Link error when subclassing string - c++

Question

for various reasons I subclassed the std::string like:
class CotengString : public string
{
public:
// Constructors
CotengString();
CotengString(const char* ch) : string(ch) {}
CotengString(const string& str) : string(str) {}
virtual ~CotengString() {};
//Operator"="
CotengString& operator=(const string& str) { return (CotengString&)this->assign(str); }
CotengString& operator=(const char* ch) { return (CotengString&)this->assign(ch); }
};
Inside the DLL I wish to use this code all compiles correctly. But inside my Google Test project I get this error after switching to the subclassed string
LNK2005 "public: __thiscall std::basic_string<char,struct std::char_traits<char>,class std::allocator<char> >::basic_string<char,struct std::char_traits<char>,class std::allocator<char> >(char const *,unsigned int)" (??0?$basic_string#DU?$char_traits#D#std##V?$allocator#D#2##std##QAE#PBDI#Z) already defined in gtest.lib(gtest-all.obj) Devices.Spectro.CppTest <path>\Devices.Spectro.CppTest\Devices.Spectro.Cpp.lib(Devices.Spectro.Cpp.dll)
My gut feeling is that I am missing something obvious. But I don't know what it could be

Answer 1

Okay, then as answer.
First cause
This seems to me to be a problem of multiple includes of header files.
This results in that the compiler wants to compile functions or in this case allocators again and detects "oh I already compiled that!".
So fix would be to add to the header file this:
#ifndef CUSTOM_STRING_IMPLEMENTATION_HEADER
#define CUSTOM_STRING_IMPLEMENTATION_HEADER
....
#endif
Second cause
However, if this isn't the case maybe you're trying to compile this file again in your code which uses the dll which already contains the compiled class.
I personally think it's the second cause according to your log file. There's said that the allocator is already compiled in your library.
Example:
If you're using only header files this results in a big problem. Look here.
Dll code:
SomeHeaderFile.hpp
class NiceClass{
//Constructor is implemented. That causes several problems now!
NiceClass{
}
...
}
Your application code (wich uses the dll):
SomeNiceApplicationCode:
//Linker error now! It's already compiled right into your dll!
#include <SomeHeaderFile.hpp>
int main(){
NiceClass niceClassInstance;
return 0;
}
Solution:
Apply those changes and create and extra cpp file for your class.
Include in your application file only the header file.
class CotengString : public string
{
public:
// Constructors
CotengString();
CotengString(const char* ch) : string(ch); //Use cpp file!
CotengString(const string& str) : string(str); //""
virtual ~CotengString(); //""
//Operator"="
CotengString& operator=(const string& str); //Same for this
CotengString& operator=(const char* ch); //""
};