Initializers

An initializer is an optional part of a data declaration that specifies an initial value of a data object.

The initializer consists of the = symbol followed by an initial expression or a braced list of initial expressions separated by commas. The number of initializers must not be more than the number of elements to be initialized. An initializer list with fewer initializers than elements, can end with a comma, indicating that the rest of the uninitialized elements are initialized to zero. The initial expression evaluates to the first value of the data object.

To assign a value to a scalar object, use the simple initializer: = expression. For example, the following data definition uses the initializer = 3 to set the initial value of group to 3:

int group = 3;

For unions, structures, and aggregate classes (classes with no constructors, base classes, virtual functions, or private or protected members), the set of initial expressions must be enclosed in { } (braces) unless the initializer is a string literal.

If the initializer of a character string is a string literal, the { } are optional. Individual expressions must be separated by commas, and groups of expressions can be enclosed in braces and separated by commas.

In an array, structure, or union initialized using a brace-enclosed initializer list, any members or subscripts that are not initialized are implicitly initialized to zero of the appropriate type.

The initialization properties of each data type are described in the section for that data type.

1. An initializer of the form (expression) can be used to initialize fundamental types in C++. For example, the following two initializations are identical:
   

   int group = 3;
   int group(3);
   

2. You can also use the (expression) form to initialize C++ classes.
3. You can initialize variables at file scope with nonconstant expressions. This is not allowed in ISO/ANSI C.
4. If your code jumps over declarations that contain initializations, the compiler generates an error. For example, the following code is not valid in C++:
   

   goto skiplabel;    // error - jumped over declaration
   int i = 3;         //   and initialization of i
   
   skiplabel: i = 4;
   

5. You can initialize classes in external, static, and automatic definitions. The initializer contains an = (equal sign) followed by a brace-enclosed, comma-separated list of values. You do not need to initialize all members of a class.

In the following example, only the first eight elements of the array grid are explicitly initialized. The remaining four elements that are not explicitly initialized are initialized as if they were explicitly initialized to zero.

static short grid[3] [4] = {0, 0, 0, 1, 0, 0, 1, 1};

The initial values of grid are:

Unions

Arrays
Enumerations
Pointers
Syntax of an Initializer
struct (Structures)