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Write a program that produces different results in C and C++

Last Updated : 23 Jul, 2025

Write a program that compiles and runs both in C and C++, but produces different results when compiled by C and C++ compilers.

There can be many such programs, following are some of them.

Character literals are treated differently in C and C++. In C character literals like 'a', 'b', ..etc are treated as integers, while as characters in C++. (See

for details) For example, the following program produces sizeof(int) as output in C, but sizeof(char) in C++.

#include<stdio.h>
int main()
{
  printf("%d", sizeof('a'));
  return 0;
}

In C, we need to use struct tag whenever we declare a struct variable. In C++, the struct tag is not necessary. For example, let there be a structure for

. In C, we must use '

' for

variables. In C++, we can omit struct and use '

' only. Following is a program that is based on the fact and produces different outputs in C and C++. It prints sizeof(int) in C and sizeof(struct T) in C++.

#include <stdio.h>
int T;
 
int main()
{
    struct T { double x; };  // In C++, this T hides the global variable T, 
                            // but not in C
    printf("%d", sizeof(T));
    return 0;
}

Types of boolean results are different in C and C++. Thanks to Gaurav Jain for suggesting this point.

 
// output = 4 in C (which is size of int)
printf("%d", sizeof(1==1)); 

// output = 1 in c++ (which is the size of boolean datatype)
cout << sizeof(1==1); 

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