20.2 Initializers

A variable’s declaration, unless it is extern, should also specify its initial value. For numeric and pointer-type variables, the initializer is an expression for the value. If necessary, it is converted to the variable’s type, just as in an assignment.

You can also initialize a local structure-type (see Structures) or local union-type (see Unions) variable this way, from an expression whose value has the same type. But you can’t initialize an array this way (see Arrays), since arrays are not first-class objects in C (see Limitations of C Arrays) and there is no array assignment.

You can initialize arrays and structures componentwise, with a list of the elements or components. You can initialize a union with any one of its alternatives.

• A component-wise initializer for an array consists of element values surrounded by ‘{…}’. If the values in the initializer don’t cover all the elements in the array, the remaining elements are initialized to zero.

  You can omit the size of the array when you declare it, and let the initializer specify the size:

  int array[] = { 3, 9, 12 };
  

• A component-wise initializer for a structure consists of field values surrounded by ‘{…}’. Write the field values in the same order as the fields are declared in the structure. If the values in the initializer don’t cover all the fields in the structure, the remaining fields are initialized to zero.
• The initializer for a union-type variable has the form { value }, where value initializes the first alternative in the union definition.

For an array of arrays, a structure containing arrays, an array of structures, etc., you can nest these constructs. For example,

struct point { double x, y; };

struct point series[]
  = { {0, 0}, {1.5, 2.8}, {99, 100.0004} };

You can omit a pair of inner braces if they contain the right number of elements for the sub-value they initialize, so that no elements or fields need to be filled in with zeros. But don’t do that very much, as it gets confusing.

An array of char can be initialized using a string constant. Recall that the string constant includes an implicit null character at the end (see String Constants). Using a string constant as initializer means to use its contents as the initial values of the array elements. Here are examples:

char text[6] = "text!";     /* Includes the null. */
char text[5] = "text!";     /* Excludes the null. */
char text[] = "text!";      /* Gets length 6. */
char text[]
  = { 't', 'e', 'x', 't', '!', 0 };  /* same as above. */
char text[] = { "text!" };  /* Braces are optional. */

and this kind of initializer can be nested inside braces to initialize structures or arrays that contain a char-array.

In like manner, you can use a wide string constant to initialize an array of wchar_t.