lvalue and rvalue in C language (original) (raw)

Last Updated : 22 Jun, 2022

lvalue:-

lvalue simply means an object that has an identifiable location in memory (i.e. having an address).

L-value: "l-value" refers to memory location which identifies an object. l-value may appear as either left hand or right hand side of an assignment operator(=). l-value often represents as identifier. Expressions referring to modifiable locations are called "modifiable l-values". A modifiable l-value cannot have an array type, an incomplete type, or a type with the const attribute. For structures and unions to be modifiable lvalues, they must not have any members with the const attribute. The name of the identifier denotes a storage location, while the value of the variable is the value stored at that location. An identifier is a modifiable lvalue if it refers to a memory location and if its type is arithmetic, structure, union or pointer. For example, if ptr is a pointer to a storage region, then *ptr is a modifiable l-value that designates the storage region to which ptr points. In C, the concept was renamed as “locator value”, and referred to expressions that locate (designate) objects. The l-value is one of the following:

  1. The name of the variable of any type i.e. , an identifier of integral, floating, pointer, structure, or union type.
  2. A subscript ([ ]) expression that does not evaluate to an array.
  3. A unary-indirection (*) expression that does not refer to an array
  4. An l-value expression in parentheses.
  5. A const object (a nonmodifiable l-value).
  6. The result of indirection through a pointer, provided that it isn't a function pointer.
  7. The result of member access through pointer(-> or .) C `

// declare 'a' an object of type 'int' int a;

// a is an expression referring to an // 'int' object as l-value a = 1;

int b = a; // Ok, as l-value can appear on right

// Switch the operand around '=' operator 9 = a;

// Compilation error: // as assignment is trying to change the // value of assignment operator

`

r-value:-

r-value simply means, an object that has no identifiable location in memory (i.e. having an address).

R-value: r-value" refers to data value that is stored at some address in memory. A r-value is an expression, that can't have a value assigned to it, which means r-value can appear on right but not on left hand side of an assignment operator(=).

C `

// declare 'a', 'b' an object of type 'int' int a = 1, b;

a + 1 = b; // Error, left expression is // is not variable(a + 1)

// declare pointer variable 'p', and 'q' int *p, *q; // *p, *q are lvalue

*p = 1; // valid l-value assignment

// below is invalid - "p + 2" is not an l-value // p + 2 = 18;

q = p + 5; // valid - "p + 5" is an r-value

// Below is valid - dereferencing pointer // expression gives an l-value *(p + 2) = 18;

p = &b;

int arr[20]; // arr[12] is an lvalue; equivalent // to *(arr+12) // Note: arr itself is also an lvalue

struct S { int m; };

struct S obj; // obj and obj.m are lvalues

// ptr-> is an lvalue; equivalent to (*ptr).m // Note: ptr and ptr are also lvalues struct S ptr = &obj;

`

Note: The unary & (address-of) operator requires an l-value as its operand. That is, &n is a valid expression only if n is an l-value. Thus, an expression such as &12 is an error. Again, 12 does not refer to an object, so it's not addressable. For instance,

C `

// declare 'a' as int variable and // 'p' as pointer variable int a, *p;

p = &a; // ok, assignment of address // at l-value

&a = p; // error: &a is an r-value

int x, y;

( x < y ? y : x) = 0; // It's valid because the ternary // expression preserves the "lvalue-ness" // of both its possible return values

`

Remembering the mnemonic, that l-values can appear on the left of an assignment operator while r-values can appear on the right.

Reference: https://msdn.microsoft.com/en-us/library/bkbs2cds.aspx