I have a solution which contains several projects. My projects (but not all of them) use precompiled headers. I decided to use protobuf and I've met a problem. After generetaing *.pb.h from *.proto by protoc.exe I'm trying to include the header and get the error - precompiled header wasn't included into *.pb.h.
How I can solve this problem? I have an idea (but I don't like it at all) - after protoc generates *.pb.h I can run some script, which'll include my precompiled header into the *.pb.h. But I don't like it because some projects may not use PCH, and PCH file name can be different.
I understand that I can just remove PCH from my projects, but I don't like that idea too.
Dont add the generated myproto.pb.cc to your project. Instead, create a myproto.cpp with
#include "pch.h"
#include "myproto.pb.cc"
I resolved my problem by creating a static library called proto-objects (without PCH) and including all my *pb.h(cpp) files there. After it I link that library to every project where I need my protobuf objects. Profit!
You can disable the pre-compiled header option on a file-by-file basis.
Given that the pch option is intended to speed up compilation, you can turn it off for the whole project, and no further changes should be necessary.
The choice of name of the header file, and the pch file are also selectable per file in the project
Update
The idea behind Microsoft's Pre-compilation PCH system is to
Speed up compilation
Make it easy to use
The header file system in C/C++ is problematic, as it is really a textual replacement.
That means that
#include "localdefs.h"
#include <windows.h>
#include "project.h"
#include "support.h"
Is in no way similar to
#include <windows.h>
#include "project.h"
#include "support.h"
That is because localdefs.h can redefine the behavior of all of the other includes.
Further to this the costs of walking through the complexities of the windows.h header files, is time consuming.
The PCH system tries to solve this by the observation that most projects have a fixed set of include files which are included by most/all of the CPP files.
Defining this set in stdafx.h allows the textual result of that parsing to be pasted in the cpp file and save a lot of work.
If most of the includes in the project are different, then there is no need to use it.
So if you are including the same qt header files in lots of places - add them to a pre-compiled header file. The more of the common includes added to this file, the better the compile speed improvements will be.
Any non-standard cpp file can be excluded by being specifically disabled - examples are "generated files". Where the template generator does not understand the MSVC system.
If all the files are different, then only limited performance benefit will be gained - as each compile would probably also include a pch recompile.
Related
First of all i want to say that I read about precompiled headers and I understand that this is an optimization that saves me the time of compiling headers over and over on every built.
I'm reading the documentation of boost and I see that in the instructions they say:
In Configuration Properties > C/C++ > Precompiled Headers, change Use Precompiled Header (/Yu) to Not Using Precompiled Headers
And then they explain it:
There's no problem using Boost with precompiled headers; these instructions merely avoid precompiled headers because it would require Visual Studio-specific changes to the source code used in the examples.
Can some explain me the sentence I marked in bold? which visual studio specific changes they are talking about ? (Here is the link to the documentation I'm reading: http://www.boost.org/doc/libs/1_55_0/more/getting_started/windows.html#pch)
Why and when I would want to turn off the precompiled headers?
what is the difference between "Create" and "Use" in the precompiled header options.
Originally a comment, but I may as well post it. Note: this is specific to VC++:
The bold sentence is their way of saying the samples don't follow the mantra of a unified use-this-lead-in-header-for-pch-generation model. IOW, their samples aren't PCH-friendly, but you can still use pch with boost in your projects if properly configured.
You would turn them off for a variety of reasons. Some source modules, particularly ones from 3rd-parties, don't follow the PCH model of including "the" pch-through-header at their outset. Their samples are such code (and thus the advise to turn them off for their samples). Sometimes source files require different preprocessor configurations only for this files and not all files int he project; another reason to disable PCH for those files.
You typically use a source/header pair to generate "the One"; the precompiled header image. This header file typically includes:
Any system standard lib headers used by your project
3rd-party SDK headers
Just about everything else that is NOT in active development for your project.
The single source file tagged as Create typically includes one line of code : #include "YourHeaderFile.h", where YourHeaderFile.h is the header you filled with stuff from the list above. Tagging it as "Create" through header YourHeaderFile.h tells VC it is the file needed for rebuilding the PCH through that header when compiling other source files. All other source files are tagged as Use (except the ones where PCH is turned off) and should include, as their first line of code, the same #include "TheHeaderFile.h".
In short (hard to believe), <boost> is telling you their samples aren't setup like described above, and as such you should turn PCH off when building them.
When you use pre-compiled headers, you need to do something like:
#include <foo>
#include <bar>
#include <baz>
#pragma hdrstop
// other code here
Everything before the #pragma goes into the precompiled header. Everything after it depends on the precompiled header. The VC++ specific "magic" to make pre-compiled header work is that #pragma.
There's a little more to the story than just that though. To make pre-compiled headers work well, you want to include exactly the same set of headers in exactly the same order in every source file.
That leads to (typically) creating one header that includes all the other common headers and has the #pragma hdrstop right at its end, then including that in all the other source files.
Then, when the compiler does its thing, there are two phases: first you need to create a pre-compiled header. This means running the compiler with one switch. The compiler only looks at what comes before the #pragma hdrstop, builds a symbol table (and such) and puts the data into a .pch file.
Then comes the phase when you do a build using the pre-compiled header. In this phase, the compiler simply ignores everything in the the file up to the #pragma hdrstop. When it gets to that, it reads the compiler's internal state from the .pch file, and then starts compiling that individual file.
This means each source file typically includes a lot of headers it doesn't actually need. That, in turn, means that if you don't use pre-compiled headers, you end up with compilation that's much slower than if you hadn't done anything to support pre-compiled headers at all.
In other words, although the only part that's absolutely required is the #pragma hdrstop, which is fairly innocuous, a great deal more file re-structuring is needed to get much benefit from them--and those changes are likely to actively harmful to compilation time if you're using anything that doesn't support pre-compiled headers (and in the same way VC++ does them at that).
When precompiled headers is on every cpp source file must start with #include "stdafx.h"
So you would turn it off if you do not want to edit all the boost source files.
When precompiled headers is on stdafx.cpp "creates" the precompiled header. All other files "use" the precompiled header.
I have a precompiled header that contains includes for various 3rd party libraries, e.g.:
#ifndef PRECOMPILED_H
#define PRECOMPILED_H
#include "booststuff.h"
#include "luastuff.h"
#endif
Where booststuff.h and luastuff.h are header files in my project that just include various boost / lua related things and set up some typedefs / usings / namespace aliases.
I set up the precompiled header in the usual way inside visual studio (2012), and use the force include option to include it in every cpp file.
In the cpp files, I've also been fairly careful to #include "booststuff.h" where I actually use it as well (I sometimes disable precompiled headers to test this). However, I've been wondering lately whether that's a good idea. So:
Does anything bad happen if I include a file again that's already included in the precompiled header (I don't see why it would, but I've seen things about headers having to be included "in the same order", and not really understood what they were on about)?
Does it affect Intellisense (unusably slow with a fairly small project)? I'd be happy to give up some portability for better Intellisense since I currently have no desire to switch platforms.
If each include file has #pragma once in it, the compiler will completely skip reading the file on the second and subsequent attempts to include it. It isn't stated explicitly but I assume the precompiled header tracks this information as well.
I am working on a project that has a vendor-provided API. I've made a class that uses that API in my project and I've included the vendors header file in my stdafx.h file. Things would not compile.
I then put the #include directly into my class' header file and now things compile (And yes, my class includes stdafx.h so that isn't the reason.
Do any of you have any guesses as to why it wouldn't compile the first time around? This isn't a project-stopper by far but I'd prefer if I could keep all vendor API files in stdafx.h where they belong.
EDIT: Problem solved, I'd created a cyclic dependency by forgetting to #ifndef a header file and then including them in the wrong order. I feel like an idiot.
stdafx.h is mainly used in the VS generated projects as the 'container' of headers to be precompiled.
When you added a new #include to stdafx.h it didn't get included because your project is probably configured to use precompiled headers, and when you add something to stdafx.h you need to regenerate the .pch file that contains the precompiled information.
One way to do that is to have a .cpp file in your project that does nothing but #include "stdafx.h". Maybe call it `precompile.cpp". Then go to the project settings for that one .cpp file and change the following setting (for all configurations):
"C/C++ | Precompiled Headers | Precompiled Header" setting
and select "Create /Yc".
That will set up the build so that when precompile.cpp needs to be built (because the stdafx.h header it includes has changed), it'll rebuild the .pch file that everything else uses.
EDIT: Wait - I don't think I read the question right. May still be helpful, though.
Another name for stdafx.h is a 'Precompiled header'
There aren't really any 'vendor specifics' in stdafx.h, what it does is it precompiles headers so that the compiler doesn't have to re-compile them every time you build the project.
It's only really helpful if you have a huge project (or a small one that includes tonnes of headers).
I use visual studio 2010 as well, generally it's not worth the fuss - I just disable it (which would solve your class inclusion issue also - make your own header, stick the vendor's in there).
In my project several STL headers are used in different files. I read that, putting all these headers into a single header and using that header in my files will allow compilers to precompile the header which may lead into faster compile time.
If I understood it correctly, I need to write like the following.
// stl.hpp
#include <string>
#include <algorithm>
#include <vector>
Now include stl.hpp in all the files that needs access to STL. Is this correct?
Few of my files will be using only functionality from vector header file. But if I follow the above method, it will include unnecessary headers. Will this make any problem? Is there any code generated if I include a header file and not used anything from that?
Any help would be great!
Basically every decent compiler uses precompiled headers. Already compiled headers will be cached and only recompiled if they were changed.
Using already compiled headers instead of recompiling them every time speeds up compilation time.
But whether you combine commonly used headers in a singe file or include them in each source file separately won't matter in terms of compilation speed.
Before attempting to speed-up you build by using pre-compiled headers, it's worth benchmarking/timing your existing builds to see if the speed-up will be worth the effort.
If you only have a few dozen files with #include <string> you may see no improvement. If you have 1000s of file, then it may be worth it.
See this article for more excellent info: www.cygnus-software.com
I've got a bunch of legacy code that I need to write unit tests for. It uses pre-compiled headers everywhere so almost all .cpp files have a dependecy on stdafx.h which is making it difficult to break dependencies in order to write tests.
My first instinct is to remove all these stdafx.h files which, for the most part, contain #include directives and place those #includes directly in the source files as needed.
This would make it necessary to turn off pre-compiled headers since they are dependent on having a file like stdafx.h to determine where the pre-compiled headers stop.
Is there a way to keep pre-compiled headers without the stdafx.h dependencies? Is there a better way to approach this problem?
Yes, there is a better way.
The problem, IMHO, with the 'wizard style' of precompiled headers is that they encourage unrequired coupling and make reusing code harder than it should be. Also, code that's been written with the 'just stick everything in stdafx.h' style is prone to be a pain to maintain as changing anything in any header file is likely to cause the whole codebase to recompile every time. This can make simple refactoring take forever as each change and recompile cycle takes far longer than it should.
A better way, again IMHO, is to use #pragma hdrstop and /Yc and /Yu. This enables you to easily set up build configurations that DO use precompiled headers and also build configurations that do not use precompiled headers. The files that use precompiled headers don't have a direct dependency on the precompiled header itself in the source file which enables them to be build with or without the precompiled header. The project file determines what source file builds the precompiled header and the #pragma hdrstop line in each source file determines which includes are taken from the precompiled header (if used) and which are taken directly from the source file... This means that when doing maintenance you would use the configuration that doesn't use precompiled headers and only the code that you need to rebuild after a header file change will rebuild. When doing full builds you can use the precompiled header configurations to speed up the compilation process. Another good thing about having the non-precompiled header build option is that it makes sure that your cpp files only include what they need and include everything that they need (something that is hard if you use the 'wizard style' of precompiled header.
I've written a bit about how this works here: http://www.lenholgate.com/blog/2004/07/fi-stlport-precompiled-headers-warning-level-4-and-pragma-hdrstop.html (ignore the stuff about /FI) and I have some example projects that build with the #pragma hdrstop and /Yc /Yu method here: http://www.lenholgate.com/blog/2008/04/practical-testing-16---fixing-a-timeout-bug.html .
Of course, getting from the 'wizard style' precompiled header usage to a more controlled style is often non-trivial...
When you normally use precompiled headers, "stdafx.h" serves 2 purposes. It defines a set of stable, common include files. Also in each .cpp file, it serves as a marker as where the precompiled headers end.
Sounds like what you want to do is:
Leave precompiled header turned on.
Leave the "stdafx.h" include in each .cpp file.
Empty out the includes from "stdafx.h".
For each .cpp file, figure out which includes were needed from the old "stdafx.h". Add these before the #include "stdafx.h" in each .cpp file.
So now you have the minimal set of dependancies, and you still are using precompiled headers. The loss is that you are not precompiling your common set of headers only once. This would be a big hit for a full rebuild. For development mode, where you are only recompiling a few files at a time, it would be less of a hit.
No, there is probably NOT a better way.
However, for a given individual .cpp file, you might decide that you don't need the precompiled header. You could modify the settings for that one .cpp file and remove the stdafx.h line.
(Actually, though, I don't how the pre-compiled header scheme is interferring with the writing of your unit tests).
No. pre-compiled headers relies on a single header included by all sources compiled this way.
you can specify for a single source (or all) not to use pre-compiled headers at all, but that's not what you want.
In the past, Borland C++ compiler did pre-compilation without a specific header. however, if two sources files included the same headers but at different order, they were compiled separately, since, indeed, the order of header files in C++ can matter...
Thus it means that the borland pre-compiled headers did save time only if you very rigidly included sources in the same order, or had a single include file included (first) by all other files... - sounds familiar ?!?!
Yes. The "stdafx.h/stdafx.pch" name is just convention. You can give each .cpp its own precompiled header. This would probably be easiest to achieve by a small script to edit the XML in your .vcproj. Downside: you end up with a large stack of precompiled headers, and they're not shared between TU's.
Possible, but smart? I can't say for sure.
My advice is - don't remove precompiled headers unless you want to make your builds painfully slow. You basically have three options here:
Get rid of precompiled headers (not recommended)
Create a separate library for the legacy code; that way you can build it separately.
Use multiple precompiled headers within a single project. You can select individual C++ files in your Solution Explorer and tell them which precomiled header to use. You would also need to setup your OtherStdAfx.h/cpp to generate a precompiled header.
Pre-compiled headers are predicated on the idea that everything will include the same set of stuff. If you want to make use of pre-compiled headers then you have to live with the dependencies that this implies. It comes down to a trade-off of the dependencies vs the build speed. If you can build in a reasonable time with the pre-compiled headers turned off then by all means do it.
Another thing to consider is that you can have one pch per library. So you may be able to split up your code into smaller libraries and have each of them have a tighter set of dependencies.
I only use pre-compiled headers for the code that needs to include the afx___ stuff - usually just UI, which I don't unit-test. UI code handles UI and calls functions that do have unit-tests (though most don't currently due to the app being legacy).
For the bulk of the code I don't use pre-compiled headers.
G.
Precompiled headers can save a lot of time when rebuilding a project, but if a precompiled header changes, every source file depending on the header will be recompiled, whether the change affects it or not. Fortunately, precompiled headers are used to compile, not link; every source file doesn't have to use the same pre-compiled header.
pch1.h:
#include <bigHeader1.h>
#include ...
pch1.cpp:
#include "pch1.h"
source1.cpp:
#include "pch1.h"
[code]
pch2.h:
#include <bigHeader2.h>
#include ...
pch2.cpp:
#include "pch2.h"
source2.cpp
#include "pch2.h"
[code]
Select pch1.cpp, right click, Properties, Configuration Properties, C/C++, Precompiled Headers.
Precompiled Header : Create(/Yc)
Precompiled Header File: pch1.h
Precompiled Header Output File: $(intDir)pch1.pch
Select source1.cpp
Precompiled Header : Use(/Yu)
Precompiled Header File: pch1.h
Precompiled Header Output File: $(intDir)pch1.pch (I don't think this matters for /Yu)
Do the same thing for pch2.cpp and source2.cpp, except set the Header File and Header Output File to pch2.h and pch2.pch. That works for me.