How to choose struct based on condition - c++

Question

I have two distinct 'struct' 'student' & 'employee'. I want to choose it according to string mentioned in the message.
struct student {
string name;
float score;
}
struct employee {
int id;
string dep;
}
I get message like; $ST,16,JOHN,A or $EM,16,IT
From the keyword like ST or EM i have to decide which structure to populate.
I managed to extract the type like ST or EM but when i write,
if (type == "ST")
student x;
else if (type == "EM")
employee x;
// long code goes on here to sort and populate `struct`
it says xis undefined. I know it is wrong but i can't solve this riddle.
How to choose struct based on the condition?

Answer 1

First of all, it looks as if your scope is wrong. So x may not be visible.
No clue how your code looks, but you could define a base class, a pointer before the conditional and assign memory via the conditional (I would not do that). Alternatively, you need to work within the right scope.
struct base {
//whatever
};
struct student : public base {
};
struct employee : public base {
};
base *ptr;
if (type == "ST")
ptr = new student;
else if (type == "EM")
ptr = new employee;

Answer 2

The if statement is equivalent to
if (type == "ST") {
student x;
} else if (type == "EM") {
employee x;
}
// other code accessing x
You can see there's a scope around student x and employee x. When the scope ends, object x goes out of scope too.
To populate the struct properly, you must process the struct inside the scope
if (type == "ST") {
student x;
// populate student
} else if (type == "EM") {
employee x;
// populate employee
}

Answer 3

If you want something common in your case. There is nothing except reading from string itself. You could create the object, then read from string in-place. OR this solution using (kind of) abstract factory pattern:
using namespace std;
// "Marker interface" common between the two structs.
// Virtual destructor to make this object polymorphic.
class ReadableFromString { public: virtual ~ReadableFromString() {} };
struct student: public ReadableFromString {
string name;
int score;
char grade;
};
struct employee: public ReadableFromString {
int id;
string dept;
};
// Abstract factory function that creates a ReadableFromString
// from a string.
shared_ptr<ReadableFromString> readFromString(const string &source) {
std::stringstream s(source);
std::stringbuf buf(ios_base::out);
string type;
s.get(buf, ',');
s.get(); // This is to skip the comma
type = buf.str();
if (type == "$ST") {
shared_ptr<student> studentObj = make_shared<student>();
s >> studentObj->score;
s.get(); // This is to skip the comma
buf = std::stringbuf(ios_base::out);
s.get(buf, ',');
studentObj->name = buf.str();
s.get(); // This is to skip the comma
studentObj->grade = s.get();
s.get(); // This is to skip the comma
return studentObj;
}
else if (type == "$EM") {
shared_ptr<employee> employeeObj = make_shared<employee>();
s >> employeeObj->id;
s.get(); // This is to skip the comma
buf = std::stringbuf(ios_base::out);
s.get(buf, ',');
employeeObj->dept = buf.str();
s.get(); // This is to skip the comma
return employeeObj;
}
return shared_ptr<ReadableFromString>(); // A null pointer
}
int main() {
string sources[] = { "$ST,16,JOHN,A", "$EM,16,IT"};
for(int i=0;i<2;i++) {
shared_ptr<ReadableFromString> firstOne = readFromString(sources[i]);
// Try to cast to student
if (dynamic_pointer_cast<student>(firstOne)) {
shared_ptr<student> studentObj = dynamic_pointer_cast<student>(firstOne);
cout << "Student: " << "\n";
cout << studentObj->name << "\n";
cout << studentObj->score << "\n";
cout << studentObj->grade << "\n";
} else if (dynamic_pointer_cast<employee>(firstOne)) {
// If not student, it could be an employee
shared_ptr<employee> employeeObj = dynamic_pointer_cast<employee>(firstOne);
cout << "Employee: " << "\n";
cout << employeeObj->id << "\n";
cout << employeeObj->dept << "\n";
} else {
cout << "Not student nor employee" << "\n";
}
}
return 0;
}