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My questions correspond to the answer from Johannes in Is there a way to instantiate objects from a string holding their class name? and the recent comment from Spencer Rose. Since I cannot add a comment there, I decided to start a new question.

Johannes suggestion is what I need. I implemented it in exact the same way but I have an unresolved external symbol linker error using VS2008 which seems to have something to do with the map. I am trying since days to solve it. Today I read the comment from Spencer and added the line he suggests

BaseFactory::map_type BaseFactory::map = new map_type();

to the Base.hpp. Now I get a LNK2005 error 

Derivedb.obj : error LNK2005:
"private: static class std::map<class std::basic_string<char,struct std::char_traits<char>,class std::allocator<char> >,class Base * (__cdecl*)(void),struct std::less<class std::basic_string<char,struct std::char_traits<char>,class std::allocator<char> > >,class std::allocator<struct std::pair<class std::basic_string<char,struct std::char_traits<char>,class std::allocator<char> > const ,class Base * (__cdecl*)(void)> > > * BaseFactory::map"
(?map@BaseFactory@@0PAV?$map@V?$basic_string@DU?$char_traits@D@std@@V?$allocator@D@2@@std@@P6APAVBase@@XZU?$less@V?$basic_string@DU?$char_traits@D@std@@V?$allocator@D@2@@std@@@2@V?$allocator@U?$pair@$$CBV?$basic_string@DU?$char_traits@D@std@@V?$allocator@D@2@@std@@P6APAVBaser@@XZ@std@@@2@@std@@A)
already defined in Switcher.obj

Project.exe : fatal error LNK1169: one or more multiply defined symbols found)

instead of the LNK2001 error 

(Switcher.obj : error LNK2001: unresolved external symbol "private: static class std::map<class std::basic_string<char,struct std::char_traits<char>,class std::allocator<char> >,class Base * (__cdecl*)(void),struct std::less<class std::basic_string<char,struct std::char_traits<char>,class std::allocator<char> > >,class std::allocator<struct std::pair<class std::basic_string<char,struct std::char_traits<char>,class std::allocator<char> > const ,class Base * (__cdecl*)(void)> > > * BaseFactory::map" (?map@BaseFactory@@0PAV?$map@V?$basic_string@DU?$char_traits@D@std@@V?$allocator@D@2@@std@@P6APAVBase@@XZU?$less@V?$basic_string@DU?$char_traits@D@std@@V?$allocator@D@2@@std@@@2@V?$allocator@U?$pair@$$CBV?$basic_string@DU?$char_traits@D@std@@V?$allocator@D@2@@std@@P6APAVBase@@XZ@std@@@2@@std@@A)
1>Derivedb.obj : error LNK2001: unresolved external symbol "private: static class std::map<class std::basic_string<char,struct std::char_traits<char>,class std::allocator<char> >,class Base * (__cdecl*)(void),struct std::less<class std::basic_string<char,struct std::char_traits<char>,class std::allocator<char> > >,class std::allocator<struct std::pair<class std::basic_string<char,struct std::char_traits<char>,class std::allocator<char> > const ,class Base * (__cdecl*)(void)> > > * BaseFactory::map" (?map@BaseFactory@@0PAV?$map@V?$basic_string@DU?$char_traits@D@std@@V?$allocator@D@2@@std@@P6APAVBase@@XZU?$less@V?$basic_string@DU?$char_traits@D@std@@V?$allocator@D@2@@std@@@2@V?$allocator@U?$pair@$$CBV?$basic_string@DU?$char_traits@D@std@@V?$allocator@D@2@@std@@P6APAVBase@@XZ@std@@@2@@std@@A)
1>Project.exe : fatal error LNK1120: 1 unresolved externals)

which means that I may have defined it twice?? Please could Spencer or somebody post the improved base.hpp code. It is such an important solution that it surely will be helpful for many more novice C++ programmers.

Second question: --> This problem is solved! Thanks!

I need some function declarations in base.hpp. Those should have been abstract in the base class and implemented in the subclasses (e.g. Derivedb.cpp ). But 

public:
       virtual ReadInFile(std::string path, std::string filename) = 0;

in base.hpp gave a compiler error. Removing "= 0" resolved the compiler error. But now I have another unresolved external symbol LNK2001 error

Derivedb.obj: error LNK2001: unresolved external symbol
"public: virtual __thiscall Base::ReadInFile(class std::basic_string<char,struct std::char_traits<char>,class std::allocator<char> >,class std::basic_string<char,struct std::char_traits<char>,class std::allocator<char> >)"
(?ReadInFile@Base@@UAE_NPAV@@V?$basic_string@DU?$char_traits@D@std@@V?$allocator@D@2@@std@@1@Z).

I call it in another cpp file

Base* importer =  BaseFactory::createInstance("DerivedB");
importer->ReadInFile(m_path, m_filename);

Maybe it is not clear which function (base or subclass) need to be called since it is not abstract in the base class??? Is there any way of solving this problem? Thank you!

Community
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Natalie
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    Errors do not vaguely mean that "something" is wrong. Errors usually have specific messages, and in the case of compiler errors, a line indicating a location for the problem. What error messages are you getting? You say you got a compiler error when you had a pure-virtual function. Was that compiler error about trying to instantiate a class that had a pure-virtual function in it? In short: what _messages_ are you getting? – Nicol Bolas Sep 25 '11 at 19:35
  • Thanks for your comment!Now I tried to get the error again by adding the "=0" and the compiler error did not come and in addition the LNK2001 error is gone. I have tried out so many things that now I have no idea what the original cause was. It did tell me that an abstract method is not allowed. Anyway I am glad for your comment since now at least that error is resolved :-). I still have the LNK2005 error. – Natalie Sep 25 '11 at 20:01
  • Now that you've gotten past the first error, you can [edit] your question. Also, compiler errors are usually quite specific, but the important details can be missed sometimes. You'll get better help if you include the *exact* compiler error when you [edit] your question. – Dan Cecile Sep 25 '11 at 20:21
  • You gotta [edit] your question to include the exact text of the LNK2001 error. – Dan Cecile Sep 25 '11 at 21:05
  • :) Thanks! Remember to include four spaces before pasting in the error messages, that way the `<` angle `>` brackets display properly. – Dan Cecile Sep 25 '11 at 22:02

2 Answers2

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From the LNK2005 error, your BaseFactory class has a static field called map, right?

Static fields in C++ have to be "declared" inside the class's header file, and "implemented" in the class's source file.

Here is a simplified example of how to set this up. In this case, in the BaseFactory.h file, you should have the static field declared:

class BaseFactory
{
private:
    static int map;
};

And in the BaseFactory.cpp file, the static field gets implemented:

int BaseFactory::map = 392;

The error message from the linker is saying that the BaseFactory::map static field got implemented in both the Derivedb.cpp file and the Switcher.cpp files.

Even if the implementation (... BaseFactory::map = ...) isn't in either of those files, you'll get the same error if you put the implementation in the BaseFactory.h header file. The C++ preprocessor just blindly includes code headers, and the linker can't tell whether the implementation is in the Switcher.cpp file or some file that Switcher.cpp included.

Just like methods need to be declared in .h files and implemented in .cpp files, the same goes for static fields.

Dan Cecile
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  • Thanks a lot Dan! This was the perfect answer to my problem. The program runs now, but in the line "importer->ReadInFile(m_path, m_filename);" I have a Debug Assertion failed error, with Expression: map/set iterator not dereferencable. The line which causes it is "retrun it->second()" in the BaseFactory::createInstance method. I am searching for a solution of this....... – Natalie Sep 26 '11 at 09:25
  • No problem. Good luck on your next challenges! – Dan Cecile Sep 26 '11 at 10:09
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map is the name of a template class in the STL, and therefore a poor choice for a variable name. This may or may not be related to the duplicate definition error you are getting.

CurtisB
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    You are right! I just named it here in the same way it was named in the example code from Johannes not to confuse anyone. But in my code it has another name. So that is unfortunately not the cause for the error. Thanks for your comment! – Natalie Sep 25 '11 at 20:18
  • `map` is such a universal name that it’s unreasonable to ban it as an identifier: a map is a core concept of programming with several distinct but related meanings. I use it freely in my code. Of course, care has to be taken to avoid conflicts but if you don’t open the whole `std` namespace and use a bit of care, this is no problem. After all, that’s what namespaces are there for. – Konrad Rudolph Sep 25 '11 at 20:19
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    I'd consider the universality of the word another shortcoming. Using `map` as a variable name doesn't tell you anything about what that variable contains. A better alternative for this problem would be `class_map` or `class_name_map` or something along those lines. – CurtisB Sep 25 '11 at 20:27