Linking error while using structures in a separate header

Question

This is a simpified version of some code.

1. The struct is defined in roughStruct.h. It has a static const char * someVar. someVar is initialized outside the struct in the same file.

2. Another class roughParse (defined in roughParse.h and implemented in roughParse.cpp) intends to make use of roughStruct(For the purposes of this questions it does absolutely nothing)

3. The main file roughMain.cpp intends to make use of the class roughParse to work on struct roughStruct.(Again here it does nothing.)

The code doesn't compile.

4. As far I understand the error is occuring because roughStruct is being include in both roughMain.cpp and roughParse.cpp. I need clarification here--I am confused as to how that can be because of the next point.
5. After a lot of trial and error, the following corrects the error:

//defining the const inline and not outside
 constexpr static const char *someVar="abcd";

or

//using constructor to initialize and not outside
roughStruct()
    {
        someVar="abcd";
    }

Both approaches don't work forconst char * replaced with const string.
Why don't they?
How do you initialize a static const string then?

The second approach doesn't seem ideal as structures are for holding data though I will use it if its the only way. But again, it doesn't solve the problem for strings.

But most importantly why do these modifications remove a linking error?

In regards to point 4, if the roughStruct was being included twice, it would still be in the above solutions as they don't change any #include directives. So point 4 is not the reason right?

Code:

The structure:

//roughStruct.h


#ifndef ROUGHSTRUCT_H
#define ROUGHSTRUCT_H

#include<string>

struct roughStruct {
    //this is the critical thing
    static const char *someVar;
//    static std::string someStringVar;
};

const char *someVar = "abcd";


#endif //ROUGHSTRUCT_H

This is the parser class:

//roughParse.h

#ifndef ROUGHPARSE_H
#define ROUGHPARSE_H

#include "roughStruct.h"

class roughParse {

    //had some code here that was implmented in roughParse.cpp
};


#endif //ROUGHPARSE_H

//roughParse.cpp

#include "roughParse.h"
#include<iostream>
using std::cout;
using std::endl;

//had some code here implementing roughParse.h

The main file:

//roughMain.cpp


#include "roughStruct.h"
#include "roughParse.h"

int main()
{
//had some code here that used roughparse and roughStruct
    return 0;
}

CMakeLists.txt:

...

add_executable(
       project
        ${SOME_DIR}roughMain.cpp
        ${SOME_DIR}roughParse.h
        ${SOME_DIR}roughParse.cpp
        ${SOME_DIR}roughStruct.h

)
...

CMake Error:

//ignore the line nos.
CMakeFiles/test/roughParse.cpp.o:project/test/roughStruct.h:13: multiple definition of `someVar'
CMakeFiles/test/roughMain.cpp.o:project/test/roughStruct.h:13: first defined here

Env
g++ (Ubuntu 7.4.0-1ubuntu1~18.04) 7.4.0
CMake: Version 3.14.3
IDE: CLion ver 2019.1.3