In programming, declarations are statements that introduce identifiers—such as variables, functions, or classes—into the program and specify their types and other attributes. A declaration provides the necessary information to the compiler or interpreter about how the identifier should be used and how it interacts with other components in the code.
Declarations serve several purposes:
Type Checking: They enable static type checking, allowing the programming language to enforce type constraints and ensure that operations are performed on compatible data types.
Storage Representation: They inform the compiler about the type of the declared data object, which helps determine the most efficient storage representation, reducing the program's storage requirements and execution time.
Storage Management: Declarations indicate the desired lifetime of the data object, enabling more effective storage management during program execution. For instance, in C, some objects can be declared at the beginning of a subprogram, while others are created dynamically using functions like malloc.
Polymorphic Operations: In languages that support polymorphism, declarations help the compiler decide at compile time the specific operation to be used for an overloaded operation symbol, based on the types of its arguments.
There are two main types of declarations:
Explicit Declaration: This is a statement in a program that lists data object names and explicitly defines them as a specific type, creating a static binding to that type. For example, int a, b; in C declares a and b as integer data objects.
Implicit Declaration: This occurs when no explicit declarations are provided, and the programming language assumes a default type based on conventions or context. For example, in older versions of FORTRAN, variables starting with certain letters were implicitly declared as integers, while others were considered to be of real type.
