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Why doesn't this work?
#include <map> #include <memory> void deleter(int* i) { delete i; } std::map<int, std::unique_ptr<int, decltype(&deleter)>> m; void foo(int* i) { m[0] = std::unique_ptr<int, decltype(&deleter)>(i, &deleter); } Check out the incomprehensible compile error https://godbolt.org/z/Uhp9NO.
In file included from <source>:1: In file included from /opt/compiler-explorer/gcc-8.2.0/lib/gcc/x86_64-linux-gnu/8.2.0/../../../../include/c++/8.2.0/map:61: In file included from /opt/compiler-explorer/gcc-8.2.0/lib/gcc/x86_64-linux-gnu/8.2.0/../../../../include/c++/8.2.0/bits/stl_map.h:63: /opt/compiler-explorer/gcc-8.2.0/lib/gcc/x86_64-linux-gnu/8.2.0/../../../../include/c++/8.2.0/tuple:1668:9: error: no matching constructor for initialization of 'std::unique_ptr<int, void (*)(int *)>' second(std::forward<_Args2>(std::get<_Indexes2>(__tuple2))...) ^ /opt/compiler-explorer/gcc-8.2.0/lib/gcc/x86_64-linux-gnu/8.2.0/../../../../include/c++/8.2.0/tuple:1655:9: note: in instantiation of function template specialization 'std::pair<const int, std::unique_ptr<int, void (*)(int *)> >::pair<int &&, 0>' requested here : pair(__first, __second, ^ /opt/compiler-explorer/gcc-8.2.0/lib/gcc/x86_64-linux-gnu/8.2.0/../../../../include/c++/8.2.0/ext/new_allocator.h:136:23: note: in instantiation of function template specialization 'std::pair<const int, std::unique_ptr<int, void (*)(int *)> >::pair<int &&>' requested here { ::new((void *)__p) _Up(std::forward<_Args>(__args)...); } ^ /opt/compiler-explorer/gcc-8.2.0/lib/gcc/x86_64-linux-gnu/8.2.0/../../../../include/c++/8.2.0/bits/alloc_traits.h:475:8: note: in instantiation of function template specialization '__gnu_cxx::new_allocator<std::_Rb_tree_node<std::pair<const int, std::unique_ptr<int, void (*)(int *)> > > >::construct<std::pair<const int, std::unique_ptr<int, void (*)(int *)> >, const std::piecewise_construct_t &, std::tuple<int &&>, std::tuple<> >' requested here { __a.construct(__p, std::forward<_Args>(__args)...); } ^ /opt/compiler-explorer/gcc-8.2.0/lib/gcc/x86_64-linux-gnu/8.2.0/../../../../include/c++/8.2.0/bits/stl_tree.h:637:23: note: in instantiation of function template specialization 'std::allocator_traits<std::allocator<std::_Rb_tree_node<std::pair<const int, std::unique_ptr<int, void (*)(int *)> > > > >::construct<std::pair<const int, std::unique_ptr<int, void (*)(int *)> >, const std::piecewise_construct_t &, std::tuple<int &&>, std::tuple<> >' requested here _Alloc_traits::construct(_M_get_Node_allocator(), ^ /opt/compiler-explorer/gcc-8.2.0/lib/gcc/x86_64-linux-gnu/8.2.0/../../../../include/c++/8.2.0/bits/stl_tree.h:654:4: note: in instantiation of function template specialization 'std::_Rb_tree<int, std::pair<const int, std::unique_ptr<int, void (*)(int *)> >, std::_Select1st<std::pair<const int, std::unique_ptr<int, void (*)(int *)> > >, std::less<int>, std::allocator<std::pair<const int, std::unique_ptr<int, void (*)(int *)> > > >::_M_construct_node<const std::piecewise_construct_t &, std::tuple<int &&>, std::tuple<> >' requested here _M_construct_node(__tmp, std::forward<_Args>(__args)...); ^ /opt/compiler-explorer/gcc-8.2.0/lib/gcc/x86_64-linux-gnu/8.2.0/../../../../include/c++/8.2.0/bits/stl_tree.h:2414:19: note: in instantiation of function template specialization 'std::_Rb_tree<int, std::pair<const int, std::unique_ptr<int, void (*)(int *)> >, std::_Select1st<std::pair<const int, std::unique_ptr<int, void (*)(int *)> > >, std::less<int>, std::allocator<std::pair<const int, std::unique_ptr<int, void (*)(int *)> > > >::_M_create_node<const std::piecewise_construct_t &, std::tuple<int &&>, std::tuple<> >' requested here _Link_type __z = _M_create_node(std::forward<_Args>(__args)...); ^ /opt/compiler-explorer/gcc-8.2.0/lib/gcc/x86_64-linux-gnu/8.2.0/../../../../include/c++/8.2.0/bits/stl_map.h:518:15: note: in instantiation of function template specialization 'std::_Rb_tree<int, std::pair<const int, std::unique_ptr<int, void (*)(int *)> >, std::_Select1st<std::pair<const int, std::unique_ptr<int, void (*)(int *)> > >, std::less<int>, std::allocator<std::pair<const int, std::unique_ptr<int, void (*)(int *)> > > >::_M_emplace_hint_unique<const std::piecewise_construct_t &, std::tuple<int &&>, std::tuple<> >' requested here __i = _M_t._M_emplace_hint_unique(__i, std::piecewise_construct, ^ <source>:11:6: note: in instantiation of member function 'std::map<int, std::unique_ptr<int, void (*)(int *)>, std::less<int>, std::allocator<std::pair<const int, std::unique_ptr<int, void (*)(int *)> > > >::operator[]' requested here m[0] = std::unique_ptr<int, decltype(&deleter)>(i, &deleter); ^ /opt/compiler-explorer/gcc-8.2.0/lib/gcc/x86_64-linux-gnu/8.2.0/../../../../include/c++/8.2.0/bits/unique_ptr.h:191:12: note: candidate template ignored: substitution failure [with _Up = void (*)(int *)]: no type named 'type' in 'std::enable_if<false, void>' constexpr unique_ptr() noexcept ^ /opt/compiler-explorer/gcc-8.2.0/lib/gcc/x86_64-linux-gnu/8.2.0/../../../../include/c++/8.2.0/bits/unique_ptr.h:204:2: note: candidate constructor template not viable: requires single argument '__p', but no arguments were provided unique_ptr(pointer __p) noexcept ^ /opt/compiler-explorer/gcc-8.2.0/lib/gcc/x86_64-linux-gnu/8.2.0/../../../../include/c++/8.2.0/bits/unique_ptr.h:236:12: note: candidate constructor template not viable: requires 1 argument, but 0 were provided constexpr unique_ptr(nullptr_t) noexcept : unique_ptr() { } ^ /opt/compiler-explorer/gcc-8.2.0/lib/gcc/x86_64-linux-gnu/8.2.0/../../../../include/c++/8.2.0/bits/unique_ptr.h:255:2: note: candidate constructor template not viable: requires single argument '__u', but no arguments were provided unique_ptr(unique_ptr<_Up, _Ep>&& __u) noexcept ^ /opt/compiler-explorer/gcc-8.2.0/lib/gcc/x86_64-linux-gnu/8.2.0/../../../../include/c++/8.2.0/bits/unique_ptr.h:265:2: note: candidate constructor template not viable: requires single argument '__u', but no arguments were provided unique_ptr(auto_ptr<_Up>&& __u) noexcept; ^ /opt/compiler-explorer/gcc-8.2.0/lib/gcc/x86_64-linux-gnu/8.2.0/../../../../include/c++/8.2.0/bits/unique_ptr.h:241:7: note: candidate constructor not viable: requires single argument '__u', but no arguments were provided unique_ptr(unique_ptr&& __u) noexcept ^ /opt/compiler-explorer/gcc-8.2.0/lib/gcc/x86_64-linux-gnu/8.2.0/../../../../include/c++/8.2.0/bits/unique_ptr.h:394:7: note: candidate constructor not viable: requires 1 argument, but 0 were provided unique_ptr(const unique_ptr&) = delete; ^ /opt/compiler-explorer/gcc-8.2.0/lib/gcc/x86_64-linux-gnu/8.2.0/../../../../include/c++/8.2.0/bits/unique_ptr.h:215:7: note: candidate constructor not viable: requires 2 arguments, but 0 were provided unique_ptr(pointer __p, ^ /opt/compiler-explorer/gcc-8.2.0/lib/gcc/x86_64-linux-gnu/8.2.0/../../../../include/c++/8.2.0/bits/unique_ptr.h:227:7: note: candidate constructor not viable: requires 2 arguments, but 0 were provided unique_ptr(pointer __p, ^ 1 error generated. Compiler returned: 1 
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A unique_ptr can only be moved. This means that the ownership of the memory resource is transferred to another unique_ptr and the original unique_ptr no longer owns it. We recommend that you restrict an object to one owner, because multiple ownership adds complexity to the program logic.
An explicit delete for a unique_ptr would be reset() . But do remember that unique_ptr are there so that you don't have to manage directly the memory they hold. That is, you should know that a unique_ptr will safely delete its underlying raw pointer once it goes out of scope.
Nullability - a scoped_ptr or unique_ptr can be null, a value object can never be. Polymorphism - a value object is always exactly its static type, but you can substitute in different derived types for a unique_ptr. The previously-held object is automatically destroyed when you do this.
Use unique_ptr when you want to have single ownership(Exclusive) of the resource. Only one unique_ptr can point to one resource. Since there can be one unique_ptr for single resource its not possible to copy one unique_ptr to another. A shared_ptr is a container for raw pointers.
The problem is that m[0] calls the default constructor of std::unique_ptr<int, decltype(&deleter)>, which is not available because it requires the deleter pointer.
Fix:
struct Deleter { void operator()(int* i) { delete i; } }; std::map<int, std::unique_ptr<int, Deleter>> m; void foo(int* i) { m[0] = std::unique_ptr<int, Deleter>(i); } This is also more efficient than std::unique_ptr<int, decltype(&deleter)> because it does not have to store the very same pointer to deleter in each instance of std::unique_ptr. I.e. sizeof(std::unique_ptr<int, Deleter>) < sizeof(std::unique_ptr<int, decltype(&deleter)>).
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