In the C-family of programming languages, declarations are often collected into header files, which are included in other source files that reference and use these declarations, but don't have access to the definition. The information in the header file provides the interface between code that uses the declaration and that which defines it, a form of information hiding. A declaration is often used in order to access functions or variables defined in different source files, or in a library. A mismatch between the definition type and the declaration type generates a compiler error. For variables, definitions assign values to an area of memory that was reserved during the declaration phase. For functions, definitions supply the function body. While a variable or function may be declared many times, it is typically defined once (in C++, this is known as the One Definition Rule or ODR). Dynamic languages such as JavaScript or Python generally allow functions to be redefined, that is, re-bound; a function is a variable much like any other, with a name and a value (the definition). Here are some examples of declarations that are not definitions, in C:  extern char example1; extern int example2; void example3(void); Here are some examples of declarations that are definitions, again in C:  char example1; /* Outside of a function definition it will be initialized to zero. */ int example2 = 5;  In some programming languages, an implicit declaration is provided the first time such a variable is encountered at compile time. In other languages, such a usage is considered to be an error, which may resulting in a diagnostic message. Some languages have started out with the implicit declaration behavior, but as they matured they provided an option to disable it (e.g. Perl's "codice_1" or Visual Basic's "codice_2").