C distinguishes between a function declared with an empty parameter list
and a function declared with a parameter list consisting of only
void.
The former is an unprototyped function taking an unspecified number of
arguments, while the latter is a prototyped function taking no arguments.
// C code
extern int foo(); // Unspecified parameters
extern int bar(void); // No parameters
void baz()
{
foo(0); // Valid C, invalid C++
foo(1, 2); // Valid C, invalid C++
bar(); // Okay in both C and C++
bar(1); // Error in both C and C++
}
C++, on the other hand, makes no distinction between the two declarations
and considers them both to mean a function taking no arguments.
// C++ code
extern int xyz();
extern int xyz(void); // Same as 'xyz()' in C++,
// Different and invalid in C
For code that is intended to be compiled as either C or C++, the best
solution to this problem is to always declare functions taking no parameters
with an explicit void prototype.
For example:
// Compiles as both C and C++
int bosho(void)
{
...
}
Empty function prototypes are a deprecated feature in C99
(as they were in C89).Refer to http://david.tribble.com/text/cdiffs.htm
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