how to do includes right

Friday, February 20. 2009

how to do includes right

Perhaps you remember such a discussion on core-devel quite some time ago. It resulted in a section on Techbase about the topic, with all the explanations why to do it that way. Too bad that the people who create the Microsoft Visual Studio stdlib headers haven't read it. This is too stupid...

I have a project at university where all the filenames start with a long prefix - and that prefix is the same for all the files. Tedious for opening files, even with auto-completion. So I 'ln -s'ed all the files to a filename without the prefix. This broke my build on Windows (the files are made available to a Windows running in VirtualBox, which is why I can use ln -s) as there now was a Math.h file in the directory. And guess what the MSVC compiler did... The <cmath> include has a

#include <math.h>

line which made MSVC include my Math.h file instead of the math.h file lying in the same directory as the cmath file. Just to prove my point I fixed the cmath file to use

#include "math.h"

and now it compiles fine.

So if you write a library and don't know the difference between #include <...> and #include "..." look at that Techbase page again, so that your library won't waste somebodies time like Microsoft just did to me.

Posted by Matthias Kretz in University at 11:59 | Comments (6) | Trackbacks (0)

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However, keep in mind it depends on the compiler what the exact meaning of <> and "" is.

#1 litb on 2009-02-20 13:19 (Reply)

However, keep in mind it depends on the compiler what the exact meaning of <> and "" is.

#2 litb on 2009-02-20 13:20 (Reply)

Kerninghan & Ritchie:

#include <filename>

[..] The named file is searched for in a sequence of implementation-dependent places.

#include "filename"

searches first in association with the original source file (a deliberately implementation-dependent phrase), and if that search fails, then as if in the first form.

This means that it is pretty safe to assume that
#include "filename"
will do a search relative to the original source file as long as the compiler is running on an OS with a traditional filesystem. (Dunno of any OS where that assumption would break.)

#2.1 Matthias Kretz (Homepage) on 2009-02-20 15:05 (Reply)

Thanks for the k&r quote. I haven't got that book here, only the C++ standard. But i would agree if it's written in k&r then it has at least some impact on implementations

It's also what i have experienced if you use "" then it's looking relative to the file first. What i wanted to say is that you just have to remind it's not a requirement for the compiler to do it. So i still agree with your blog fully, mate

#2.1.1 litb on 2009-02-20 15:16 (Reply)

Unfortunately, the problem goes deeper.

Imagine that you have a C file that has to use math.h:
#include <math.h>

That breaks.

Or let's imagine that your code is using a third-party C library that needs math.h. It'll do:
#include <math.h>

That breaks too.

Put the blame in the case-insensitiveness of the filesystem.

#3 Thiago Macieira on 2009-02-20 23:59 (Reply)

Yes, I noticed that problem, too. But OTOH I'd expect that I had found that problem a lot faster than what cmath compilation was telling me about. Lots of lines like this:
c:\Program Files\Microsoft Visual Studio .NET\Vc7\include\cmath(22) : error C2039: 'acos' : is not a member of 'operator``global namespace''
c:\Program Files\Microsoft Visual Studio .NET\Vc7\include\cmath(22) : error C2873: 'acos' : symbol cannot be used in a using-declaration
c:\Program Files\Microsoft Visual Studio .NET\Vc7\include\cmath(22) : error C2039: 'asin' : is not a member of 'operator``global namespace''
c:\Program Files\Microsoft Visual Studio .NET\Vc7\include\cmath(22) : error C2873: 'asin' : symbol cannot be used in a using-declaration
c:\Program Files\Microsoft Visual Studio .NET\Vc7\include\cmath(23) : error C2039: 'atan' : is not a member of 'operator``global namespace''
c:\Program Files\Microsoft Visual Studio .NET\Vc7\include\cmath(23) : error C2873: 'atan' : symbol cannot be used in a using-declaration

#3.1 Matthias Kretz (Homepage) on 2009-02-21 08:41 (Reply)

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