In C++, large programs are typically divided into multiple files to keep code modular, maintainable, and reusable. One of the most important tools in that structure is the header file, usually marked with the .h or .hpp extension.

Header files declare the interfaces of your program—such as function prototypes, class definitions, and constants—while leaving the implementations in separate .cpp source files.

This separation allows different source files to share the same declarations without duplicating the code, enabling both clarity and compiler efficiency.

The purpose of a header file is to act like a blueprint. When included using #include, it tells the compiler what to expect: what functions exist, what classes are available, and how objects should be constructed or used. However, blindly including headers in multiple places can lead to serious problems at compile time.

That’s where the One Definition Rule (ODR) comes in. The ODR is a fundamental C++ principle stating that every entity in your program must have exactly one definition. Violating this rule—by defining the same function or variable in more than one translation unit—leads to linker errors or undefined behavior.

To prevent this, developers use include guards or the #pragma once directive. Include guards are conditional preprocessor macros that wrap around the contents of a header file to ensure it is only included once per translation unit. For example:

#ifndef MY_HEADER_H

#define MY_HEADER_H

// declarations go here

#endif

This pattern guarantees that if the header is included multiple times across different files, the compiler will process it only once. Alternatively, modern compilers support #pragma once, which serves the same purpose with less code and fewer mistakes, though it’s not part of the official C++ standard.

In projects like Bitcoin Core, headers play a vital role in exposing cryptographic routines, networking classes, and consensus logic while separating them cleanly from their complex implementations.

This approach makes the codebase easier to maintain, test, and scale. For developers working on large C++ systems, understanding the role of headers and strictly following the One Definition Rule is non-negotiable. It ensures consistent builds, prevents duplicate symbols, and protects against a whole class of subtle and frustrating bugs.

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