volatile void func(...) 함수의 컴파일 warning 에 대해서...

pure
    Found in versions: 3.0-3.4 
    Introduced in version 2.96. (according to documentation) 
    Description:
 
         Many functions have no effects except the return value and their
         return value depends only on the parameters and/or global
         variables.  Such a function can be subject to common subexpression
         elimination and loop optimization just as an arithmetic operator
         would be.  These functions should be declared with the attribute
         `pure'.  For example,
 
              int square (int) __attribute__ ((pure));
 
         says that the hypothetical function `square' is safe to call fewer
         times than the program says.
 
         Some of common examples of pure functions are `strlen' or `memcmp'.
         Interesting non-pure functions are functions with infinite loops
         or those depending on volatile memory or other system resource,
         that may change between two consecutive calls (such as `feof' in a
         multithreading environment).
 
         The attribute `pure' is not implemented in GCC versions earlier
         than 2.96.

int square (int) __attribute__ ((pure));
int square (int) 
{
 ... 
}

 __attribute__ ((pure)) int square (int)
{
 ... 
}

class My {
public:
	void foo() volatile {};
	void bar() {};
	int i;
};
 
int main() 
{
    volatile My x;
    x.foo();       // ok
    int j = x.i;   // ok - i는 자동으로 volatile
    x.bar();       // error: passing ‘volatile My’ as ‘this’ argument of ‘void My::bar()’ discards qualifiers
}