Structure vs class in C++

In C++, a structure is the same as a class except for a few differences. The most important of them is security. A Structure is not secure and cannot hide its implementation details from the end user while a class is secure and can hide its programming and designing details. Following are the points that expound on this difference:

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

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// 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;
}

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// 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;
}

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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.

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// 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;
}

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// 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;
}

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

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