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

Here we will see some programs that produce different results when compiled with C and C++ compilers. These differences arise from how C and C++ handle certain language features differently.

Difference 1: Character Literal Size

In C, character literals are treated as int type, while in C++, they are treated as char type. This affects the result of the sizeof() operator −

Example: C Compiler

#include <stdio.h>

int main() {
    printf("The character: %c, size(%d)\n", 'a', sizeof('a'));
    return 0;
}

The character: a, size(4)

Example: C++ Compiler

#include <stdio.h>

int main() {
    printf("The character: %c, size(%d)\n", 'a', sizeof('a'));
    return 0;
}

The character: a, size(1)

Difference 2: Struct Usage

In C, the struct keyword is required when declaring variables of struct type. In C++, the struct name can be used directly without the struct keyword −

Example: C Compiler (Requires struct keyword)

#include <stdio.h>

struct MyStruct {
    int x;
    char y;
};

int main() {
    struct MyStruct st;  /* struct keyword required in C */
    st.x = 10;
    st.y = 'd';
    printf("Struct (%d|%c)\n", st.x, st.y);
    return 0;
}

Struct (10|d)

Example: C++ Compiler (struct keyword optional)

#include <stdio.h>

struct MyStruct {
    int x;
    char y;
};

int main() {
    MyStruct st;  // struct keyword not required in C++
    st.x = 10;
    st.y = 'd';
    printf("Struct (%d|%c)\n", st.x, st.y);
    return 0;
}

Struct (10|d)

Difference 3: Boolean Expression Size

Boolean expressions in C evaluate to int type (4 bytes), while in C++ they evaluate to bool type (1 byte) −

Example: C Compiler

#include <stdio.h>

int main() {
    printf("Bool size: %d\n", sizeof(1 == 1));
    return 0;
}

Bool size: 4

Example: C++ Compiler

#include <stdio.h>

int main() {
    printf("Bool size: %d\n", sizeof(1 == 1));
    return 0;
}

Bool size: 1