[apaprocki]

The more modern gcc handling is with "#pragma once":

http://en.wikipedia.org/wiki/Pragma_once

His comments on include files are not the way the world has gone. In all my years writing C, I have never seen a C/C++ header which does not include the other headers it needs in order to compile cleanly.

He states: "Simple rule: include files should never include include files."

If you can find me one C project out there which has headers with zero #includes and forces each compilation unit including the header to include all the prerequisites needed for that header I would be genuinely interested. Even all OS header files include their pre-requisites and it is considered a bug if they do not do so. A more modern 21st century rule would be:

"Simple rule: include file order does not matter. Use include guards (or #pragma once) and always include only the prerequisites needed to cleanly compile and nothing more."

[keithflower]

Our experiences don't match. Avoiding nested includes has been a part of the rigorous and useful standards of a lot of important, mission-critical, large-scale projects, and is something I try to enforce in my own code.

There are numerous examples of projects which follow that recommendation. I'm told, for example, that the recommendation on the excellent Blender 3-D graphics system is to avoid nested includes. Other examples:

NASA: http://sunland.gsfc.nasa.gov/info/cstyle.html

European Molecular Biology Open Software Suite (EMBOSS) http://emboss.sourceforge.net/developers/c-standards_new.htm...

Atacama Large Millimeter Array (astronomy): http://www.alma.nrao.edu/development/computing/docs/joint/00...

Clinical Neutron Therapy System (for radiation oncology): http://staff.washington.edu/jon/cnts/impl.pdf

[tptacek]

I take your point, but I'm not sure you're really taking his. As another data point, I just did a find/xargs-grep across my "~/codebase/3p" tree, which has ~5500 .h files for projects ranging from MongoDB to Ruby and Python to OpenSSL to valgrind. Virtually all of them have nested includes. Many of them are components of mission-critical software.

He's right; nested includes are a modern C idiom.

[keithflower]

> In all my years writing C, I have never seen a C/C++ header which does not include the other headers it needs in order to compile cleanly.

> He's right; nested includes are a modern C idiom.

No doubt they are found a lot. The request was for counterexamples. Fashion/style vs engineering practice assertions may be passing like ships on a nighttime C here.

>...for projects ranging from MongoDB to Ruby and Python...

Python....why, even Tim Peters asserts that "Flat is better than nested"...wink...

http://www.python.org/dev/peps/pep-0020/

/irrelevant quote

[apaprocki]

To be fair here, NASA does say to avoid nesting headers. But then it follows up with this ridiculous statement:

  In extreme cases, where a large number of header files are 
  to be included in several different source files, it is 
  acceptable to put all common #includes in one include file. 

I would not trust anything else the author of that guide writes. EMBOSS says not to nest. The Atacama code suggests not nesting but then says:

  To avoid nested includes, use #define wrappers as follows:

The last pdf states:

  This prevents including the same item more than once when 
  a .c file includes several .h files (we rejected
  alternatives involving conditional compilation as too 
  complicated).

I guess it boils down to preference since the language allows you to do either, I just don't put a lot of stock in these style guides.

[luriel]

> If you can find me one C project out there which has headers with zero #includes and forces each compilation unit including the header to include all the prerequisites needed for that header I would be genuinely interested.

A whole OS does this: Plan 9 from Bell Labs :)

For details see the paper by Rob Pike: How to Use the Plan 9 C Compiler

http://doc.cat-v.org/plan_9/4th_edition/papers/comp

Also the Plan 9 libraries are much cleaner and leaner than those in 'modern' *nix systems, which makes keeping track of includes much easier: http://man.cat-v.org/plan_9/2/intro

[apaprocki]

Well it stands to reason that the Plan 9 code would work that way if Rob Pike made the comment. I'm just wondering if anyone else found this way of coding attractive. Everything I've seen points to "no".

[catwell]

There is even a relatively popular piece of software dedicated to helping developers use this style: http://code.google.com/p/include-what-you-use/

[apaprocki]

That is not the stated purpose of the tool (to write code in the aforementioned style):

  "Include what you use" means this: for every symbol (type,  
  function variable, or macro) that you use in foo.cc, either 
  foo.cc or foo.h should #include a .h file that exports the 
  declaration of that symbol.

I agree 100% with that statement. According to the style, foo.h can never #include anything.

[charolastrauno]

That's not true - you can't forward-declare types used in class member variables for instance.

[4ad]

Dennis Ritchie, creator of C? Ken Thompson, creator of Unix?

[nitrogen]

The more modern gcc handling is with "#pragma once":

I've considered switching to this, but in the interest of maximum hypothetical portability, I've decided to stick with the #ifndef include guard. I've just checked the C1X standard draft I have, and #pragma once is still not in the standard (6.10.6.1 in the version I have).

As for your other points, I agree with the notion that a header file should include its dependencies, but no more. I suppose my thinking on this is influenced by the dependency tracking of Linux package managers such as apt.

[apaprocki]

We use both internally, actually. GNU gcc/g++ and IBM xlc/xlC all support doing this:

  #if defined(... all the compilers above ...)
  # define PRAGMA_ONCE _Pragma("once")
  #else
  # define PRAGMA_ONCE
  #endif

Specifically, MSVC and HP cc/aCC do not support it. Oracle cc/CC does the #ifndef/#define detection and internally does something similar to what the pragma enables in other compilers.