Compatibility of C and C++

The C and C++ programming languages are closely related. C++ grew out of C, as it was designed to be source-and-link compatible with C.[1] Due to this, development tools for the two languages (such as IDEs and compilers) are often integrated into a single product, with the programmer able to specify C or C++ as their source language. However, due to minor semantic differences, most non-trivial C programs will not compile as C++ code without modification — C is not a subset of C++.[2]

Likewise, C++ introduces many features that are not available in C and in practice almost all code written in C++ is not conforming C code. This article, however, focuses on differences that cause conforming C code to be ill-formed C++ code, or to be conforming/well-formed in both languages, but to behave differently in C and C++.

Bjarne Stroustrup, the creator of C++, has suggested[3] that the incompatibilities between C and C++ should be reduced as much as possible in order to maximize inter-operability between the two languages. Others have argued that since C and C++ are two different languages, compatibility between them is useful but not vital; according to this camp, efforts to reduce incompatibility should not hinder attempts to improve each language in isolation. The official rationale for the 1999 C standard (C99) "endorse[d] the principle of maintaining the largest common subset" between C and C++ "while maintaining a distinction between them and allowing them to evolve separately", and stated that the authors were "content to let C++ be the big and ambitious language."[4]

Several additions of C99 are or were not supported in C++ or conflicted with C++ features, such as variadic macros, compound literals, designated initializers, variable-length arrays, and native complex number types. The long long int datatype and restrict type qualifier defined in C99 were not included in the C++03 standard, but most mainstream compilers such as the GNU Compiler Collection,[5] Microsoft Visual C++, and Intel C++ Compiler provided them as an extension. The long long datatype along with variadic macros are present in the new C++ standard, C++11. On the other hand, C99 has reduced some other incompatibilities by incorporating C++ features such as // comments and mixed declarations and code.[6]

Constructs valid in C but not in C++

void* ptr;
/* Implicit conversion from void* to int* */
int *i = ptr;
or similarly:
int *j = malloc(sizeof(int) * 5);     /* Implicit conversion from void* to int* */
In order to make the code compile in C++, one must use an explicit cast:
void* ptr;
int *i = static_cast<int*>(ptr);
int *j = static_cast<int*>(malloc(sizeof(int) * 5));
struct template 
{
    int new;
    struct template* class;
};
is valid C code, but is rejected by a C++ compiler, since the keywords "template", "new" and "class" are reserved.
void fn(void)
{
    goto flack;
    int i = 1;
    flack:
        ;
}
enum BOOL {FALSE, TRUE};
typedef int BOOL;
int N;
int N = 10;

Constructs that behave differently in C and C++

There are a few syntactical constructs that are valid in both C and C++, but produce different results in the two languages.

For example, character literals such as 'a' are of type int in C and of type char in C++, which means that sizeof 'a' will generally give different results in the two languages: in C++, it will be 1, while in C it will be sizeof(int) which on architectures with 8 bit wide char will be at least 2. As another consequence of this type difference, in C, 'a' will always be a signed expression, regardless of whether or not char is a signed or unsigned type, whereas for C++ this is compiler implementation specific.

C++ implicitly treats any const global as file scope unless it is explicitly declared extern, unlike C in which extern is the default. Conversely, inline functions in C are of file scope whereas they have external linkage by default in C++.[7]

Several of the other differences from the previous section can also be exploited to create code that compiles in both languages but behaves differently. For example, the following function will return different values in C and C++:

extern int T;

int size(void)
{
    struct T {  int i;  int j;  };
    
    return sizeof(T);
    /* C:   return sizeof(int)
     * C++: return sizeof(struct T)
     */
}

This is due to C requiring struct in front of structure tags (and so sizeof(T) refers to the variable), but C++ allowing it to be omitted (and so sizeof(T) refers to the implicit typedef). Beware that the outcome is different when the extern declaration is placed inside the function: then the presence of an identifier with same name in the function scope inhibits the implicit typedef to take effect for C++, and the outcome for C and C++ would be the same. Observe also that the ambiguity in the example above is due to the use of the parenthesis with the sizeof operator. Using sizeof T would expect T to be an expression and not a type, and thus the example would not compile with C++.

Both C99 and C++ have a boolean type bool with constants true and false, but they behave differently. In C++, bool is a built-in type and a reserved keyword. In C99, a new keyword, _Bool, is introduced as the new boolean type. In many aspects, it behaves much like an unsigned int, but conversions from other integer types or pointers always constrained to 0 and 1. Other than for other unsigned types, and as one would expect for a boolean type, such a conversion is 0 if and only if the expression in question evaluates to 0 and it is 1 in all other cases. The header stdbool.h provides macros bool, true and false that are defined as _Bool, 1 and 0, respectively.

Linking C and C++ code

While C and C++ maintain a large degree of source compatibility, the object files their respective compilers produce can have important differences that manifest themselves when intermixing C and C++ code. Notably:

For these reasons, for C++ code to call a C function foo(), the C++ code must prototype foo() with extern "C". Likewise, for C code to call a C++ function bar(), the C++ code for bar() must be declared with extern "C".

A common practice for header files to maintain both C and C++ compatibility is to make its declaration be extern "C" for the scope of the header:[10]

/* Header file foo.h */
#ifdef __cplusplus /* If this is a C++ compiler, use C linkage */
extern "C" {
#endif

/* These functions get C linkage */
void foo();
 
struct bar { /* ... */ };

#ifdef __cplusplus /* If this is a C++ compiler, end C linkage */
}
#endif

Differences between C and C++ linkage and calling conventions can also have subtle implications for code that uses function pointers. Some compilers will produce non-working code if a function pointer declared extern "C" points to a C++ function that is not declared extern "C".[11]

For example, the following code:

void my_function();
extern "C" void foo(void (*fn_ptr)(void));

void bar()
{
   foo(my_function);
}

Using Sun Microsystems' C++ compiler, this produces the following warning:

$ CC -c test.cc
"test.cc", line 6: Warning (Anachronism): Formal argument fn_ptr of type
extern "C" void(*)() in call to foo(extern "C" void(*)()) is being passed
void(*)().

This is because my_function() is not declared with C linkage and calling conventions, but is being passed to the C function foo().

References

  1. Stroustrup, Bjarne. "An Overview of the C++ Programming Language" (PDF). p. 4. Retrieved 12 August 2009.
  2. "Bjarne Stroustrup's FAQ – Is C a subset of C++?". Retrieved 5 March 2014.
  3. "Bjarne Stroustrup's Homepage" (PDF). Research.att.com. Retrieved 18 August 2013.
  4. Rationale for International Standard—Programming Languages—C, revision 5.10 (April 2003).
  5. Restricted Pointers from Using the GNU Compiler Collection (GCC)
  6. "C Dialect Options - Using the GNU Compiler Collection (GCC)". gnu.org.
  7. 1 2 "IBM Knowledge Center". ibm.com.
  8. "Const correctness, C++ FAQ". Parashift.com. 4 July 2012. Retrieved 18 August 2013.
  9. "IBM Knowledge Center". ibm.com.
  10. "IBM Knowledge Center". ibm.com.
  11. "Oracle Documentation". Docs.sun.com. Retrieved 18 August 2013.

External links

The Wikibook C++ Programming has a page on the topic of: Programming Languages/Comparisons/C
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