Question

According to this answer boost and STL headers belong into the precompiled header file (stdafx.h in the MSVC world). So I changed the headers of my dynamic link library project and moved all STL/Boost headers into the stdafx.h of my project.

Before

#include <boost/smart_ptr.hpp>

namespace XXX
{
  class CLASS_DECL_BK CExampleClass // CLASS_DECL_BK is just a standard dll import/export macro
  {
    private:
      boost::scoped_ptr<Replica> m_replica;
  }
}

After

namespace XXX
{
  class CLASS_DECL_BK CExampleClass
  {
    private:
      boost::scoped_ptr<Replica> m_replica;
  }
}

Now I have the advantage of decreased compile times, but all the users of my library are getting build errors (e.g. unknown boost::scoped_ptr…) because of the missing includes (which are now moved to my stdafx.h).

---

What could be a solution for this dilemma?

I want reduced compile times and compile errors after including my headers files are not acceptable for any users of the dll.

Could this help?

• leave all includes directives as they are but duplicate them in my ‘stdafx.h’? Since the stdafx.h is always included first inside any cpp file of my project I should be fine, and the users won’t get any errors. Or do I loose the speed advantage if multiple includes of the same header occur in one translation unit (got header guards)?

Thanks for any hints!

Answer 1

You should get nearly the same speed increase when you leave the header-includes in place in the library headers and just additionally put them into stdafx.h.

Alternatively, you could add an additional define (a bulk external include guard)

// stdafx.h
#define MY_LIB_STD_HEADERS_ALREADY_INCLUDED

// library_file.h
#ifndef MY_LIB_STD_HEADERS_ALREADY_INCLUDED
#include <boost/smart_ptr.hpp>
...
#endif

But I would only do that if you are sure it helps. Just take a stopwatch and run a few re-compilations. (No need to link.) Then you’ll see if there’s any difference.

Aside

I’m not sure if adding all boost headers that are needed somewhere in the project is such a good idea. I’d say shared_ptrand friends, boost/foreach, maybe Boost.Format, … are a good idea, but I’d already think twice for the Boost.RegExp headers. _{Note: I did not do any speed measurements, but I dimly remember a problem with the size of the pch file and some compiler hiccup. I really should do some tests.}

Also check if the Boost Library in question provides forwarding headers and whether you should include them instead. Bloating the precompiled header file can have it’s downsides.