Write a 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 errors when compiled with a C++ compiler. Following is the list of the C programs that won’t compile in C++:

1. Calling a function before the declaration
2. Using normal pointer with const variable
3. Using typecasted pointers
4. Declaring constant values without initializing
5. Using specific keywords as variable names
6. Strict type checking
7. The return type of main()

These points are discussed in detail below:

1) Calling a function before declaration: In C++, it is a compiler error to call a function before it is declared. But in C, it may compile. (See What happens when a function is called before its declaration in C?) 

C

// C Program to demonstrate calling
// a function before declaration
#include <stdio.h>

// Main starts
int main()
{
    // fun() is called before its
    // declaration/definition
    fun();
}

// Function Declaration
int fun()
{
    printf("Hello");
    return 0;
}

2) Using a normal pointer with const variable: In C++, a compiler error is generated when a normal pointer is used to point to a const variable, however, it is allowed in C. (Must Read – Const Qualifier in C) 

C

// C Program to demonstrate using a
// normal pointer with const variable
#include <stdio.h>

// Main starts
int main()
{

    // A normal pointer points to const
    int const j = 20;

    int* ptr = &j;

    // The below assignment is invalid in C++,
    // results in error.In C, the compiler may
    // throw a warning, but casting is implicitly allowed
    printf("*ptr: %d\n", *ptr);

    return 0;
}

3) Using typecasted pointers: 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 typed cast. 

C

// C Program to demonstrate
// using typecasted pointers
#include <stdio.h>

// Main starts
int main()
{
    void* vptr;

    // In C++, it must be
    // replaced with int *iptr=(int *)vptr;
    int* iptr = vptr;

    return 0;
}

Note: 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) Declaring constant values without initializing: In C++, the const variable must be initialized but in C it is not necessary. The following program compiles & runs fine in C, but fails in the compilation in C++. 

C

// C Program to demonstrate declaring
// constant values without initializing:
#include <stdio.h>

// Main starts
int main()
{
    const int a;
    return 0;
}

5) Using specific keywords as variable names: In C, specific keywords can be used as variable names, however, it is not possible in C++. The following program won’t compile in C++ but would compile in C.  

C

// C Program to demonstrate using
// specific keywords as variable names
#include <stdio.h>

// Main starts
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) Strict type checking: 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*'”. 

C

// C Program to demonstrate
// strict type checking
#include <stdio.h>

// Main starts
int main()
{
    char *c = 333;
    printf("c = %u", c);
    return 0;
}

7) Return type of main(): In C++, the main function requires the return type of ‘int’, however not the case in C. In C++, we cannot use the return type as ‘void’.

C

// C Program to demonstrate that
// 'void' can be used as a return type
// for main()
#include <stdio.h>

// Main starts
void main()
{
    printf("Hello World");
}

8) The following program compiles in C but doesn’t compile in C++. (See this article for more reference.) 

C

// C Program that won't compile in C++
#include <stdio.h>
void func()
{
    // definition
}

// Main starts
int main()
{
    func();
    func(2);
}

Explanation: In C++, func() is equivalent to func(void), however in C, func() is equivalent to func(…).

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