C program that won’t compile in C++

Although C++ is designed to have backward compatibility with C there can be many C programs that would produce compiler error when compiled with a C++ compiler. Following are some 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()

{

   foo(); // foo() is called before its declaration/definition

}

int foo()

{

   printf("Hello");

   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 j = 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 = &j;  // 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;

    int *iptr = vptr; // In C++, it must be replaced with int *iptr = (int *)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) 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)
{
    int new = 5;  // new is a keyword in C++, but not in C
    printf("%d", new);
}

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

Please write comments if you find anything incorrect, or you want to share more information about the topic discussed above