C Definitions and Declarations: Headers or .cpp Files?

C Best Practices: Where to Place Definitions and Declarations

In the realm of C programming, the question of where to place definitions and declarations has sparked many debates. Traditional practice dictates separating declarations and definitions into header and .cpp files, respectively. However, some argue that modern coding norms have shifted towards placing definitions directly in header files.

According to conventional wisdom, the standard approach is to maintain a separation between declarations and definitions. Declarations reside in header files (.h), providing an interface for other parts of the program to interact with. Definitions, on the other hand, are stored in companion .cpp files, encapsulating the actual implementation details. This separation enables header files to remain lightweight and portable, while .cpp files handle the complexities of the program's functions and data structures.

Some proponents of placing definitions in header files claim that it enhances compiler optimization by promoting inlining. However, this benefit comes at a cost. If the header files contain a substantial amount of code, it can significantly inflate compile times. Additionally, introducing circular object relationships can be challenging in such scenarios.

While "header-only" libraries, such as Boost, make use of templates to achieve this approach, such libraries are not representative of average user code. In general, separating declarations and definitions into header and .cpp files remains the recommended practice.

It is worth noting that C 17's introduction of inline variables may mitigate some of the downsides of header-only code in the future. However, for the time being, the traditional separation of concerns is still considered the best practice for maintaining code modularity, code readability, and efficient compilation.

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