Re: [Boost-users] [Bind] Placeholders passed by reference?

Subject: Re: [Boost-users] [Bind] Placeholders passed by reference?
From: Steven Watanabe (watanabesj_at_[hidden])
Date: 2010-07-07 12:08:40

AMDG

Robert Jones wrote:
> This works exactly as expected...
>
> void f( unsigned & val ) { val *= 2; }
>
> unsigned i = 3;
> boost::bind( f, _1)( i );
>
> with i being doubled. What I don't get is why I don't need to
> say
>
> boost::bind( f, boost::ref( _1 ) )( i );
>
> and conversely how I specify that I want bind to pass by value
> rather than by reference for the placeholder.
>   

boost::ref is only needed for objects that are stored
in the function object.  The arguments are always
passed by reference.  If you want pass by value,
either make f pass by value, or explicitly copy the
argument:

template<class R>
struct copy {
    typedef R result_type;
    R operator()(R r) const { return r; }
};

bind(f, bind(copy<int>(), _1))(i)

In Christ,
Steven Watanabe