Include directive

Many programming languages and other computer files have a directive, often called include, import, or copy, that causes the contents of the specified file to be inserted into the original file. These included files are called s or copybooks. They are often used to define the physical layout of program data, pieces of procedural code, and/or forward declarations while promoting encapsulation and the reuse of code or data. In computer programming, a header file is a file that allows programmers to separate certain elements of a program's source code into reusable files. Header files commonly contain forward declarations of classes, subroutines, variables, and other identifiers. Programmers who wish to declare standardized identifiers in more than one source file can place such identifiers in a single header file, which other code can then include whenever the header contents are required. This is to keep the interface in the header separate from the . The C standard library and the C++ standard library traditionally declare their standard functions in header files. Some recently created compiled languages (such as Java and C#) do not use forward declarations; identifiers are recognized automatically from s and read directly from dynamic library symbols. This means header files are not needed. The include directive allows libraries of code to be developed which help to: ensure that everyone uses the same version of a data layout definition or procedural code throughout a program, easily cross-reference where components are used in a system, easily change programs when needed (only one file must be edited), and save time by reusing data layouts. An example situation which benefits from the use of an include directive is when referring to functions in a different file. Suppose there is some C source file containing a function add, which is referred to in a second file by first declaring its external existence and type (with a function prototype) as follows: int add(int, int); int triple(int x) { return add(x, add(x, x)); } One drawback of this approach is that the function prototype must be present in all files that use the function.