Re: Errors with ->*

[Glynn Clements <glynn@xxxxxxxxxxxxxxxxxx>]

Shriramana Sharma wrote:

> 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 ;

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

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

> 	mystruct () 
> 	{ 
> 		xptr = & x ; 
> 		yptr = & y ; 
> 	}

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.

> 	a .* xptr = 3 ;
> 	a .* yptr = 4 ;
> 
> 	aptr ->* xptr = 5 ;
> 	aptr ->* yptr = 6 ;

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 <glynn@xxxxxxxxxxxxxxxxxx>
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