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I am working on my honors thesis and want to change my threading method from std::threads to include boost::threadpool. I am able to accomplish this easily by just doing a
#include "../include/threadpool.hpp"
within each of my files, but this becomes very repetitive with many files / changes to thread systems. Is there some way to give a compiler flag (or something else) so that all files natively #include that file?
My naive thought would be something like
//dependencies.h
#include "../include/threadpool.hpp"
#include //some other dependency I need.h
And pass a
-I "path_to_dependencies.h"
flag to compiler. Would something like this work or am I off base?
Thanks!
The latter is the preferred way to do it. In fact you can go one step farther and make it a precompiled header, which will cut down compilation time by a lot, especially if your project gets large.
I assume you're using gcc, you can read about precompiled headers here: https://gcc.gnu.org/onlinedocs/gcc/Precompiled-Headers.html
For Microsoft's compiler you have the stdafx.h file everyone knows about since it enables precompiled headers by default.
I am currently working on program with a lot of source files. Sometimes it is difficult to keep track of what libraries I have already #included. Theoretically, I could make a single header file called Headers.h that just contains all the #include statements I need, then make all other header files #include "Headers.h".
Why is this a good/bad idea?
Pros:
Slightly less maintenance as you don't have to keep track of which of your files are including headers from which libraries or other compoenents.
Cons:
Definitions in included files might conflict with each other. Especially in C where you don't have namespaces (you tagged with C and C++)
Macros in particular can cause hard to debug problems, where a macro definition unexpectedly conflicts with some name in your file or one of the other included files
Depending on which compiler you use, compilation times might blow out. If using a compiler that pre-compiles headers it might actually reduce compilation time, but if not the opposite will happen
You will often unnecessarily trigger rebuilds of files. If you have your build system set up correctly, then each source file will get rebuilt if any of the included files gets modified. If you always include all headers in your project, then a change to any of your headers will force recompilation of all your source files. Not likely to be an issue for system headers but it will be if you include your own headers in the master file as well.
On the whole I would not recommend that approach. The last con listed above it particularly important.
Best practice would be to include only headers that are needed for the code in each file.
In complement of Harmic's answer, indeed the main issue is the build system (most builders work on file timestamp, not on file contents. omake is a notable exception).
Notice that if you only care about many dependencies, GNU make can be used with autodependencies, together with -M* options passed to GCC (i.e. to g++ and actually to the preprocessor).
However, many libraries are offering to their user a single header (e.g. <gtk/gtk.h>)
Also, a single header file is more friendly to precompiled headers technology. In particular, GCC wants a single header for precompilation.
See also ccache.
Tracking all the required includes would be more difficult as they are abstracted from their c source files and not really supporting modularisation pus all the cons from #harmic
Is there an automated way to take a large amount of C++ header files and combine them in a single one?
This operation must, of course, concatenate the files in the right order so that no types, etc. are defined before they are used in upcoming classes and functions.
Basically, I'm looking for something that allows me to distribute my library in two files (libfoo.h, libfoo.a), instead of the current bunch of include files + the binary library.
As your comment says:
.. I want to make it easier for library users, so they can just do one single #include and have it all.
Then you could just spend some time, including all your headers in a "wrapper" header, in the right order. 50 headers are not that much. Just do something like:
// libfoo.h
#include "header1.h"
#include "header2.h"
// ..
#include "headerN.h"
This will not take that much time, if you do this manually.
Also, adding new headers later - a matter of seconds, to add them in this "wrapper header".
In my opinion, this is the most simple, clean and working solution.
A little bit late, but here it is. I just recently stumbled into this same problem myself and coded this solution: https://github.com/rpvelloso/oneheader
How does it works?
Your project's folder is scanned for C/C++ headers and a list of headers found is created;
For every header in the list it analyzes its #include directives and assemble a dependency graph in the following way:
If the included header is not located inside the project's folder then it is ignored (e.g., if it is a system header);
If the included header is located inside the project's folder then an edge is create in the dependency graph, linking the included header to the current header being analyzed;
The dependency graph is topologically sorted to determine the correct order to concatenate the headers into a single file. If a cycle is found in the graph, the process is interrupted (i.e., if it is not a DAG);
Limitations:
It currently only detects single line #include directives (e.g., #include );
It does not handles headers with the same name in different paths;
It only gives you a correct order to combine all the headers, you still need to concatenate them (maybe you want remove or modify some of them prior to merging).
Compiling:
g++ -Wall -ggdb -std=c++1y -lstdc++fs oneheader.cpp -o oneheader[.exe]
Usage:
./oneheader[.exe] project_folder/ > file_sequence.txt
(Adapting an answer to my dupe question:)
There are several other libraries which aim for a single-header form of distribution, but are developed using multiple files; and they too need such a mechanism. For some (most?) it is opaque and not part of the distributed code. Luckily, there is at least one exception: Lyra, a command-line argument parsing library; it uses a Python-based include file fuser/joiner script, which you can find here.
The script is not well-documented, but they way you use it is with 3 command-line arguments:
--src-include - The include file to convert, i.e. to merge its include directives into its body. In your case it's libfoo.h which includes the other files.
--dst-include - The output file to write - the result of the merging.
--src-include-dir - The directory relative to which include files are specified (i.e. an "include search path" of one directory; the script doesn't support the complex mechanism of multiple include paths and search priorities which the C++ compiler offers)
The script acts recursively, so if file1.h includes another file under the --src-include-dir, that should be merged in as well.
Now, I could nitpick at the code of that script, but - hey, it works and it's FOSS - distributed with the Boost license.
If your library is so big that you cannot build and maintain a single wrapping header file like Kiril suggested, this may mean that it is not architectured well enough.
So if your library is really huge (above a million lines of source code), you might consider automating that, with tools like
GCC make dependency generator preprocessor options like -M -MD -MF etc, with another hand made script sorting them
expensive commercial static analysis tools like coverity
customizing a compiler thru plugins or (for GCC 4.6) MELT extensions
But I don't understand why you want an automated way of doing this. If the library is of reasonable size, you should understand it and be able to write and maintain a wrapping header by hand. Automating that task will take you some efforts (probably weeks, not minutes) so is worthwhile only for very large libraries.
If you have a master include file that includes all others available, you could simply hack a C preprocessor re-implementation in Perl. Process only ""-style includes and recursively paste the contents of these files. Should be a twenty-liner.
If not, you have to write one up yourself or try at random. Automatic dependency tracking in C++ is hard. Like in "let's see if this template instantiation causes an implicit instantiation of the argument class" hard. The only automated way I see is to shuffle your include files into a random order, see if the whole bunch compiles, and re-shuffle them until it compiles. Which will take n! time, you might be better off writing that include file by hand.
While the first variant is easy enough to hack, I doubt the sensibility of this hack, because you want to distribute on a package level (source tarball, deb package, Windows installer) instead of a file level.
You really need a build script to generate this as you work, and a preprocessor flag to disable use of the amalgamate (that could be for your uses).
To simplify this script/program, it helps to have your header structures and include hygiene in top form.
Your program/script will need to know your discovery paths (hint: minimise the count of search paths to one if possible).
Run the script or program (which you create) to replace include directives with header file contents.
Assuming your headers are all guarded as is typical, you can keep track of what files you have already physically included and perform no action if there is another request to include them. If a header is not found, leave it as-is (as an include directive) -- this is required for system/third party headers -- unless you use a separate header for external includes (which is not at all a bad idea).
It's good to have a build phase/translation that includes header alone and produces zero warnings or errors (warnings as errors).
Alternatively, you can create a special distribution repository so they never need to do more than pull from it occasionally.
What you want to do sounds "javascriptish" to me :-) . But if you insist, there is always "cat" (or the equivalent in Windows):
$ cat file1.h file2.h file3.h > my_big_file.h
Or if you are using gcc, create a file my_decent_lib_header.h with the following contents:
#include "file1.h"
#include "file2.h"
#include "file3.h"
and then use
$ gcc -C -E my_decent_lib_header.h -o my_big_file.h
and this way you even get file/line directives that will refer to the original files (although that can be disabled, if you wish).
As for how automatic is this for your file order, well, it is not at all; you have to decide the order yourself. In fact, I would be surprised to hear that a tool that orders header dependencies correctly in all cases for C/C++ can be built.
usually you don't want to include every bit of information from all your headers into the special header that enables the potential user to actually use your library. The non-trivial removal of type definitions, further includes or defines, that are not necessary for the user of your interface to know can not be automatedly done. As far as I know.
Short answer to your main question:
No.
My suggestions:
manually make a new header, that contains all relevant information (nothing more, nothing less) for the user of your library interface. Add nice documentation comments for each component it contains.
use forward declarations where possible, instead of full-fledged included definitions. Put the actual includes in your implementation files. The less include statements you have in your headers, the better.
don't build a deeply nested hierarchy of includes. This makes it extremely hard to keep an overview on the contents of every bit you include. The user of your library will look into the header to learn how to use it. And he will probably not be able to distinguish relevant code from irrelevant on the first sight. You want to maximize the ratio of relevant code per total code in the main header for your library.
EDIT
If you really do have a toolkit library, and the order of inclusion really does not matter, and you have a bunch of independent headers, that you want to enumerate just for convenience into a single header, then you can use a simple script. Like the following Python (untested):
import glob
with open("convenience_header.h", 'w') as f:
for header in glob.glob("*.h"):
f.write("#include \"%s\"\n" % header)
I'm having several "undefined reference" (during linkage) and "unresolved symbol" (during runtime after dlopen) issues where I work. It is quite a large makefile system.
Are there general rules and guidelines for linking libraries and using compiler flags/options to evade these types of errors?
IF YOU WERE USING MSVC :
You cannot evade this type of error by setting a flag : it means some units (.cpp) dont' have definitions of declared identifiers. It's certainly caused by missing includes or missing object definitions (often static objects) somewhere.
While developing you can follow those guidelines ( from those articles ) to be sure all your cpp includes all the headers they need but no more :
Every cpp file includes its own header file first. This is the most
important guideline; everything else
follows from here. The only exception
to this rule are precompiled header
includes in Visual Studio; those
always have to be the first include in
the file. More about precompiled
headers in part two of this article.
A header file must include all the header files necessary to parse it.
This goes hand in hand with the first
guideline. I know some people try to
never include header files within
header files claiming efficiency or
something along those lines. However,
if a file must be included before a
header file can be parsed, it has to
be included somewhere. The advantage
of including it directly in the header
file is that we can always decide to
pull in a header file we’re interested
in and we’re guaranteed that it’ll
work as is. We don’t have to play the
“guess what other headers you need”
game.
A header file should have the bare minimum number of header files
necessary to parse it. The previous
rule said you should have all the
includes you need in a header file.
This rule says you shouldn’t have any
more than you have to. Clearly, start
by removing (or not adding in the
first place) useless include
statements. Then, use as many forward
declarations as you can instead of
includes. If all you have are
references or pointers to a class, you
don’t need to include that class’
header file; a forward reference will
do nicely and much more efficiently.
But as commenter have suggested, it seem you're using g++...
Setting up a build system where X depends on Y which depends on Z helps. It's when you get into circles (Z depends on X) that things get ugly.
Oftentimes it's the order libraries are linked ("-lZ -lY -lX" vs "-lX -lY -lZ") that causes grief. More rarely, you have the same library-name in multiple places on your search path, or your linking against outdated versions that have not yet been recompiled.
"nm --demangle" can let you see where things are defined/used.
"ldd" can be used to see what dynamic libraries you depend on.
The gcc/g++ flag -print-file-name=LIBRARY can help track down exactly which library is being used.
Afterthought: (Since you ask about rules/guidelines.)
It is possible to set up a makefile system such that:
If module=D depends on modules A,B,&C.
Then trying to make module=D would first make modules A,B,&C.
And, more importantly, module=D would automatically determine its libraries (-lA,etc), library paths (-LA), and include paths (-IA) from the makefiles for modules A,B,&C.
That can get a little hairy to set up. Last time I did it, I favored merely caching the information rather than forking an excessive number of make subprocesses. Coupled with makefile-importing and a little perl script to remove duplicates. Kludgey, I know. (Powers that be didn't want to spend time on infrastructure.) But it can be done.
Then again, I was using GNU-make, which has a few extensions.
I am working on a large C++ project in Visual Studio 2008, and there are a lot of files with unnecessary #include directives. Sometimes the #includes are just artifacts and everything will compile fine with them removed, and in other cases classes could be forward declared and the #include could be moved to the .cpp file. Are there any good tools for detecting both of these cases?
While it won't reveal unneeded include files, Visual studio has a setting /showIncludes (right click on a .cpp file, Properties->C/C++->Advanced) that will output a tree of all included files at compile time. This can help in identifying files that shouldn't need to be included.
You can also take a look at the pimpl idiom to let you get away with fewer header file dependencies to make it easier to see the cruft that you can remove.
PC Lint works quite well for this, and it finds all sorts of other goofy problems for you too. It has command line options that can be used to create External Tools in Visual Studio, but I've found that the Visual Lint addin is easier to work with. Even the free version of Visual Lint helps. But give PC-Lint a shot. Configuring it so it doesn't give you too many warnings takes a bit of time, but you'll be amazed at what it turns up.
There's a new Clang-based tool, include-what-you-use, that aims to do this.
!!DISCLAIMER!! I work on a commercial static analysis tool (not PC Lint). !!DISCLAIMER!!
There are several issues with a simple non parsing approach:
1) Overload Sets:
It's possible that an overloaded function has declarations that come from different files. It might be that removing one header file results in a different overload being chosen rather than a compile error! The result will be a silent change in semantics that may be very difficult to track down afterwards.
2) Template specializations:
Similar to the overload example, if you have partial or explicit specializations for a template you want them all to be visible when the template is used. It might be that specializations for the primary template are in different header files. Removing the header with the specialization will not cause a compile error, but may result in undefined behaviour if that specialization would have been selected. (See: Visibility of template specialization of C++ function)
As pointed out by 'msalters', performing a full analysis of the code also allows for analysis of class usage. By checking how a class is used though a specific path of files, it is possible that the definition of the class (and therefore all of its dependnecies) can be removed completely or at least moved to a level closer to the main source in the include tree.
I don't know of any such tools, and I have thought about writing one in the past, but it turns out that this is a difficult problem to solve.
Say your source file includes a.h and b.h; a.h contains #define USE_FEATURE_X and b.h uses #ifdef USE_FEATURE_X. If #include "a.h" is commented out, your file may still compile, but may not do what you expect. Detecting this programatically is non-trivial.
Whatever tool does this would need to know your build environment as well. If a.h looks like:
#if defined( WINNT )
#define USE_FEATURE_X
#endif
Then USE_FEATURE_X is only defined if WINNT is defined, so the tool would need to know what directives are generated by the compiler itself as well as which ones are specified in the compile command rather than in a header file.
Like Timmermans, I'm not familiar with any tools for this. But I have known programmers who wrote a Perl (or Python) script to try commenting out each include line one at a time and then compile each file.
It appears that now Eric Raymond has a tool for this.
Google's cpplint.py has an "include what you use" rule (among many others), but as far as I can tell, no "include only what you use." Even so, it can be useful.
If you're interested in this topic in general, you might want to check out Lakos' Large Scale C++ Software Design. It's a bit dated, but goes into lots of "physical design" issues like finding the absolute minimum of headers that need to be included. I haven't really seen this sort of thing discussed anywhere else.
Give Include Manager a try. It integrates easily in Visual Studio and visualizes your include paths which helps you to find unnecessary stuff.
Internally it uses Graphviz but there are many more cool features. And although it is a commercial product it has a very low price.
You can build an include graph using C/C++ Include File Dependencies Watcher, and find unneeded includes visually.
If your header files generally start with
#ifndef __SOMEHEADER_H__
#define __SOMEHEADER_H__
// header contents
#endif
(as opposed to using #pragma once) you could change that to:
#ifndef __SOMEHEADER_H__
#define __SOMEHEADER_H__
// header contents
#else
#pragma message("Someheader.h superfluously included")
#endif
And since the compiler outputs the name of the cpp file being compiled, that would let you know at least which cpp file is causing the header to be brought in multiple times.
PC-Lint can indeed do this. One easy way to do this is to configure it to detect just unused include files and ignore all other issues. This is pretty straightforward - to enable just message 766 ("Header file not used in module"), just include the options -w0 +e766 on the command line.
The same approach can also be used with related messages such as 964 ("Header file not directly used in module") and 966 ("Indirectly included header file not used in module").
FWIW I wrote about this in more detail in a blog post last week at http://www.riverblade.co.uk/blog.php?archive=2008_09_01_archive.xml#3575027665614976318.
Adding one or both of the following #defines
will exclude often unnecessary header files and
may substantially improve
compile times especially if the code that is not using Windows API functions.
#define WIN32_LEAN_AND_MEAN
#define VC_EXTRALEAN
See http://support.microsoft.com/kb/166474
If you are looking to remove unnecessary #include files in order to decrease build times, your time and money might be better spent parallelizing your build process using cl.exe /MP, make -j, Xoreax IncrediBuild, distcc/icecream, etc.
Of course, if you already have a parallel build process and you're still trying to speed it up, then by all means clean up your #include directives and remove those unnecessary dependencies.
Start with each include file, and ensure that each include file only includes what is necessary to compile itself. Any include files that are then missing for the C++ files, can be added to the C++ files themselves.
For each include and source file, comment out each include file one at a time and see if it compiles.
It is also a good idea to sort the include files alphabetically, and where this is not possible, add a comment.
If you aren't already, using a precompiled header to include everything that you're not going to change (platform headers, external SDK headers, or static already completed pieces of your project) will make a huge difference in build times.
http://msdn.microsoft.com/en-us/library/szfdksca(VS.71).aspx
Also, although it may be too late for your project, organizing your project into sections and not lumping all local headers to one big main header is a good practice, although it takes a little extra work.
If you would work with Eclipse CDT you could try out http://includator.com to optimize your include structure. However, Includator might not know enough about VC++'s predefined includes and setting up CDT to use VC++ with correct includes is not built into CDT yet.
The latest Jetbrains IDE, CLion, automatically shows (in gray) the includes that are not used in the current file.
It is also possible to have the list of all the unused includes (and also functions, methods, etc...) from the IDE.
Some of the existing answers state that it's hard. That's indeed true, because you need a full compiler to detect the cases in which a forward declaration would be appropriate. You cant parse C++ without knowing what the symbols mean; the grammar is simply too ambiguous for that. You must know whether a certain name names a class (could be forward-declared) or a variable (can't). Also, you need to be namespace-aware.
Maybe a little late, but I once found a WebKit perl script that did just what you wanted. It'll need some adapting I believe (I'm not well versed in perl), but it should do the trick:
http://trac.webkit.org/browser/branches/old/safari-3-2-branch/WebKitTools/Scripts/find-extra-includes
(this is an old branch because trunk doesn't have the file anymore)
If there's a particular header that you think isn't needed anymore (say
string.h), you can comment out that include then put this below all the
includes:
#ifdef _STRING_H_
# error string.h is included indirectly
#endif
Of course your interface headers might use a different #define convention
to record their inclusion in CPP memory. Or no convention, in which case
this approach won't work.
Then rebuild. There are three possibilities:
It builds ok. string.h wasn't compile-critical, and the include for it
can be removed.
The #error trips. string.g was included indirectly somehow
You still don't know if string.h is required. If it is required, you
should directly #include it (see below).
You get some other compilation error. string.h was needed and isn't being
included indirectly, so the include was correct to begin with.
Note that depending on indirect inclusion when your .h or .c directly uses
another .h is almost certainly a bug: you are in effect promising that your
code will only require that header as long as some other header you're using
requires it, which probably isn't what you meant.
The caveats mentioned in other answers about headers that modify behavior
rather that declaring things which cause build failures apply here as well.