Disable_if_type / is_pair
Hi, I've got a find wrapper (returning a pointer instead of an iterator) that works for map, unordered map, ptr map, etc. I'd like to extent it to set and mult_index. What is the recommended way to disable the second overload? I didn't find a is_pair type_trait or disable_if_type. I call the find() member, which for example vector doesn't have. How should I use the free find() if a member find() isn't available? template <class T, class U> typename T::value_type::second_type* find_ptr(T& c, U v) { typename T::iterator i = c.find(v); return i == c.end() ? NULL : &i->second; } template <class T, class U> typename iterator_traits<typename T::iterator>::pointer find_ptr(T& c, U v) { typename T::iterator i = c.find(v); return i == c.end() ? NULL : &*i; } Greetings, Olaf
Olaf van der Spek wrote:
[reordering to compress]
template <class T, class U> typename T::value_type::second_type* find_ptr(T& c, U v);
template <class T, class U> typename iterator_traits<typename T::iterator>::pointer find_ptr(T& c, U v);
What is the recommended way to disable the second overload? I didn't find a is_pair type_trait or disable_if_type.
pair doesn't have a nested type "iterator", so SFINAE would disable the second.
I call the find() member, which for example vector doesn't have. How should I use the free find() if a member find() isn't available?
The TypeTraits extension doesn't have a has_member trait, but probably uses some techniques that could be employed to detect T::find. The Type Traits Introspection library provides BOOST_TTI_HAS_MEMBER_FUNCTION which, based upon the name, would apply. _____ Rob Stewart robert.stewart@sig.com Software Engineer using std::disclaimer; Dev Tools & Components Susquehanna International Group, LLP http://www.sig.com ________________________________ IMPORTANT: The information contained in this email and/or its attachments is confidential. If you are not the intended recipient, please notify the sender immediately by reply and immediately delete this message and all its attachments. Any review, use, reproduction, disclosure or dissemination of this message or any attachment by an unintended recipient is strictly prohibited. Neither this message nor any attachment is intended as or should be construed as an offer, solicitation or recommendation to buy or sell any security or other financial instrument. Neither the sender, his or her employer nor any of their respective affiliates makes any warranties as to the completeness or accuracy of any of the information contained herein or that this message or any of its attachments is free of viruses.
On Tue, Oct 11, 2011 at 8:40 PM, Stewart, Robert <Robert.Stewart@sig.com> wrote:
template <class T, class U> typename T::value_type::second_type* find_ptr(T& c, U v);
template <class T, class U> typename iterator_traits<typename T::iterator>::pointer find_ptr(T& c, U v);
What is the recommended way to disable the second overload? I didn't find a is_pair type_trait or disable_if_type.
pair doesn't have a nested type "iterator", so SFINAE would disable the second.
T would be for example map or vector. If it's vector, the first overload is disabled. But if it's map, both overloads are available and the second one needs to be disabled 'manually'. Olaf
Olaf van der Spek wrote:
On Tue, Oct 11, 2011 at 8:40 PM, Stewart, Robert <Robert.Stewart@sig.com> wrote:
template <class T, class U> typename T::value_type::second_type* find_ptr(T& c, U v);
template <class T, class U> typename iterator_traits<typename T::iterator>::pointer find_ptr(T& c, U v);
What is the recommended way to disable the second overload? I didn't find a is_pair type_trait or disable_if_type.
pair doesn't have a nested type "iterator", so SFINAE would disable the second.
T would be for example map or vector. If it's vector, the first overload is disabled. But if it's map, both overloads are available and the second one needs to be disabled 'manually'.
Right. I was focused on the "is_pair" part of your question. It seems like you should be able to use the usual trait class technique to determine whether c.find(v) is valid syntax and, if so, dispatch to that implementation preferentially, falling back on std::find() otherwise. _____ Rob Stewart robert.stewart@sig.com Software Engineer using std::disclaimer; Dev Tools & Components Susquehanna International Group, LLP http://www.sig.com ________________________________ IMPORTANT: The information contained in this email and/or its attachments is confidential. If you are not the intended recipient, please notify the sender immediately by reply and immediately delete this message and all its attachments. Any review, use, reproduction, disclosure or dissemination of this message or any attachment by an unintended recipient is strictly prohibited. Neither this message nor any attachment is intended as or should be construed as an offer, solicitation or recommendation to buy or sell any security or other financial instrument. Neither the sender, his or her employer nor any of their respective affiliates makes any warranties as to the completeness or accuracy of any of the information contained herein or that this message or any of its attachments is free of viruses.
On Wed, Oct 12, 2011 at 1:33 PM, Stewart, Robert <Robert.Stewart@sig.com> wrote:
Olaf van der Spek wrote:
pair doesn't have a nested type "iterator", so SFINAE would disable the second.
T would be for example map or vector. If it's vector, the first overload is disabled. But if it's map, both overloads are available and the second one needs to be disabled 'manually'.
Right. I was focused on the "is_pair" part of your question.
So there's no proper solution?
It seems like you should be able to use the usual trait class technique to determine whether c.find(v) is valid syntax and, if so, dispatch to that implementation preferentially, falling back on std::find() otherwise.
Are such traits available or would that require me to write traits for every single class that has a find() member? Olaf
Olaf van der Spek wrote:
On Wed, Oct 12, 2011 at 1:33 PM, Stewart, Robert <Robert.Stewart@sig.com> wrote:
Olaf van der Spek wrote:
It seems like you should be able to use the usual trait class technique to determine whether c.find(v) is valid syntax and, if so, dispatch to that implementation preferentially, falling back on std::find() otherwise.
Are such traits available or would that require me to write traits for every single class that has a find() member?
I pointed you to the libraries that might offer help in this regard. However, here I was suggesting that it might be possible to write a generic trait, like those in TypeTraits, that indicates whether c.find(v), given the c and v in your find_ptr() call, is valid. _____ Rob Stewart robert.stewart@sig.com Software Engineer using std::disclaimer; Dev Tools & Components Susquehanna International Group, LLP http://www.sig.com ________________________________ IMPORTANT: The information contained in this email and/or its attachments is confidential. If you are not the intended recipient, please notify the sender immediately by reply and immediately delete this message and all its attachments. Any review, use, reproduction, disclosure or dissemination of this message or any attachment by an unintended recipient is strictly prohibited. Neither this message nor any attachment is intended as or should be construed as an offer, solicitation or recommendation to buy or sell any security or other financial instrument. Neither the sender, his or her employer nor any of their respective affiliates makes any warranties as to the completeness or accuracy of any of the information contained herein or that this message or any of its attachments is free of viruses.
On Wed, Oct 12, 2011 at 2:22 PM, Stewart, Robert <Robert.Stewart@sig.com> wrote:
I pointed you to the libraries that might offer help in this regard. However, here I was suggesting that it might be possible to write a generic trait, like those in TypeTraits, that indicates whether c.find(v), given the c and v in your find_ptr() call, is valid.
Ah. Sorry, I don't have much experience with traits yet. I thought you were suggesting a non-generic way. Thanks! Olaf
participants (2)
-
Olaf van der Spek -
Stewart, Robert