[dcl.fct]
Change: In C++, a function declared with an empty parameter list takes no arguments.
In C, an empty parameter list means that the number and type of the function arguments are unknown.
Example:
int f(); // means int f(void) in C++ // int f( unknown ) in C
Rationale:
This is to avoid erroneous function calls (i.e., function calls
with the wrong number or type of arguments).
Effect on original feature:
Change to semantics of well-defined feature.
This feature was marked as “obsolescent” in C.
Difficulty of converting:
Syntactic transformation.
The function declarations using C incomplete declaration style must
be completed to become full prototype declarations.
A program may need to be updated further if different calls to the
same (non-prototype) function have different numbers of arguments or
if the type of corresponding arguments differed.
How widely used:
Common.
[dcl.fct] [see [expr.sizeof]]
Change: In C++, types may not be defined in return or parameter types.
In C, these type definitions are allowed.
Example:
void f( struct S { int a; } arg ) {} // valid C, invalid C++
enum E { A, B, C } f() {} // valid C, invalid C++
Rationale:
When comparing types in different translation units, C++ relies
on name equivalence when C relies on structural equivalence.
Regarding parameter types: since the type defined in a parameter list
would be in the scope of the function, the only legal calls in C++
would be from within the function itself.
Effect on original feature:
Deletion of semantically well-defined feature.
Difficulty of converting:
Semantic transformation.
The type definitions must be moved to file scope, or in header files.
How widely used:
Seldom.
This style of type definition is seen as poor coding style.
[dcl.fct.def]
Change: In C++, the syntax for function definition excludes the “old-style” C function.
In C, “old-style” syntax is allowed, but deprecated as “obsolescent”.
Rationale:
Prototypes are essential to type safety.
Effect on original feature:
Deletion of semantically well-defined feature.
Difficulty of converting:
Syntactic transformation.
How widely used:
Common in old programs, but already known to be obsolescent.
[dcl.init.string]
Change: In C++, when initializing an array of character with a string, the number of
characters in the string (including the terminating '\0') must not exceed the
number of elements in the array. In C, an array can be initialized with a string even if
the array is not large enough to contain the string-terminating '\0'.
Example:
char array[4] = "abcd"; // valid C, invalid C++
Rationale:
When these non-terminated arrays are manipulated by standard
string functions, there is potential for major catastrophe.
Effect on original feature:
Deletion of semantically well-defined feature.
Difficulty of converting:
Semantic transformation.
The arrays must be declared one element bigger to contain the
string terminating '\0'.
How widely used:
Seldom.
This style of array initialization is seen as poor coding style.