Program that Produces Different Results in C and C++

February 3, 2023

Program in C and C++

On this page we will discuss about the program that produces different results in C and C++ . There are a few key differences between C and C++ that can lead to different behavior in programs written in these languages.
Program that Produces Different Results in C and C++

Different Program in C and C++

Here, we will write a program that compiles and runs in both C and C++ but give different output when compiled by the C and C++ compilers.

There could be many such programs, here are a few examples.

1) Character literals: In C and C++, character literals such as ‘a’, ‘b’, etc. are handled in distinct ways. In C, they are considered as integers, while in C++ they are considered as characters.

This distinction can be seen in following example by the fact that the size of a character literal in a C program results in sizeof(int), but it results in sizeof(char) in a C++ program.

C Program

Run 
#include <stdio.h>

int main()
{
  printf("sizeof('f') =  %lu" , sizeof('f'));

  return 0;
}

Output:

sizeof('f') =  4

C++ Program

Run 
#include <iostream>
using namespace std;

int main()
{
    cout << "sizeof('f') = " << sizeof('f') << endl;
    
    return 0;
}

Output:

sizeof('f') = 1

2) Struct variables: When declaring struct variables in C, it is necessary to include the struct tag, for example using “struct Employee” to refer to a struct of type Employee. In C++, this tag is not required and the struct name “Employee” can be used on its own.

This difference can be observed by comparing the output of a program in C and C++ . In C it would print sizeof(int) and in C++ it would print sizeof(struct AB).

C Program

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

    return 0;
}

Output:

sizeof(AB) = 4
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Function in C++

Static Members in C++

virtual in C++

Pointers in C++

References in C++

C++ Program

Run 
#include<iostream>
using namespace std;

int AB;

int main()
{
  struct AB
  {
    double y;
  };
  
  cout << "sizeof(AB) =  " << sizeof (AB);
  
  return 0;
}

Output:

sizeof(AB) = 8

3) Boolean literals: C++ supports the literals “true” and “false” for boolean values, whereas in C, you would typically use the integers 1 and 0 to represent true and false.

C Program

Run 
#include <stdio.h>

int main()
{
    printf("sizeof(2==2) =  %d", sizeof(2==2)); // which is size of int

    return 0;
}

Output:

sizeof(2==2) =  4

C++ Program

Run 
#include <iostream>
using namespace std;

int main()
{
    cout << "sizeof(2==2) = " << sizeof(2==2); 

    return 0;
}

Output:

sizeof(2==2) = 1

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