Annex C (informative) Compatibility [diff]

Change: Converting void* to a pointer-to-object type requires casting.

char a[10];
void* b=a;
void foo() {
  char* c=b;
}

ISO C will accept this usage of pointer to void being assigned to a pointer to object type.

C++ will not.

Rationale: C++ tries harder than C to enforce compile-time type safety.

Effect on original feature: Deletion of semantically well-defined feature.

Difficulty of converting: Could be automated.

Violations will be diagnosed by the C++ translator.

The fix is to add a cast.

For example:

char* c = (char*) b;

How widely used: This is fairly widely used but it is good programming practice to add the cast when assigning pointer-to-void to pointer-to-object.

Some ISO C translators will give a warning if the cast is not used.

Change: Implicit declaration of functions is not allowed.

Rationale: The type-safe nature of C++.

Effect on original feature: Deletion of semantically well-defined feature.

Note: the original feature was labeled as “obsolescent” in ISO C.

Difficulty of converting: Syntactic transformation.

Facilities for producing explicit function declarations are fairly widespread commercially.

How widely used: Common.

Change: Decrement operator is not allowed with bool operand.

Rationale: Feature with surprising semantics.

Effect on original feature: A valid ISO C expression utilizing the decrement operator on a bool lvalue (for instance, via the C typedef in <stdbool.h> ([depr.stdbool.h.syn])) is ill-formed in this International Standard.

Change: Types must be defined in declarations, not in expressions.

In C, a sizeof expression or cast expression may define a new type.

For example,

p = (void*)(struct x {int i;} *)0;

defines a new type, struct x.

Rationale: This prohibition helps to clarify the location of definitions in the source code.

Effect on original feature: Deletion of semantically well-defined feature.

Difficulty of converting: Syntactic transformation.

How widely used: Seldom.

Change: The result of a conditional expression, an assignment expression, or a comma expression may be an lvalue.

Rationale: C++ is an object-oriented language, placing relatively more emphasis on lvalues.

For example, function calls may yield lvalues.

Effect on original feature: Change to semantics of well-defined feature.

Some C expressions that implicitly rely on lvalue-to-rvalue conversions will yield different results.

For example,

char arr[100];
sizeof(0, arr)

yields 100 in C++ and sizeof(char*) in C.

Difficulty of converting: Programs must add explicit casts to the appropriate rvalue.

How widely used: Rare.