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7.2 Lvalues

An expression that identifies a memory space that holds a value is called an lvalue, because it is a location that can hold a value.

The standard kinds of lvalues are:

• A variable.
• A pointer-dereference expression (see Dereferencing Pointers) using unary ‘*’, if its type is not a non-function type.
• A structure field reference (see Structures) using ‘.’, if the structure value is an lvalue.
• A structure field reference using ‘->’. This is always an lvalue since ‘->’ implies pointer dereference.
• A union alternative reference (see Unions), on the same conditions as for structure fields.
• An array-element reference using ‘[…]’, if the array is an lvalue.
• A string constant (see String Constants).
• An array constructor (see Constructing Array Values).
• A structure or union constructor (see Structure Constructors).

If an expression’s outermost operation is any other operator, that expression is not an lvalue. Thus, the variable x is an lvalue, but x + 0 is not, even though these two expressions compute the same value (assuming x is a number).

It is rare that a structure value or an array value is not an lvalue, but that does happen—for instance, the result of a function call or a conditional operator can have a structure or array type, but is never an lvalue.

If an array is an lvalue, using the array in an expression still converts it automatically to a pointer to the zeroth element. The result of this conversion is not an lvalue. Thus, if the variable a is an array, you can’t use a by itself as the left operand of an assignment. But you can assign to an element of a, such as a[0]. That is an lvalue since a is an lvalue.