HeaderDatei

HeaderDatei, or header file, is a source file that contains declarations to be shared by multiple source files in a program. In C and C++, header files typically carry declarations of functions and variables, type definitions, constants, and macros. They may also declare class and template types in C++. The actual executable code usually resides in separate implementation files (source files).

Header files serve two main purposes: enabling separate compilation by allowing compilers to verify interfaces without needing function definitions, and providing a central point to declare interfaces that can be included in multiple translation units. They usually have extensions such as .h, .hpp, or .hh.

Contents typically include function prototypes, class or struct declarations, enums, typedefs, and macro definitions. In C++, templates and inline function definitions are often placed in header files because they must be visible to the compiler at the point of use. To avoid multiple definition errors, header files are protected by include guards or by #pragma once.

The include mechanism uses the preprocessor directive #include to insert the header’s contents into a translation unit. Proper header design minimizes dependencies, uses forward declarations where possible, and avoids pulling in heavy or unrelated headers. Circular dependencies should be avoided.

In build systems, changes to a header can trigger recompilation of all dependent source files, so strategies such as forward declarations, precompiled headers (pch), and careful dependency management are important. This article provides a concise overview of the role and common practices around header files in C and C++ contexts.