Include .cpp instead of header(.h)

Question

There are some cases when we include .cpp file instead of standard header file (.h), for example:

#include "example.cpp"

instead of

#include "example.h"

It seems to work but is this safe or should I avoid it?

What about the compilation time?

It's as safe as you make it. It shouldn't generally be necessary. Programming isn't about blindly following some cargo plane, but rather about understanding what you're doing and making decisions based on understanding and reasoning. — Kerrek SB
– Kerrek SB, Commented Feb 12, 2012 at 23:38
The only valid reason I've ever seen for doing this is to save on link time. Unity Build is explained here. — Ben Voigt
– Ben Voigt, Commented Feb 12, 2012 at 23:40
@sysop: As I said: understand what you're doing, and asking, first. Includes are processed by the preprocessor, long before any compiling happens. Understand the preprocessor and you will know when it's OK to include things. — Kerrek SB
– Kerrek SB, Commented Feb 12, 2012 at 23:42

zellio · Accepted Answer · 2012-02-12 23:47:57Z

22

It's lazy coding. Use header files. Yes they can increase compile time but they mean that you can easily re-implement chunks of your code, or better yet, another developer could at anytime. The header file serves as a template for what your C/C++ code is going to do. It's a bad idea to discard or ignore it.

edited Feb 12, 2012 at 23:47

answered Feb 12, 2012 at 23:42

zellio

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3 Comments

sysop Over a year ago

I'm always use .h, but we have quite debate here and that's why I ask.

zellio Over a year ago

@sysop -- to be fair, "It's lazy coding" is a bit hyperbolic. There are cases where it might actually be the right thing to do, but in the general case I would argue that it's poor form.

tom Over a year ago

Including .h files instead of the .cpp files can speed up compile time a lot because it lets you do partial recompilations.

Dan K · Accepted Answer · 2013-07-24 16:43:46Z

8

I agree with Kerrek SB.

I did this once. I was building an excellent, widely used compression library that needed to be built separately for 8-bit images and for 12-bit images. The cleanest way I could come up with to fit this into the build system was (oversimplifying a bit) to have two master .cpp files, one that set #defines for an 8-bit build, the other for a 12-bit build. The master .cpp files then #included the source files of the compression library.

It's okay to not follow a general rule if you understand the rule well enough to know the reasons for it and why it might not apply in your case. (But those cases ought to be rare.)

answered Jul 24, 2013 at 16:43

Dan K

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Dima Tisnek · Accepted Answer · 2012-02-15 11:42:03Z

3

There are legitimate uses for #include "impl.cpp":

testing for access to static/etc variables
ad hoc templates like these if c++ template mechanism proves inadequate (rare)

#define MACRO (...)

#include "impl.cpp" // uses MACRO

Note that #include "impl.cpp" can be unsafe it same file is included in separate compilation units that are later linked together.

answered Feb 15, 2012 at 11:42

Dima Tisnek

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Joqus · Accepted Answer · 2012-02-12 23:40:53Z

1

I have used it before and had no problem but I cannot ensure that this is safe. Sometimes this was the only option for me so I used it, otherwise I will use the .h file.

answered Feb 12, 2012 at 23:40

Joqus

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1 Comment

LihO Over a year ago

There is always another option a.k.a. never say never.

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