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:
- The name of the variable of any type i.e, an identifier of integral, floating, pointer, structure, or union type.
- A subscript ([ ]) expression that does not evaluate to an array.
- A unary-indirection (*) expression that does not refer to an array
- An l-value expression in parentheses.
- A const object (a nonmodifiable l-value).
- The result of indirection through a pointer, provided that it isn’t a function pointer.
- The result of member access through pointer(-> or .)
// 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” 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(=).
// 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 lvalue as its operand. That is, &n is a valid expression only if n is an lvalue. Thus, an expression such as &12 is an error. Again, 12 does not refer to an object, so it’s not addressable. For instance,
// 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 lvalues can appear on the left of an assignment operator while rvalues can appear on the right
Reference:
https://msdn.microsoft.com/en-us/library/bkbs2cds.aspx
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