Template class and derived classes - c++

Question

I have templated MyDatabase class and two derived classes Database_A and Database_B. I have another class C which is using Database_A and Database_B. So, whenever I am including headers of both derived class (Database_A and Database_B) in class C, it gives me this error "Redefinition of MyDatabase". My guessing is, because both derived class (Database_A and Database_B) are dragging template class MyDatabase, it could be the reason for redefinition of MyDatabase. But I don't know how to resolve this problem.
My code:
template <typename U, class T> class My_DataBase {
protected:
std::map<U,T> Container
public:
void add();
T* getNameToPointer(string key);
};
class Database_A: public My_Database <string,A> {
add();
A* getNameToPointer(name);
};
class Database_B: public My_DataBase <string,B> {
add();
B* getNameToPointer(name);
};
class C {
private:
Database_A Db_A; // drage template class on back_end
Database_B Db_B; // drage template class on back_end
};
How can I solve this problem?

Answer 1

It seems like you have two header files, where each header file contains the base class and a dereived class. That's why you get a redefinition error.
To avoid this issue do this: Use three header files and include guards. The base class will be in one header file, named "My_DataBase.h". The two other headers files (one for every dereived class) will include "My_DataBase.h".
However, since you may want to include both header files (of the derived classes), you need to make use of the include guards to avoid redefinition of the base class.
So here is an example:
My_DataBase.h
#ifndef MY_DATABASE_H
#define MY_DATABASE_H
template <typename U, class T> class My_DataBase {
protected:
std::map<U,T> Container
public:
void add();
T* getNameToPointer(string key);
}
#endif
Database_A.h
#ifndef DATABASE_A_H
#define DATABASE_A_H
#include "My_DataBase.h"
class Database_A: public My_Database <string,A> {
add();
A* getNameToPointer(name);
}
Database_B.h
#ifndef DATABASE_B_H
#define DATABASE_B_H
#include "My_DataBase.h"
class Database_B:public My_DataBase <string,B> {
add();
B* getNameToPointer(name);
}

Answer 2

You should prevent header files from multiple inclusion.
You can do this either by specifiying #pragma once instruction on the top of your header files (however I think this is Microsoft specific).
A better way is to surround your header file by code similar to the following :
#ifndef MYHEADER_FILENAME
#define MYHEADER_FILENAME
/// whatever contained in your header file
...
#endif // MYHEADER_FILENAME