Fw: [C++-sig] Pyste and STL types

[David Abrahams]

Nicodemus <nicodemus at globalite.com.br> writes:

> David Abrahams wrote:
>
>>Nicodemus <nicodemus at globalite.com.br> writes:
>>
>>
>>>What do you mean by "active field"? I ask because my knowledge of
>>>unions is limited.
>>>
>>
>>The one someone stored data in.
>>
>
> I know, but from your comment I understood that C/C++ took care of
> this for you, like this:
>
> union Test
> {
>     float d;
>     int i;
> };
>
> void main()
> {
>     Test t;
>     t = 3.0;       // use the d member
>     float x = t;  // get the d member
>     t = 1;         // set the i member
>     int z = t;     // get the i member
> }
>
> Which I don't think is the case. 

I didn't meant that.

> The programmer has to take care of
> what field is active:

Yes.

> void main()
> {
>     Test t;
>     t.d = 3.0;     // set the d member
>     int z = t.i;   // member i has garbage
>     float x = t.d; // ok
> }
>
> What am I missing?

Nothing; I am just suggesting that the Python interface ought to work
like the first example when the union is a member of another class, as
it commonly will be.  In order to handle a union coherently there
must be some additional data telling you which field is active, so
the union is commonly packaged in a struct:

    struct Test
    {
        bool is_float;
        union val
        {
            float d;
            char* s;
        };
    };

In Python, we ought to be able to say:

   Test.val = 3.14

or

   Test.val = 'hello'
    
...just IMO.

-- 
Dave Abrahams
Boost Consulting
www.boost-consulting.com