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C Programs which are not compiled with C++ ( C Vs C++ )

We all know that C++ is designed to have backward compatibility with C programming but there can be many C programs that would produce compiler error when compiled with a C++ compiler.
Following are few of them.

1) In C++, it is a compiler error to call a function before it is declared. But in C, it may compile

#include<stdio.h>

int main()
{
 /* sum() is called before its declaration or definition */
sum(5, 10);
}
 
int sum(int a, int b)
{ 
printf(" sum is : %d", a+b); 
return 0; 
}

2) In C++, it is compiler error to make a normal pointer to point a const variable, but it is allowed in C

#include <stdio.h>
 
int main(void)
{ 
int const a = 20; 

/* The below assignment is invalid in C++, results in error
In C, the compiler *may* throw a warning, but casting is
implicitly allowed */
int *ptr = &a;  // A normal pointer points to const

printf("*ptr: %d\n", *ptr); 

return 0; 
}

3) In C, a void pointer can directly be assigned to some other pointer like int *, char *. But in C++, a void pointer must be explicitly typcasted.

#include <stdio.h>
int main()
{
   void *vptr;
   
   /*In C++, it must be replaced with int *iptr=(int *)vptr;*/
   int *iptr = vptr; 
   
   return 0; 
}

This is something we notice when we use malloc(). Return type of malloc() is void *. In C++, we must explicitly typecast return value of malloc() to appropriate type, e.g., “int *p = (void *)malloc(sizeof(int))”. In C, typecasting is not necessary.

4) Following program compiles & runs fine in C, but fails in compilation in C++. const variable in C++ must be initialized but in c it isn’t necessary.

#include <stdio.h>
int main()
{
const int a;   // LINE 4
return 0; 
} 

Line 4 [Error] uninitialized const 'a' [-fpermissive]

5) This is the worst answer among all, but still a valid answer. We can use one of the C++ specific keywords as variable names. The program won’t compile in C++, but would compiler in C.

#include <stdio.h>
int main(void)
{
/*new is a keyword in C++, but not in C*/
int new = 5;  
printf("%d", new);
}

Similarly, we can use other keywords like delete, explicit, class, .. etc.

6) C++ does more strict type checking than C. For example the following program compiles in C, but not in C++. In C++, we get compiler error invalid conversion from ‘int’ to ‘char*'


#include <stdio.h>
int main()
{
char *c = 333;
printf("c = %u", c);
return 0;
}