldest is incremented from 55 to 56.

Definitions

Definition 1: Lvalue
The requirement that the operand on the left side of the assignment operator is modifiable, usually a variable.
Definition 2: Rvalue
Pulls or fetches the value stored in a variable or constant.

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