Errors with ->*

Hello -- long time since I posted on this list.

I'm trying to understand the use of ->* but in compiling the attached program which I thought was straightforward I am getting the following errors:

foo.cpp: In constructor ‘mystruct::mystruct()’:
foo.cpp:9:12: error: invalid conversion from ‘int*’ to ‘int’ foo.cpp: In function ‘int main()’:
foo.cpp:21:7: error: ‘xptr’ was not declared in this scope foo.cpp:22:7: error: ‘yptr’ was not declared in this scope

Please help.

Shriramana Sharma

struct mystruct
{
public:

	int x, y ;
	int * xptr, yptr ;
	mystruct () 
	{ 
		xptr = & x ; 
		yptr = & y ; 
	}
} ;

int main ( void )

{
	mystruct a ;
	mystruct * aptr = & a ;

	a . x = 1 ; 
	a . y = 2 ;

	a .* xptr = 3 ;
	a .* yptr = 4 ;

	aptr ->* xptr = 5 ;
	aptr ->* yptr = 6 ;
}

1) a .* xptr = 3 ;
Above line's syntax is wrong.
Should be

        *a.xptr = 3;

2) Also declare the structure member variables as
int *xptr, *yptr ; (otherwise yptr will be of type int, not of int*)

3)  aptr ->* xptr = 5 ;
is also of wrong syntax.
Should be
*aptr->xptr = 5 ;

Which is equivalent to

               *(aptr->xptr) = 5;

the above code DOES NOT mean this one :

             (*aptr)->xptr   // this one dereference aptr first, then

does nothing to xptr

Thanks.

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typo correction :

       (*aptr).xptr = 5;

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Line 9 should be:
int * xptr, * yptr ;
You forgot a '*'. About the rest, you are using the pointer to members incorrectly, look at
http://www.google.com/search?q=pointer+to+member for examples and usage. Darío

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If these are supposed to have pointer-to-member type, the correct syntax is:

	int mystruct::* xptr, mystruct::* yptr;

There's no point in using pointer-to-member instance variables like this. They're normally used when you need to identify a specific field within a class (rather than an instance). If you want to point to a specific member of a specific instance; you would just use plain pointers.

xptr and yptr are members of mystruct, so you need e.g.:

    aptr ->* aptr->xptr = 5 ;
    aptr ->* aptr->yptr = 6 ;

    a .* a.xptr = 3 ;
    a .* a.yptr = 4 ;

The whole point about pointer-to-member is that the value is the same for all instances of the class, so you could just do:

	int main ( void )
	{
	    int mystruct::* xptr = &mystruct::x;
	    int mystruct::* yptr = &mystruct::y;
	    mystruct a, b;
	    mystruct *aptr = &a;
	
	    a .* xptr = 1 ; 
	    a .* yptr = 2 ;
	
	    b .* xptr = 1 ; 
	    b .* yptr = 2 ;
	
	    aptr ->* xptr = 1 ; 
	    aptr ->* yptr = 2 ;
	}

In any case, given that this isn't specific to Linux, and has nothing to do with C, there are better places for asking such questions than the linux-c-programming list (e.g. the comp.lang.c++ newsgroup).

Glynn Clements <[email protected]>
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Thanks all for your replies. I now have a better idea of the things. And Glynn's note about comp.lang.c++ is noted.

Shriramana Sharma