Hello Watanabe, I'm not sure if what you mean is that in your use case: boost::function f = make_less_by_func(boost::bind(...)); the temporarily constructed bind obj will go out of its lifetime before we use it via a refence to it? But if I cannot declare const F& func_; instead of F func_; then why bother using another ref lib? or, in other words, is there a way to save this 'unneeded' copy construction and allow us to use make_less_by_func(boost::bind(...)) ? btw, I'm not using ref lib frequently, but should boost::unwrap_ref be boost::unwrap_reference? Thank you so much for your continuous help and patience to my probable stupid questions. Best regards Max ----- Original Message ----- From: Steven Watanabe To: boost-users@lists.boost.org Subject: Re: [Boost-users] [bind] Need Help - Function Composition Date: 2008-12-21 02:39:38 AMDG loadcom wrote:

One step further, will the following version save one copy construction

in make_less_by_func()?

template struct less_by_func

{

typedef bool result_type;

const F & func_;

less_by_func(const F & func) : func_(func) {}

template

bool operator()(const T& lhs, const T& rhs) const

{ return func_(lhs) };

// helper function

template less_by_func make_less_by_func(const F& func)

{ return less_by_func(func); }

I don't advise this in general because it requires the argument of make_less_by_func to survive for a while. boost::function f = make_less_by_func(boost::bind(...)); f(1, 2); // undefined behavior because of the dangling reference. If you really want this capability #include template struct less_by_func { typedef bool result_type; F func_; less_by_func(const F & func) : func_(func) {} template bool operator()(const T& lhs, const T& rhs) const { return boost::unwrap_ref(func_)(lhs) http://lists.boost.org/mailman/listinfo.cgi/boost-users ------------------------------------------------------------------- 新浪空间——与朋友开心分享网络新生活！(http://space.sina.com.cn/ )