Re: Return value of functor object in for_each algorithm

James Kanze <>
Sun, 31 May 2009 02:21:23 -0700 (PDT)
On May 29, 10:53 am, Maxim Yegorushkin <>

On May 29, 9:29 am, mthread <> wrote:

    I am using a functor object in for_each algorithm. I would like to
know if there is any way for me to find out the return value of the
functor object. I would like to check this return value to carry out
further processing(Say exit the application when return value is -1).

std::for_each returns a copy of your functor, after it has been
applied to the sequence. Please see notes at the end of


I suspect that that's the intent, but it's certainly not
required by the standard. for_each returns some copy of the
functional object; what exactly is unspecified. Thus, for
example, the following would be a perfectly legal

    template< typename InputIterator, typename Function >
    for_each( InputIterator first, InputIterator last, Function f )
        if ( first != last ) {
            f( *first ) ;
            ++ first ;
            for_each( first, last, f ) ;
        return f ;

In general, it's best to make all copies of functional objects
idempotent, using an additional level of indirection if
necessary. For functional objects with mutable state, there are
two more or less standard solutions:

 -- The client defines an instance of the state, and passes an
    address of it to the constructor of the functional object,
    which contains a pointer to it. This is the simplest and
    most efficient solution; if there is mutable state which the
    client doesn't need to know about, however, it breaks

 -- The functional object defines a nested class type with
    state. The non-copy constructors allocate an instance of
    the state dynamically, and the functional object uses
    reference counting (boost::shared_ptr works fine here,
    although other reference counted pointers may be more
    efficient) so that copies share the instance of the state.

James Kanze (GABI Software)
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