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

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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.

1) 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 this for details)

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

int main()
  printf("%d", sizeof('a'));
  return 0;

2) 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 Student. In C, we must use ‘struct Student‘ for Student variables. In C++, we can omit struct and use ‘Student‘ 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;

3) 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); 

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

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Last Updated : 29 May, 2017
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