Thread safety - again...

[Phil Edwards]

On Sat, Feb 23, 2002 at 06:27:27PM +0100, Stefan Olsson wrote:
> 
> string shared::find_in_shared_map( string key )
> {
>         string tmp = "";
> 
>         if( shared_map.find( key ) != shared_map.end() )
>         {
>                 tmp = shared_map[ key ];
>         }
> 
>         return tmp;
> }

If you /really/ want to make certain that this won't change the object,
mark it as a const member function, and pass the string by reference-to-
const for efficiency:

    class shared
    {
        ....
        string find_in_shared_map( string const& key ) const ;
                                          ^^^^^^       ^^^^^
    };

(You may already have this stuff; I'm mainly writing it for the audience.)

Doing this, you'll find that map's operator[] cannot be used for non-const
objects.  That should lead you to use the return value from find()
rather than making, essentially, a second call to find() inside op[].
Something like

    string shared::find_in_shared_map (string const& key) const
    {
        string tmp;
        map<.....>::iterator i = shared_map.find(key);

        if (i != shared_map.end())
            tmp = i->second;

        return tmp;
    }

(unchecked, off-the-top-of-my-head code)


> In this case there is no method that alters the map container, in which 
> case a mutex would have been needed, but is the above according to the 
> rules or not?

I /think/ so, but I'm not the threading expert.


Phil

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