StackOverflow2013

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plurals

PO
primarykey
Id
16907394
data
AcceptedAnswerId
0
AnswerCount
0
ClosedDate
CommentCount
6
CommunityOwnedDate
CreationDate
2013-06-03T23:19:28.070
FavoriteCount
0
LastActivityDate
2013-06-03T23:51:05.477
LastEditDate
2013-06-03T23:51:05.477
LastEditorUserId
1932150
OwnerUserId
1932150
ParentId
16907331
PostTypeId
2
Score
10
ViewCount
0
LastEditorDisplayName
text
Body
While the compiler can perform derived-to-base conversions when doing type deduction, <code>std::shared_ptr<Derived></code> does not itself derive from <code>std::shared_ptr<Base<int>></code>. There is a user-defined conversion between the two that allows <code>shared_ptr</code> to behave like regular pointers with respect to polymorphism, but the compiler won't take into consideration user-defined conversions when performing type deduction. Without considering user-defined conversiosn, the compiler can't deduce a <code>T</code> that would make <code>shared_ptr<Base<T>></code> either identical to <code>shared_ptr<Derived></code> or a base class of <code>shared_ptr<Derived></code> (once again, <code>shared_ptr<Base<int>></code> is not a base class of <code>shared_ptr<Derived></code>). Therefore, type deduction fails. To work around this problem, you could let the parameter of your function be a simple <code>shared_ptr<T></code> and add a SFINAE-constraint that would make sure your overload is picked only when the type of the argument is derived from (or is) an instance of the <code>Base</code> class template: <pre><code>#include <type_traits> namespace detail { template<typename T> void deducer(Base<T>); bool deducer(...); } template <typename T, typename std::enable_if< std::is_same< decltype(detail::deducer(std::declval<T>())), void >::value>::type* = nullptr> void func(std::shared_ptr<T> ptr) { // ... } </code></pre> Here is a <a href="http://coliru.stacked-crooked.com/view?id=f4988955d3b682b6e1185a37194b20e2-e54ee7a04e4b807da0930236d4cc94dc">live example</a>.
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1. This table or related slice is empty.
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1. POIssue with std::shared_ptr, inheritance, and template argument deduction
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PostTypePostTypeId
1. PTAnswer
UserLastEditorUserId
1. USAndy Prowl
UserOwnerUserId
1. USAndy Prowl
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1. POIssue with std::shared_ptr, inheritance, and template argument deduction
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2. VO
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