cpp c++ linker warnings which look like name mangling problems but are cause by classes not being exposed in the dll

Today, I wanted to write some tests for a cpp dynamic library under windows.
Such a build still generates the .lib file, like for static libs, but this .lib file only contains some header information or somesuch.
In the end, the .dll will be used.

Then, when I tried to compile the tests, I got this:
1>SwarmTests.obj : error LNK2001: unresolved external symbol "public: __thiscall Swarm::SpecificChainer::SpecificChainer(long,class std::basic_string<wchar_t,struct std::char_traits<wchar_t>,class std::allocator<wchar_t> > const &,class std::basic_string<wchar_t,struct std::char_traits<wchar_t>,class std::allocator<wchar_t> > const &,class Swarm::ZergLink *)" (??0SpecificChainer@Swarm@@QAE@JABV?$basic_string@_WU?$char_traits@_W@std@@V?$allocator@_W@2@@std@@0PAVZergLink@1@@Z) 1>C:\dev\mycode\SwarmTests\Debug\SwarmTests.exe : fatal error LNK1120: 1 unresolved externals

After several hours of despair, I found out that the class:
Swarm::SpecificChainer
was declared like this:
class SpecificChainer {
 ctor...
  ...
}

But should have been declared like this:
class DLL_PUBLIC SpecificChainer {
 ctor...
  ...
}

The keyword is DLL_PUBLIC which will expose this class to the users of the dynamic library. See here on gnu.gcc.org for further information. All non-exposed classes are only visible internally.

I still don't know, why it's done like this, whether this is only on windows (can't remember that on linux) and if it's common for all dynamic cpp libs, why they didn't invent some smart keyword for that, like in c#: internal.