C vs C++ – Differences in ‘const’ Keyword

The trick is to read the declaration backwards (right-to-left):

const int a = 1; // read as "a is an integer which is constant"
int const a = 1; // read as "a is a constant integer"

Both are the same thing. Therefore:

a = 2; // Can't do because a is constant

The reading backwards trick especially comes in handy when you're dealing with more complex declarations such as:

const char *s;      // read as "s is a pointer to a char that is constant"
char c;
char *const t = &c; // read as "t is a constant pointer to a char"

*s = 'A'; // Can't do because the char is constant
s++;      // Can do because the pointer isn't constant
*t = 'A'; // Can do because the char isn't constant
t++;      // Can't do because the pointer is constant

const in C cannot be used to build constant expressions.

For example :

#include <stdio.h>
int main()
{
   int i = 2;
   const int C = 2;
   switch(i)
   {
      case C  : printf("Hello") ;
      break;

      default : printf("World");
   }
}

doesn't work in C because case label does not reduce to an integer constant.