Why Use Header Files Instead of Directly Including CPP Files in C ?

Why Include Headers Instead of CPP Files?

When embarking on your programming journey, it's understandable to encounter unfamiliar concepts. One such question that recently surfaced in a student's grading experience was the recommendation to use header files instead of directly including CPP files.

Delving into the Issue

Initially, the student had created CPP files without establishing corresponding header files, believing that this approach would suffice. However, the grader emphasized the importance of header files, prompting the student's inquiry into the matter.

Specifically, the student had chosen to #include "mycppfile.cpp" in their code, a practice that deviates from recommended coding techniques. The reasons behind this decision were:

• The CPP files contained all the necessary information that would typically go into header files.
• The student had observed header files being #included in other programs and followed suit.

Understanding the Distinction

It's crucial to recognize that the C standard treats header and source files equally. Both are considered legal code files. However, while including source files into your program is not illegal, it undermines the benefits of separating source files.

Consequences of Direct CPP Inclusion

The #include directive instructs the preprocessor to copy the entire specified file into the active file before compilation. This implies that including all source files together removes the advantages of utilizing separate source files altogether.

Performance Implications

In small-scale programs, the impact of direct CPP inclusion may not be noticeable. However, in real-world scenarios involving projects with millions of lines of code, compilation times can be astronomical, especially on modern CPUs.

Maintainability Concerns

When changes are made to source files, having everything merged into a single entity makes it laborious to make targeted corrections and test changes efficiently. For instance, a simple bug fix may trigger a recompilation of the entire project.

The Header File Solution

To address these challenges, the concept of header files was introduced. Header files provide a way to separate interface information (function prototypes, class definitions) from implementation details (actual function code). By only including the necessary information in header files, one can #include them into different source files, allowing for modular compilation and easy modification of individual code segments.

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