在C / C ++中包含未使用的头文件会有性能影响吗？

Question

I have a project where each C/C++ file uses a bunch of header files. But about 70-80% of the header files that each C/C++ file uses is the same. So to make my code more readable, I am planning to include all the headers that I will need in the project into a single header file say common_headers.h and include this in all my C/C++ files like this:

#include "common_headers.h"

Now this will include all the necessary headers but also few extra headers that will not be used by an individual file. I want to know if doing this way, would it hit the performance at run time by any chance?

I am fine with a few milliseconds extra delay for compiling the code, but I want to know if this will impact my runtime performance?

Description of headers used:

1. Most of them are standard C/C++ headers.
2. The user defined headers have inline template functions in them .
3. No static functions in user defined headers.

This is my Compiler: g++ (GCC) 4.4.7 20120313 (Red Hat 4.4.7-3)

Answer 1

COMPILATION:

If something is included then it has to analyzed even if it will never be actually compiled and linked, so compilation time will increase for sure - Do not include unused headers .

RUNTIME:

It has been already mentioned by @DonReba that unused headers may include some pragma directives that can change the resulting executable, but usually it won't be the case.

Most of the unused functions and declaraions will be optimized out, excluding some specific cases - Do unused functions get optimized out? . The resulting exe may become a bit bigger, but those functions and variables won't be used, so overall impact will be minimal. - - Nonetheless, do not include unused headers .

SUMMARY:

If you can modify your source code to not include anything unneeded - modify it.

Personnaly I prefer to have self-contained modules(headers), that include everything they need - nothing more, nothing less. Such modules can be added and removed without hindsight and possibility that some unneeded dependency has been left. They are still not panacea, but coupled with attentiveness and some code analysis they will keep your program free from deadweight headers.

EDIT:

Precompiled headers:

Precompiled headers are used to reduce compilation time for often used, but rarely changed headers(system headers, huge project headers), so if those unused headers are included in the precompiled header, then the compilation time effect during subsequent compilations will be minimized. Still, all runtime issues, no matter how small they are, stay the same as for simple header includes.

Answer 2

Short answer to the question asked: No.

Long answer:

More headers mean marginally more chance of some problem appearing that might manifest as a performance issue, but it's really not a concern.

Many consider it poor style to do as you plan, but there are also those that consider it acceptable.

One of the key reasons for avoiding this style is that it will make it easier to get circular dependencies.

I would discourage it due to the compile time issue where I do not have pre-compiled headers.

Answer 3

Depends on compiler. Today's most compilers are smart and use precompiled headers to improve performance. I use GCC compiler which supports precompiled header and AFAIK does not affect performance.

Answer 4

It is possible that inclusion of an extra header would produce different runtime code due to overriding a preprocessor directive. But this would not be a normal situation.

Visual C++, GCC, and Clang support precompiled headers for improving compilation times for use cases like yours.