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 (See http://sh.st/qMeGg)
#include<stdio.h>
int main()
{
func(); // func() is called before its declaration/definition
return 0;
}
int func()
{
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.
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