An object is a region of storage that can be examined and stored into. An lvalue is an expression that refers to such an object. An lvalue does not necessarily permit modification of the object it designates. For example, a const object is an lvalue that cannot be modified. The term modifiable lvalue is used to emphasize that the lvalue allows the designated object to be changed as well as examined. The following object types are lvalues, but not modifiable lvalues:
- An array type
- An incomplete type
- A const-qualified type
- An object is a structure or union type and one of its members has a const-qualified type
Because these lvalues are not modifiable, they cannot appear on the left side of an assignment statement.
The term rvalue refers to a data value that is stored at some address in memory. An rvalue is an expression that cannot have a value assigned to it. Both a literal constant and a variable can serve as an rvalue. When an lvalue appears in a context that requires an rvalue, the lvalue is implicitly converted to an rvalue. The reverse, however, is not true: an rvalue cannot be converted to an lvalue. Rvalues always have complete types or the void type.
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