The const type qualifier

The const qualifier explicitly declares a data object as something that cannot be changed. Its value is set at initialization. You cannot use const data objects in expressions requiring a modifiable lvalue. For example, a const data object cannot appear on the lefthand side of an assignment statement.

Beginning of C only.

A const object cannot be used in constant expressions. A global const object without an explicit storage class is considered extern by default.

End of C only.

C++ Beginning of C++ only.

In C++, all const declarations must have initializers, except those referencing externally defined constants. A const object can appear in a constant expression if it is an integer and it is initialized to a constant. The following example demonstrates this:

const int k = 10;
int ary[k];     /* allowed in C++, not legal in C */

In C++ a global const object without an explicit storage class is considered static by default, with internal linkage.

const int k = 12;  /* Different meanings in C and C++ */

static const int k2 = 120;  /* Same meaning in C and C++ */
extern const int k3 = 121;  /* Same meaning in C and C++ */

Because its linkage is assumed to be internal, a const object can be more easily defined in header files in C++ than in C.

C++ End of C++ only.

An item can be both const and volatile. In this case the item cannot be legitimately modified by its own program but can be modified by some asynchronous process.

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