Code valid in both C and C++ but produce different output

Here we will see programs that return different results when compiled with C or C++ compilers. These differences arise due to fundamental variations in how C and C++ handle certain language features.

Character Literal Size Differences

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

Example 1: C Code

#include <stdio.h>

int main() {
    printf("Character: %c, Size: %d bytes\n", 'a', sizeof('a'));
    return 0;
}

Character: a, Size: 4 bytes

Example 2: Equivalent C++ Code

The same code compiled as C++ would produce different output −

#include <iostream>
#include <cstdio>

int main() {
    printf("Character: %c, Size: %d bytes\n", 'a', sizeof('a'));
    return 0;
}

Character: a, Size: 1 bytes

Structure Declaration Differences

In C, you must use the struct keyword when declaring structure variables, but C++ allows omitting it −

Example 3: C Structure Usage

#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 values: x = %d, y = %c\n", st.x, st.y);
    return 0;
}

Struct values: x = 10, y = d

Example 4: C++ Structure Usage

In C++, the struct keyword is optional −

#include <iostream>
#include <cstdio>

struct MyStruct {
    int x;
    char y;
};

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

Boolean Expression Size

Boolean expressions have different sizes in C and C++. In C, boolean expressions return int (typically 4 bytes), while C++ has a dedicated bool type (1 byte) −

Example 5: Boolean Size in C

#include <stdio.h>

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

Boolean expression size: 4 bytes

Key Differences Summary

Conclusion

These differences highlight the importance of understanding language-specific behaviors when writing code that needs to be compatible with both C and C++ compilers. Character literals, structure declarations, and boolean types are key areas where the languages diverge.