Sheikh Hasina National Institute of Youth Development — Instructor ICT — Exam: 20.05.2023প্রশ্ন ও সমাধান

Definition. A virtual function is a member function declared in a base class with the virtual keyword, which a derived class is expected to override; calling it through a base-class pointer or reference resolves to the derived class's version at runtime, not the base class's, enabling runtime polymorphism.

Why it's needed: without virtual, C++ resolves a function call using the static (declared) type of the pointer at compile time. With virtual, the compiler instead installs a vtable (virtual function table) pointer in each object, and the call is dispatched through that table based on the object's actual runtime type.

Example:

class Shape {
public:
    virtual double area() { return 0; }
};
class Circle : public Shape {
    double r;
public:
    Circle(double r) : r(r) {}
    double area() override { return 3.1416 * r * r; }
};

int main() {
    Shape* s = new Circle(5);
    std::cout << s->area();   // prints 78.54 -- Circle's version, via the base pointer
}

Without virtual, s->area() would call Shape::area() (returning 0), because s is statically typed as Shape*.

Related term: a pure virtual function (virtual double area() = 0;) has no body and forces derived classes to implement it, making the base class abstract.

Exam takeaway: virtual shifts function resolution from compile time (static binding) to runtime (dynamic binding via the vtable) — it is the mechanism C++ uses to implement true runtime polymorphism through base-class pointers/references.

Definitions.

• Super key: any set of one or more attributes that uniquely identifies a tuple in a relation; it may contain extra, unnecessary attributes.
• Candidate key: a minimal super key — no attribute can be removed from it without losing the uniqueness property. A table can have several candidate keys.
• Primary key: the candidate key that the database designer chooses as the main identifier for the table; every other candidate key not chosen becomes an alternate key.

Worked example — table Student(RollNo, Email, NationalID, Name, Dept):

• Super keys: {RollNo}, {Email}, {NationalID}, {RollNo, Name}, {RollNo, Email, NationalID}, etc. — any superset of a uniquely-identifying set.
• Candidate keys: {RollNo}, {Email}, {NationalID} — each alone is minimal and unique; {RollNo, Name} is a super key but not a candidate key, since {Name} is redundant (RollNo alone already suffices).
• Primary key: {RollNo} — chosen by the designer; {Email} and {NationalID} then become alternate keys.

graph TD
  SK["Super Keys: all uniquely-identifying sets"] --> CK["Candidate Keys: minimal super keys"]
  CK --> PK["Primary Key: the one chosen"]
  CK --> AK["Alternate Keys: candidates not chosen"]

Exam takeaway: every candidate key is a super key, but not every super key is a candidate key (minimality is the filter); every primary key is a candidate key, but not every candidate key is the primary key (the designer's choice is the filter).