Jun 112012
 

What is l-value and r-value in C/C++ languages. What is the difference between the two?

Solution:

The name l-value and r-value are given based on the side of assignment operator that an expression can come. For example:

    int x;

    // x is l-value, 2 is r-value.
    x = 2;  
    
    // y is used as l-value, x as r-value 
    // (though it can be used as l-value also as shown above).
    int y = x;

    // ERROR. 2 does not have l-value 
    //(and hence cannot appear on left side of assignment) 
    2 = x; 

if ptr is an int * like below

    int *ptr;

then *ptr is an l-value.

Any of the following C expressions can be l-value expressions:

    • A variable of integral, floating, pointer, structure, or union type
    • A subscript ([ ]) expression that does not evaluate to an array
    • A member-selection expression (–> or .)
    • A unary-indirection (*) expression that does not refer to an array (dereferencing a pointer)
    • An l-value expression in parentheses

If arr is an array, like below:

    int arr[5];

then we can assign to arr[i] but arr itself cannot be changed.

    arr[2] = 3; //OK
    arr = &intVar; // ERROR. Can't modify Array name.

So, arr can appear on the right side of assignment but not on the left side. Should we say arr is r-value? According to me:

YES.

But, Some authors also talk about “modifiable l-value” and “non-modifiable l-value”.

They say that literals (constants like 2, ‘A’, “Hello”, 4.5, etc.) are r-values. Array names and constant identifiers are non-modifiable l-values and variables (objects) are modifiable l-values. I leave the choice of terminologies to you.

Example:

    // function returning an l-value
    int fun1(){
        int x = 3;
        return x;
    }

    // function returning an r-value
    int * fun2(){
        static int y = 2;
        return &y;
    }

    main()
    {
        fun1() = 5;  // ERROR. fun1 returns an l-value
        *fun2() = 5;  // OK. fun2 returns an l-value
    }

Note that an l-value expression can be equally complex (and may not necessarily be a simple variable/object).

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