Is C++0x Compatible with C?

C++0x (later standardized as C++11) is not fully compatible with C, just as previous C++ standards were not. While C++ was designed to be largely compatible with C, there are several key differences that prevent full compatibility.

Syntax

// C code that may not compile in C++
// or behaves differently

Key Compatibility Issues

Example 1: Implicit void* Conversion

In C, you can implicitly convert from void* to other pointer types. C++ requires explicit casting −

#include <stdio.h>
#include <stdlib.h>

int main() {
    /* This works in C but not in C++ */
    int *ptr = malloc(sizeof(int) * 5);
    
    if (ptr != NULL) {
        *ptr = 42;
        printf("Value: %d\n", *ptr);
        free(ptr);
    }
    
    return 0;
}

Value: 42

Example 2: Different Keywords

C++ has additional keywords that are valid identifiers in C −

#include <stdio.h>

int main() {
    /* These are valid variable names in C but keywords in C++ */
    int class = 10;
    int new = 20;
    int delete = 30;
    
    printf("class = %d, new = %d, delete = %d\n", class, new, delete);
    
    return 0;
}

class = 10, new = 20, delete = 30

Example 3: Character Literal Size

The size of character literals differs between C and C++ −

#include <stdio.h>

int main() {
    /* In C, 'A' is of type int (usually 4 bytes) */
    /* In C++, 'A' is of type char (1 byte) */
    printf("Size of 'A' in C: %zu bytes\n", sizeof('A'));
    printf("Size of char: %zu bytes\n", sizeof(char));
    
    return 0;
}

Size of 'A' in C: 4 bytes
Size of char: 1 bytes

Major Differences Summary

Conclusion

C++0x/C++11 maintains the same compatibility limitations with C as previous C++ standards. While most C code can be compiled as C++, full compatibility was never a design goal, and certain C constructs will always require modification to work in C++.