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
Related
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.
Would it be optimizable in a large program if a .cpp file loaded all the needed headers for the application rather than pre-loading it in the main source file?
Like instead of having
Main.cpp
#include <header.h>
#include <header1.h>
#include <header2.h>
#include <header3.h>
//main code
Can I just have a .cpp file that does this and just loads .cpp file in the main.cpp? Like this
Loader.cpp
#include <header.h>
#include <header1.h>
#include <header2.h>
#include <header3.h>
Main.cpp
#include "Loader.cpp"
//main code
Preprocessing simply generates the text that gets compiled. Your suggestion leads to the same body of source code, so it will have no effect on optimization.
Including all the headers, all the time (call it a "super-header") may lead to slow compilation.
However, precompiled headers are a popular solution to allow such super-headers to work quickly. You might check your IDE or compiler's documentation to learn its precompiled header facility.
In any case, typically the super-header is still named with .h; since it implements nothing a .cpp name would not be appropriate.
You can, but you may want to reconsider.
You may have trouble with accidentally trying to compile Loader.cpp by itself since you've named it as if it was a source file. Since you're using it as a header - and it is a concatenation of multiple headers - it would make sense to name it according to the convention and use .h file name extension.
Would it be optimizable
It would have zero effect on the compiled program, so in that sense optimization is irrelevant.
This will bring problems with compilation speed however. And those problems are not "optimizable". Copying every header into the source file - needed or not - slow the time (and the slowdown is bound to the hard drive speed) needed to compile the source file. Much bigger problem is that it prevents incremental building because a change in any header file would force the source file to be recompiled, because its content will have changed.
These problems are compounded over multiple source files assuming you intend to use this "technique" with all source files that you have. Having to recompile the entire project when any header is modified is usually non acceptable for any project that is not trivially small.
It really depends on the compiler.
Visual studio has a thing called stdafx.h.
What's the use for "stdafx.h" in Visual Studio?
I have a question regarding where to #include iosteam and vector? In the main.cpp, header.h or in the memberfunction.cpp? Seems I needed using namespace std in main.cpp but #include< functional > in the header file>. Is there any robust way to do it? Thanks!
Simple Rule:
Include the header file only in those files which need it.
If your source or header file does not use any constructs defined/declared in the header file then there is no need to include that header. Doing so only brings unnecessary code in to those translation units thereby corrupting the namespace and possibly increasing the compilation time.
For readability reasons you want to include headers only in those translation units using them. So in a source code not using at all std::vector template you would not #include <vector>, hence you would put that include before your own #include "myheader.h"
However, you may want to decrease compilation time by pre-compiling the header.
As I explain in this answer, precompiled headers with GCC works only if all your program has one single header containing all the includes. For that reason you would want to have a single myheader.h which is itself including <vector> (even for the few compilation units not using it).
Pre-compilation of header files is compiler (and perhaps system) specific.
If you use vectors, or input/output streams in any way in your header.h (for instance, parameters of that type etc.), then it is better to include iostream and/or vector there. If you use them only internally in your memberfunction.cpp, include it there (it is of no use to the rest of the code).
Looking at an open source code base i came across this code:
#include "StableHeaders.h"
#include "polygon.h"
#include "exception.h"
#include "vector.h"
...
Now the StableHeaders.h is a precompiled header which is included by a 'control' cpp to force it's generation. The three includes that appear after the precompiled header are also included in the StableHeaders.h file anyway. My question is, are these files included twice so that the code base will build on compilers that don't support precompiled headers? As im assuming that include guards/header caching will make the multiple includes redundant anyway...
EDIT btw, the stableheaders.h file has a check for win32 (roughly) so again im assuming that the includes inside stableheaders.h wont be included on compilers that don't support precompiled headers.
Compilers that don't support precompiled headers would just include StableHeaders.h and reparse it every time (rather than using the precompiled file). It won't cause any problems neither does it fix any problems for certain compilers as you asked. I think its just a minor 'mistake' that probably happened over time during development.
I think you yourself answered the question question! Pre-compiled headers is a compiler feature. If the guard is present the headers will not be included twice, in any case.
The only reason I can think of to protect the precompiled header and include the stuff anyway is speed. The reason to use precompiled headers is to speed up the compile times, this works by including and compiling the contents of the precompiled header, when you do this you can include headers that are only used by 75% of the source files and it is still quicker than no precompiled headers.
However if the other platforms down support prepcompiled headers you only want to include the header files that are required for this source file. So if the precompiled header contains include files that are only required for by some source files it is quicker to just include and compile the header files you need.
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.