How Are Partially Initialized Automatic Structures and Arrays Handled in C and C ?

Partial Initialization in Automatic Structures and Arrays in C/C

When initializing an automatic structure or array with fewer initializers than elements, it's important to understand the behavior specified by the C and C standards.

Complete and Partial Initialization

The C standard defines two types of initialization for automatic variables: complete initialization and no initialization. Partial initialization is a non-standard term that refers to a situation where only a subset of the elements or members is initialized.

Rules for Partial Initialization

C99 Standard

In C99, for automatic arrays and structures, if there are fewer initializers than elements, the remaining elements are initialized implicitly the same as objects with static storage duration. This means they are initialized to 0 for integer types.

C 03 Standard

In C , for automatic arrays and structures, uninitialized members are value-initialized. For class types, this means invoking the default constructor. For built-in types like int, it means zero-initialization.

Example

In C, initializing an automatic array of integers with a single value, e.g.:

int arr[10] = {123,};

will initialize the first element to 123 and all the remaining elements to 0, as specified by the C standard.

Compatibility with Different Compilers

Most mainstream compilers follow the rules for partial initialization as specified by the C and C standards. However, to ensure compatibility across different compilers, it's recommended to initialize all elements or members explicitly.

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