Compatibility of C and C++ - Constructs That Behave Differently in C and C++

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++.

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.