Re: Mutex in ostream operator - doesn't work

On Jan 29, 5:08 am, "Christopher Pisz" <some...@somewhere.net> wrote:


    [...]


That doesn't solve the problem for things like:

    logstream << "whatever = " << whatever << std::endl ;

Another thread can still intervene between the different <<
operators.

I use a wrapper for this, with a template operator<< which
forwards to the actual stream. The constructor of the wrapper
acquires the lock, and the destructor frees it. (Since the
wrapper will usually be used as a temporary, the lock will be
freed at the end of the full expression.)

Something like (off the top of my head):

    class LockedStream
    {
    public:
        LockedStream( ostream& dest, Mutex& mutex )
            : myStream( dest )
            , myMutex( mutex )
            , myCounter( new int( 1 ) )
        {
            myMutex->lock() ;
        }
        LockedStream( LockedStream const& other )
            : myStream( other.myStream )
            , myMutex( other.myMutex )
            , myCounter( other.myCounter )
        {
            ++ (*myCounter) ;
        }
        ~LockedStream()
        {
            -- (*myCounter) ;
            if ( *myCounter == 0 ) {
                myMutex->unlock() ;
                delete myCounter ;
            }
        }

        template< typename T >
        LockedStream& operator<<( T const& obj )
        {
            myStream << obj ;
            return *this ;
        }

        LockedStream& operator<<( std::ios& (*manip)( std::ios ))
        {
            myStream << manip ;
        }

        LockedStream& operator<<( std::ostream& (*manip)
( std::ostream ) )
        {
            myStream << manip ;
        }

        LockedStream& operator<<( char const* s )
        {
            myStream << s ;
        }

    private:
        ostream& myStream ;
        Mutex& myMutex ;
        int* myCounter ;
    } ;

You can then write something like:

    LockedStream( myStream ) << "whatever = " << whatever <<
std::endl ;

and hold the lock for the entire duration.

--
James Kanze (GABI Software) email:james.kanze@gmail.com
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