Link error when subclassing string

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

Solution

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); //""
};