Re: warning: invalid offsetof from non-POD type

[Oscar Fuentes <ofv at wanadoo dot es>]

John Quigley <johnw at lowestplane dot org> writes:

> Here you go.  Note that it is a warning, not an error.  
>
> // BEGIN CODE
> #include <cstddef>
> #include <iostream>
> using namespace std;
>
> class Foo
> {
>    public:
>       int x;
>       char fillerdata[256];
>       int y;
>
>       Foo()
>       {
>          x = 0;
>          y = 0;
>       }
> };
>
> int main()
> {
>    int yoffset = offsetof(Foo, y); // triggers warning
>    cout << "yoffset: " << yoffset << endl;
>
>    Foo foo;
>    
>    // get a pointer to y using the computed offset
>    int* yptr = (int*)(((unsigned char*)&foo) + yoffset);
>    // test it by assigning value
>    *yptr = 15;
>    cout << "foo values: " << foo.x << ", " << foo.y << endl;
>
>    return 0;
> }
> // END CODE

The C++ Standard says that offsetof is only required to work for POD
types. The warning is remembering that to you.

-- 
Oscar