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      <title>Guidelines for Authors of Boost Libraries Containing Separate Source</title>
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               <h3><A href="../index.htm"><IMG height="86" alt="C++ Boost" src="../boost.png" width="277" border="0"></A></h3>
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               <H1 align="center">Guidelines for Authors of Boost Libraries Containing Separate 
                  Source</H1>
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      <BR>
      <HR>
      <P>These guidelines are designed for the authors of Boost libraries which have 
         separate source that need compiling in order to use the library. Throughout, 
         this guide refers to a fictitious "whatever" library, so replace all 
         occurrences of "whatever" or "WHATEVER" with your own library's name when 
         copying the examples.</P>
      <H2>Contents</H2>
      <dl class="index">
         <dt><A href="#source_changes">Changes Affecting Source Code</A>
            <dd>
               <dl class="index">
                  <dt><A href="#abi">Preventing Compiler ABI Clashes</A> <DT><A href="#static_or_dynamic">Static 
                           or Dymanic Libraries</A>&nbsp; <dt><A href="#dlls">Supporting Windows Dll's</A> <dt>
                              <a href="#auto-link">Automatic Library Selection and Linking with auto_link.hpp</a>
                           </dt>
               </dl>
               <dt><A href="#build_changes">Changes Affecting the Build System</A>
                  <dd>
                     <dl class="index">
                        <dt><A href="#jamfile">Creating the Library Jamfile</A> <dt><A href="#testing">Testing 
                                 Auto-linking</A> </dt>
                     </dl>
                     <dt><A href="#copyright">Copyright</A></dt>
      </dl>
      <h2><A name="source_changes"></A>Changes Affecting Source Code</h2>
      <H3><A name="abi"></A>Preventing Compiler ABI Clashes</H3>
      <P>There are some compilers (mostly Microsoft Windows compilers again!), which 
         feature a range of compiler switches that alter the ABI of C++ classes and 
         functions. By way of example, consider Borland's compiler which has the 
         following options:</P>
      <PRE>-b    (on or off - effects enum sizes).
-Vx   (on or off - empty members).
-Ve   (on or off - empty base classes).
-aX   (alignment - 5 options).
-pX   (Calling convention - 4 options).
-VmX  (member pointer size and layout - 5 options).
-VC   (on or off, changes name mangling).
-Vl   (on or off, changes struct layout). 
</PRE>
      <P>These options are provided in addition to those affecting which runtime library 
         is used (more on which later); the total number of combinations of options can 
         be obtained by multiplying together the individual options above, so that gives 
         2*2*2*5*4*5*2*2 = 3200 combinations!
      </P>
      <P>The problem is that users often expect to be able to build the Boost libraries 
         and then just link to them and have everything just plain work, no matter what 
         their project settings are. Irrespective of whether this is a reasonable 
         expectation or not, without some means of managing this issue, the user may 
         well find that their program will experience strange and hard to track down 
         crashes at runtime unless the library they link to was built with the same 
         options as their project (changes to the default alignment setting are a prime 
         culprit). One way to manage this is with "prefix and suffix" headers: these 
         headers invoke compiler specific #pragma directives to instruct the compiler 
         that whatever code follows was built (or is to be built) with a specific set of 
         compiler ABI settings.</P>
      <P>Boost.config provides the macro BOOST_HAS_ABI_HEADERS which is set whenever 
         there are prefix and suffix headers available for the compiler in use, typical 
         usage in a header like this:</P>
      <PRE>#ifndef BOOST_WHATEVER_HPP
#define BOOST_WHATEVER_HPP

#include &lt;boost/config.hpp&gt;

// this must occur after all of the includes and before any code appears:
#ifdef BOOST_HAS_ABI_HEADERS
#  include BOOST_ABI_PREFIX
#endif
//
// this header declares one class, and one function by way of examples:
//
class whatever
{
   // details.
};

whatever get_whatever();

// the suffix header occurs after all of our code:
#ifdef BOOST_HAS_ABI_HEADERS
#  include BOOST_ABI_SUFFIX
#endif

#endif
</PRE>
      <P>You can include this code in your library source files as well if you want, 
         although you probably shouldn't need to:&nbsp;&nbsp;</P>
      <UL>
         <LI>
            If you <EM>don't</EM>
         use these in the library source files (but do in your library's headers) and 
         the user attempts to compile the library source with a non-default ABI setting, 
         then they will get compiler errors if there are any conflicts.
         <LI>
            If you <EM>do </EM>include them in both the library's headers and the library 
            source files, then the code should always compile no matter what the compiler 
            settings used, although the result might not match what the user was expecting: 
            since we've forced the ABI back into default mode.</LI></UL>
      <H4>Rationale:</H4>
      <P>Without some means of managing this issue, users often report bugs along the 
         line of "Your silly library always crashes when I try and call it" and so on. 
         These issues can be extremely difficult and time consuming to track down, only 
         to discover in the end that it's a compiler setting that's changed the ABI of 
         the class and/or function types of the program compared to those in the 
         pre-compiled library. The use of prefix/suffix headers can minimize this 
         problem, although probably not remove it completely.</P>
      <H5>Counter Argument #1:</H5>
      <P>Trust the user, if they want 13-byte alignment (!) let them have it.</P>
      <H5>Counter Argument #2:</H5>
      <P>Prefix/suffix headers have a tendency to "spread" to other boost libraries - 
         for example if boost::shared_ptr&lt;&gt; forms part of your class's ABI, then 
         including prefix/suffix headers in your code will be of no use unless 
         shared_ptr.hpp also uses them. Authors of header-only boost libraries may not 
         be so keen on this solution - with some justification - since they don't face 
         the same problem.</P>
      <H3><A name="static_or_dynamic"></A>Static or Dynamic Libraries</H3>
      <P>When the users runtime is dynamically linked the Boost libraries can be built 
         either as dynamic libraries (.so's on Unix platforms, .dll's on Windows) or as 
         static libraries (.a's on Unix, .lib's on Windows).&nbsp; So we have a choice 
         as to which is supported by default:</P>
      <UL>
         <LI>
         On Unix platforms it typically makes no difference to the code: the user just 
         selects in their makesfile which library they prefer to link to.
         <LI>
         On Windows platforms, the code has to be specially annotated to support DLL's, 
         so we need to pick one option as the default and one as an alternative.
         <LI>
            On Windows platforms, we can inject special code to automatically select which 
            library variant to link against: so again we need to decide which is to be the 
            default (see the section on auto-linking below).</LI></UL>
      <P>The recomendation is to pick static linking by default.</P>
      <H4>Rationale:</H4>
      <P>There is no one policy that fits all here.
      </P>
      <P>The rationale for the current behaviour was inherited from Boost.Regex (and 
         it's ancestor regex++): this library&nbsp;originally used dynamic linking by 
         default whenever the runtime was dynamic. It's actually safer that way should 
         you be using regex from a dll for example. However,&nbsp;this 
         behavior&nbsp;brought a persistent stream of user complaints: mainly about 
         deployment, all asking if static linking could be the default. After&nbsp;regex 
         changed behavior the complaints stopped, and the author hasn't had one 
         complaint about static linking by default being the wrong choice.</P>
      <P>Note that other libraries might need to make other choices: for example 
         libraries that are intended to be used to implement dll pluggin's would like 
         need to use dynamic linking in almost all cases.</P>
      <H3>Supporting Windows Dll's</H3>
      <p>On most Unix-like platforms no special annotations of source code are required 
         in order for that source to be compiled as a shared library because all 
         external symbols are exposed. However the majority of Windows compilers require 
         that symbols that are to be imported or exported from a dll, be prefixed with 
         __declspec(dllimport) or __declspec(dllexport). Without this mangling of source 
         code, it is not possible to correctly build shared libraries on Windows 
         (historical note - originally these declaration modifiers were required on 
         16-bit Windows where the memory layout for exported classes was different from 
         that of "local" classes - although this is no longer an issue, there is still 
         no way to instruct the linker to "export everything", it also remains to be 
         seen whether 64-bit Windows will resurrect the segmented architecture that led 
         to this problem in the first place. Note also that the mangled names of 
         exported symbols are different from non-exported ones, so __declspec(dllimport) 
         is required in order to link to code within a dll).</p>
      <p>In order to support the building of shared libraries on MS Windows your code 
         will have to prefix all the symbols that your library exports with a macro 
         (lets call it BOOST_WHATEVER_DECL) that your library will define to expand to 
         either __declspec(dllexport) or __declspec(dllimport) or nothing, depending 
         upon how your library is being built or used. Typical usage would look like 
         this:</p>
      <pre>#ifndef BOOST_WHATEVER_HPP
#define BOOST_WHATEVER_HPP

#include &lt;boost/config.hpp&gt;

#ifdef BOOST_HAS_DECLSPEC // defined in config system
// we need to import/export our code only if the user has specifically
// asked for it by defining either BOOST_ALL_DYN_LINK if they want all boost
// libraries to be dynamically linked, or BOOST_WHATEVER_DYN_LINK
// if they want just this one to be dynamically liked:
#if defined(BOOST_ALL_DYN_LINK) || defined(BOOST_WHATEVER_DYN_LINK)
// export if this is our own source, otherwise import:
#ifdef BOOST_WHATEVER_SOURCE
# define BOOST_WHATEVER_DECL __declspec(dllexport)
#else
# define BOOST_WHATEVER_DECL __declspec(dllimport)
#endif  // BOOST_WHATEVER_SOURCE
#endif  // DYN_LINK
#endif  // BOOST_HAS_DECLSPEC
//
// if BOOST_WHATEVER_DECL isn't defined yet define it now:
#ifndef BOOST_WHATEVER_DECL
#define BOOST_WHATEVER_DECL
#endif

//
// this header declares one class, and one function by way of examples:
//
class BOOST_WHATEVER_DECL whatever
{
   // details.
};

BOOST_WHATEVER_DECL whatever get_whatever();

#endif
</pre>
      And then in the source code for this library one would use:
      <pre> 
// 
// define BOOST_WHATEVER SOURCE so that our library's 
// setup code knows that we are building the library (possibly exporting code), 
// rather than using it (possibly importing code): 
// 
#define BOOST_WHATEVER_SOURCE 
#include &lt;boost/whatever.hpp&gt; 

// class members don't need any further annotation: 
whatever::whatever() { } 
// but functions do: 
BOOST_WHATEVER_DECL whatever get_whatever() 
{
   return whatever();
}
</pre>
      <H4>Importing/exporting dependencies</H4>
      <P>As well as exporting your main classes and functions (those that are actually 
         documented), Microsoft Visual C++ will warn loudly and often if you try to 
         import/export a class whose dependencies are not also exported. Dependencies 
         include: any base classes, any user defined types used as data members, plus 
         all of the dependencies of your dependencies and so on. This causes particular 
         problems when a dependency is a template class, because although it is 
         technically possible to export these, it is not at all easy, especially if the 
         template itself has dependencies which are implementation-specific details. In 
         most cases it's probably better to simply suppress the warnings using:</P>
      <PRE>#ifdef BOOST_MSVC
#  pragma warning(push)
#  pragma warning(disable : 4251 4231 4660)
#endif

// code here

#ifdef BOOST_MSVC
#pragma warning(pop)
#endif
</PRE>
      <p>This is safe provided that there are no dependencies that are (template) 
         classes with non-constant static data members, these really do need exporting, 
         otherwise there will be multiple copies of the static data members in the 
         program, and that's really really bad.
      </p>
      <p>Historical note: on 16-bit Windows you really did have to export all 
         dependencies or the code wouldn't work, however since the latest Visual Studio 
         .NET supports the import/export of individual member functions, it's a 
         reasonably safe bet that Windows compilers won't do anything nasty - like 
         changing the class's ABI - when importing/exporting a class.</p>
      <h4>Rationale:</h4>
      <p><EM>Why bother - doesn't the import/export mechanism take up more code that the 
            classes themselves?</EM></p>
      <P>A good point, and probably true, however there are some circumstances where 
         library code must be placed in a shared library - for example when the 
         application consists of multiple dll's as well as the executable, and more than 
         one those dll's link to the same Boost library - in this case if the library 
         isn't dynamically linked and it contains any global data (even if that data is 
         private to the internals of the library) then really bad things can happen - 
         even without global data, we will still get a code bloating effect. 
         Incidentally, for larger applications, splitting the application into multiple 
         dll's can be highly advantageous - by using Microsoft's "delay load" feature 
         the application will load only those parts it really needs at any one time, 
         giving the impression of a much more responsive and faster-loading application.</P>
      <p><EM>Why static linking by default? </EM>
      </p>
      <P>In the worked example above, the code assumes that the library will be 
         statically linked unless the user asks otherwise. Most users seem to prefer 
         this (there are no separate dll's to distribute, and the overall distribution 
         size is often significantly smaller this way as well: i.e. you pay for what you 
         use and no more), but this is a subjective call, and some libraries may even 
         only be available in dynamic versions (Boost.threads for example).</P>
      <h3><A name="auto-link"></A>Automatic Library Selection and Linking with <a href="../boost/config/auto_link.hpp">
            auto_link.hpp</a></h3>
      <p>Many Windows compilers ship with multiple runtime libraries - for example 
         Microsoft Visual Studio .NET comes with 6 versions of the C and C++ runtime. It 
         is essential that the Boost library that the user links to is built against the 
         same C runtime as the program is built against. If that is not the case, then 
         the user will experience linker errors at best, and runtime crashes at worst. 
         The Boost build system manages this by providing different build variants, each 
         of which is build against a different runtime, and gets a slightly different 
         mangled name depending upon which runtime it is built against. For example the 
         regex libraries get named as follows when built with Visual Studio .NET 2003:</p>
      <pre>boost_regex-vc71-mt-1_31.lib
boost_regex-vc71-mt-gd-1_31.lib
libboost_regex-vc71-mt-1_31.lib
libboost_regex-vc71-mt-gd-1_31.lib
libboost_regex-vc71-mt-s-1_31.lib
libboost_regex-vc71-mt-sgd-1_31.lib
libboost_regex-vc71-s-1_31.lib
libboost_regex-vc71-sgd-1_31.lib
</pre>
      <p>The difficulty now is selecting which of these the user should link his or her 
         code to.</p>
      <p>In contrast, most Unix compilers typically only have one runtime (or sometimes 
         two if there is a separate thread safe option). For these systems the only 
         choice in selecting the right library variant is whether they want debugging 
         info, and possibly thread safety.
      </p>
      <p>Historically Microsoft Windows compilers have managed this issue by providing a 
         #pragma option that allows the header for a library to automatically select the 
         library to link to. This makes everything automatic and extremely easy for the 
         end user: as soon as they include a header file that has separate source code, 
         the name of the right library build variant gets embedded in the object file, 
         and as long as that library is in the linker search path, it will get pulled in 
         by the linker without any user intervention.</p>
      <p>Automatic library selection and linking can be enabled for a Boost library by 
         including the header &lt;boost/config/auto_link.hpp&gt;, after first defining 
         BOOST_LIB_NAME and, if applicable, BOOST_DYN_LINK.</p>
      <pre>//
// Automatically link to the correct build variant where possible. 
// 
#if !defined(BOOST_ALL_NO_LIB) &amp;&amp; !defined(BOOST_WHATEVER_NO_LIB) &amp;&amp; !defined(BOOST_WHATEVER_SOURCE)
//
// Set the name of our library, this will get undef'ed by auto_link.hpp
// once it's done with it:
//
#define BOOST_LIB_NAME boost_whatever
//
// If we're importing code from a dll, then tell auto_link.hpp about it:
//
#if defined(BOOST_ALL_DYN_LINK) || defined(BOOST_WHATEVER_DYN_LINK)
#  define BOOST_DYN_LINK
#endif
//
// And include the header that does the work:
//
#include &lt;boost/config/auto_link.hpp&gt;
#endif  // auto-linking disabled
</pre>
      <p>The library's user documentation should note that the feature can be disabled 
         by defining either BOOST_ALL_NO_LIB or BOOST_WHATEVER_NO_LIB:</p>
      <P>If for any reason you need to debug this feature, the header 
         &lt;boost/config/auto_link.hpp&gt; will output some helpful diagnostic messages 
         if you first define BOOST_LIB_DIAGNOSTIC.</P>
      <H2><A name="build_changes"></A>Changes Affecting the Build System</H2>
      <H3><a name="build"></a><A name="jamfile"></A>Creating the library Jamfile</H3>
      <P>The Jamfile for building library "whatever" typically lives in 
         boost-root/libs/whatever/build, the only extra step required is to add a 
         &lt;define&gt; requirement to the library target so that your code knows 
         whether it's building a dll or static library, a typical Jamfile would like 
         like this:</P>
      <PRE>
lib boost_regex : ../src/whatever.cpp : 
  &lt;link&gt;shared:&lt;define&gt;BOOST_WHATEVER_DYN_LINK=1 ;
 </PRE>
      <H3><A name="testing"></A>Testing Auto-linking</H3>
      <P>Testing the auto-link feature&nbsp;is somewhat convoluted, and requires access 
         to a compiler that supports the feature: refer to <A href="../libs/config/test/link/test/Jamfile.v2">
            libs/config/test/link/test/Jamfile.v2</A> for an example.</P>
      <HR>
      <p><A name="copyright"></A>Revised 
         <!--webbot bot="Timestamp" S-Type="EDITED" S-Format="%d %B, %Y" startspan --> 
         26 November, 2003<!--webbot bot="Timestamp" endspan i-checksum="39365" --></p>
      <p><i>© Copyright John Maddock&nbsp;1998- 
            <!--webbot bot="Timestamp" S-Type="EDITED" S-Format="%Y" startspan -->  2003<!--webbot bot="Timestamp" endspan i-checksum="746" --></i></p>
      <P><I>Distributed under the Boost Software License, Version 1.0. (See accompanying 
            file <a href="../LICENSE_1_0.txt">LICENSE_1_0.txt</a> or copy at <a href="http://www.boost.org/LICENSE_1_0.txt">
               http://www.boost.org/LICENSE_1_0.txt</a>)</I></P>
      <P><EM>The use of code snippets from this article does not require the reproduction 
            of this copyright notice and license declaration; if you wish to provide 
            attribution then please provide a link to this article.</EM></P>
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