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### 2399. `shared_ptr`'s constructor from `unique_ptr` should be constrained

Section: 20.11.3.2 [util.smartptr.shared.const] Status: C++17 Submitter: Stephan T. Lavavej Opened: 2014-06-14 Last modified: 2017-07-30

Priority: 0

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Discussion:

Consider the following code:

>     #include <iostream>
>     #include <memory>
>     #include <string>

>     using namespace std;

>     void meow(const shared_ptr<int>& sp) {
>       cout << "int: " << *sp << endl;
>     }

>     void meow(const shared_ptr<string>& sp) {
>       cout << "string: " << *sp << endl;
>     }

>     int main() {
>       meow(make_unique<int>(1729));
>       meow(make_unique<string>("kitty"));
>     }

This fails to compile due to ambiguous overload resolution, but we can easily make this work. (Note: `shared_ptr`'s constructor from `auto_ptr` is also affected, but I believe that it's time to remove `auto_ptr` completely.)

[2014-06-16 Rapperswil]

Move to Ready

Proposed resolution:

This wording is relative to N3936.

1. Change 20.11.3.2 [util.smartptr.shared.const] around p33 as indicated:

   >     template <class Y, class D> shared_ptr(unique_ptr<Y, D>&& r);

   > -?- Remark: This constructor shall not participate in overload resolution unless `unique_ptr<Y, D>::pointer` is convertible to `T*`.

   > -33- Effects: Equivalent to `shared_ptr(r.release(), r.get_deleter())` when `D` is not a reference type, otherwise `shared_ptr(r.release(), ref(r.get_deleter()))`.