if (postgres()) {
//
} else if (interbase()) {
//
} else {
//
}

class IDataBase
{
public:
  //Набор операций с базами данных предопределен и фиксирован
  virtual void perform(class Query&) = 0;
  virtual void perform(class Insert&) = 0;
  virtual void perform(class CreateTable&) = 0;
  virtual void perform(class DropTable&) = 0;
};

class IOperation
{
public:
  virtual void accept(IDataBase&) = 0; 
};

class Query       : public IOperation { void accept(IDataBase& db) { db.perform(*this); } };
class Insert      : public IOperation { void accept(IDataBase& db) { db.perform(*this); } };
class CreateTable : public IOperation { void accept(IDataBase& db) { db.perform(*this); } };
class DropTable   : public IOperation { void accept(IDataBase& db) { db.perform(*this); } };

void perfom_operation(IDataBase& db, IOperation& op)
{
  op.accept(db);
}

//Расширяемый набор классов баз данных
class OracleDb : public IDataBase {/*...*/};
class InterbaseDb : public IDataBase {/*...*/};
class MsSqlDb : public IDataBase {/*...*/};
class PostgreSQLDb : public IDataBase {/*...*/};
//...

А>if (postgres()) {
А>//
А>} else if (interbase()) {
А>//
А>} else {
А>//
А>}
А>

template<typename dbT>
class database
{
public:
  // ...
  void make_query0(...);
  void make_query1(...);
  // ...
};

struct interbase_tag {};
struct oracle_tag {};
struct postgress_tag {};
// ...

template<>
class database<interbase_tag>
{
public:
  void make_query0()
  {
    // implementation for Interbase...
  }
  void make_query1()
  {
    // implementation for Interbase...
  }
};

template<>
class database<oracle_tag>
{
public:
  void make_query0()
  {
    // implementation for Oracle...
  }
  void make_query1()
  {
    // implementation for Oracle...
  }
};

#if defined(USE_ORACLE)
  typedef database_tag oracle_tag
#elif defined(USE_INTERBASE)
  typedef database_tag interbase_tag
#else
#  error Database type isn't set...
#endif

typedef database<database_tag> database_type;

int main(int, char* [])
{
  database_type db;
  db.make_query1(...);
  return 0;
}