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Write a C program that won’t compile in C++
  • Difficulty Level : Easy
  • Last Updated : 13 Nov, 2018

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 https://www.geeksforgeeks.org/g-fact-95/)




#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. (See Const Qualifier 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 = (int *)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. Thanks to Pravasi Meet for suggesting this point.




#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)
{
    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.

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*'”. Thanks to Pravasi Meet for adding this point.




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

7) C++ require main return ‘int’ type




#include <stdio.h>
void main()
{
    printf("Hello World");
}

8) A C/C++ Function Call Puzzle
This article is contributed by Abhay Rathi. Please write comments if you find anything incorrect, or you want to share more information about the topic discussed above

Want to learn from the best curated videos and practice problems, check out the C++ Foundation Course for Basic to Advanced C++ and C++ STL Course for foundation plus STL.  To complete your preparation from learning a language to DS Algo and many more,  please refer Complete Interview Preparation Course.



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