
I deliberately used this std::bind syntax since it accepts any callable (including things like pointer-to-member [functions]). I tried lambdas with my code and they just work. E.g. auto opt_i3 = map(opt_a, [](const A& a){ return a.i;}); assert(*opt_i3 == 42); Furthermore without the std::decay boost::optional<decltype(f(*o))> will be a boost::optional<T const&>. Tobias Von: Andrzej Krzemienski [via Boost] [mailto:ml-node+s2283326n4677436h22@n4.nabble.com] Gesendet: Montag, 22. Juni 2015 16:44 An: Löw, Tobias (STEAG Energy Services GmbH) Betreff: Re: [Optional] Monadic bind 2015-06-22 16:17 GMT+02:00 Tobias Loew <[hidden email]</user/SendEmail.jtp?type=node&node=4677436&i=0>>:
Sorry for that,
I've made in implementation with free functions which compiles on VS 2012:
#include <assert.h> #include <boost/optional.hpp>
template<class T, class B> auto bind(const boost::optional<T>& optional, B&& binder) -> typename std::decay<decltype(std::bind(binder, *optional)())>::type { if(optional) { return std::bind(binder, *optional)(); } else { return boost::none; } }
template<class T, class B> auto map(const boost::optional<T>& optional, B&& binder) -> boost::optional<typename std::decay<decltype(std::bind(binder, *optional)())>::type> { if(optional) { return std::bind(binder, *optional)(); } else { return boost::none; } }
void foo() { struct A { int i; }; boost::optional<A> opt_a;
auto opt_i = map(opt_a, &A::i); assert(!opt_i);
A a = {42}; opt_a = a;
auto opt_i2 = map(opt_a, &A::i); assert(*opt_i2 == 42);
}
Would you find the following an acceptable solution? ``` #include <assert.h> #include <boost/bind.hpp> #include <boost/optional.hpp> template <typename T, typename F> auto map2(const boost::optional<T>& o, F f) -> boost::optional<decltype(f(*o))> { if (o) return f(*o); else return boost::none; } void foo() { struct A { int i; }; boost::optional<A> opt_a; auto opt_i = map2(opt_a, boost::bind(&A::i, _1)); assert(!opt_i); A a = {42}; opt_a = a; auto opt_i2 = map2(opt_a, boost::bind(&A::i, _1)); assert(opt_i2); assert(*opt_i2 == 42); } int main() { foo(); } ``` It requires of you to type a bit more, but at the same time it is more flexible: you can type any free function on T, including a lambda. Regards, &rzej _______________________________________________ Unsubscribe & other changes: http://lists.boost.org/mailman/listinfo.cgi/boost ________________________________ If you reply to this email, your message will be added to the discussion below: http://boost.2283326.n4.nabble.com/Optional-Monadic-bind-tp4677431p4677436.h... To unsubscribe from [Optional] Monadic bind, click here<http://boost.2283326.n4.nabble.com/template/NamlServlet.jtp?macro=unsubscribe_by_code&node=4677431&code=VG9iaWFzLkxvZXdAc3RlYWcuY29tfDQ2Nzc0MzF8MTI1MDAzMDE5Mg==>. NAML<http://boost.2283326.n4.nabble.com/template/NamlServlet.jtp?macro=macro_viewer&id=instant_html%21nabble%3Aemail.naml&base=nabble.naml.namespaces.BasicNamespace-nabble.view.web.template.NabbleNamespace-nabble.view.web.template.NodeNamespace&breadcrumbs=notify_subscribers%21nabble%3Aemail.naml-instant_emails%21nabble%3Aemail.naml-send_instant_email%21nabble%3Aemail.naml> -- View this message in context: http://boost.2283326.n4.nabble.com/Optional-Monadic-bind-tp4677431p4677437.h... Sent from the Boost - Dev mailing list archive at Nabble.com.