WHAT ARE THE THINGS WHICH ARE ALLOWED IN C AND C++ BUT WITH DIFFERENT MEANINGS

Some incompatibilities unavoidably arose between C and C++ .They are:


1:

C specifies that a variable declared with a constant qualifier is not a modifiable object and in C it is an external linkage.

ex:

  const int           i = 4;  // External linkage
extern const int      j = 7;  // 'extern' optional
 static const int     k = 8;  // 'static' required 

C++ specifies that a Constant object with file scope has internal linkage by default, meaning that the object's name is not visible outside the source file in which it is declared

ex:

 const int           i = 4;  // Internal linkage
 extern const int    j = 7;  // 'extern' required
static const int    k = 8;  // 'static' optional

2:

sizeof (’1′) == sizeof (int) in C; but it is sizeof (char) in C++.

3:

C does not allow a given typedef to appear more than once in the same scope.

C++ handles typedefs and type names differently than C, and allows redundant occurrences of a given typedef within the same scope.

Thus the following code is valid in C++ but invalid in C:

ex:

 typedef int  i
typedef int  i;     // Valid C++, invalid C


4:


Nested structure types may be declared within other structures.
The scope of the inner structure tag extends outside the scope of the
outer structure in C,but does not do so in C++.Structure declarations possess their own scope in C++

ex:

struct Outer
{
    struct Inner        // Nested structure declaration
    {
        int         a;
        float       f;
    }           in;

    enum E              // Nested enum type declaration
    {
        UKNOWN, OFF, ON
    }           state;
};

struct Inner    si;     // Nested type is visible in C,
                        // Not visible in C++

enum E          et;     // Nested type is visible in C,
                        // Not visible in C++