cpp_notes.md

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• Reference
  • When should we use reference over pointer and vice versa ?
• friend function
• difference between plain Enum and Enum class in C++ ?
• How to prevent someone from taking address of your object?
• How to prevent someone from inheriting from your class ?
• How to return mulitple values ?
• How to assign object to int ?
• Why use override keyword ?
• Why copy constructor take argument as reference ?
• Function Hiding in C++

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Reference

1. alias , same memory address
2. Reassignment is NOT possible with reference
3. can NOT make a reference with value NULL

When should we use reference over pointer and vice versa ?

• Use reference in function parameter and return type(best use for reference)

  1. Pass big objects ( avoid object value copy )

     void fun(BigClass& obj) {
         obj->_data ;
     }
     int main() {
         BigClass obj;
         fun(obj);
         return 0;
     }

  2. To modify local variable of caller function

     void change(int & x) {
         x = 10 ;
     }
     int main() {
         int x = 5 ;
         change(x); 
         return 0;
     }

  3. To achieve run-time polymorphism in a function
• Use pointers in algorithms and data structures like linked list, tree, graph etc...
  • Why?
    1. sometime we put NULL/nullptr in node
    2. sometime we change pointers to point some another node

friend function

• mainly used for giving rights explicitly
  • i.e. used for testing
• friend function don't have the permission to access the parent Class.
  • you invited a friend , he can enter into your room, but not your parents' room.

difference between plain Enum and Enum class in C++ ?

// plain Enum
enum Color1{red, green, blue};
Color1 c1 = red ;

// Class Enum
enum class Color2{red, green, blue};
Color2 c2 = Color2::red ;

How to prevent someone from taking address of your object?

• A1. Overlaod & operateor and keep it private

  class Base {
      int x; 
  public:
      Base() {}
      Base(int x):x{x} {} 
  private:
      Base* operator & () {
          return this ;
      }
  } ;

• A2. Delete & operator from you class.

  class Base {
      int x ;
  public:
      Base () {} 
      Base (int x):x{x} {}
      Base* operator & () = delete ;
  } ;

How to prevent someone from inheriting from your class ?

• just use final keyword

class Base final {
    ...
}

How to return mulitple values ?

1. use some struct/class and fill the values in that.
2. use std::tuple in c++11

How to assign object to int ?

class Base {
    ...
public:
    operator int() const {
        return var; 
    }
}

Why use override keyword ?

class Base {
    ...
public:
    virtual void fun() {
        ...
    }
}
class Derived: public Base {
    ...
public:
    void fun() override {
        ...
    }
}

1. Testing become easy with this ... ( easy maintainance )
2. Compile-time check can be performed. ( future error could be reduced )
   • void fun(int a) will pass compilation, while void fun(int a) override failed.

Why copy constructor take argument as reference ?

    Foo(const Foo obj) {
        ...
    }

A: To avoid infinite recursive.

Function Hiding in C++

• in a class, function with same name and different parameters will be overloaded.
• in a base class and a derived class, same name function will be hidden from derived class.