Structure vs class in C++

2.2

In C++, a structure is same as class except the following differences:

1) Members of a class are private by default and members of struct are public by default.
For example program 1 fails in compilation and program 2 works fine.

 
// Program 1
#include <stdio.h>

class Test {
    int x; // x is private
};
int main()
{
  Test t;
  t.x = 20; // compiler error because x is private
  getchar();
  return 0;
}
 
// Program 2
#include <stdio.h>

struct Test {
    int x; // x is public
};
int main()
{
  Test t;
  t.x = 20; // works fine because x is public
  getchar();
  return 0;
}

2) When deriving a struct from a class/struct, default access-specifier for a base class/struct is public. And when deriving a class, default access specifier is private.
For example program 3 fails in compilation and program 4 works fine.

 
// Program 3
#include <stdio.h>

class Base {
public:
    int x;
};

class Derived : Base { }; // is equilalent to class Derived : private Base {}

int main()
{
  Derived d;
  d.x = 20; // compiler error becuase inheritance is private
  getchar();
  return 0;
}
 
// Program 4
#include <stdio.h>

class Base {
public:
    int x;
};

struct Derived : Base { }; // is equilalent to struct Derived : public Base {}

int main()
{
  Derived d;
  d.x = 20; // works fine becuase inheritance is public
  getchar();
  return 0;
}

Related Article: Difference between C structures and C++ structures

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