PImpl without user defined constructor using std::unique_ptr

Question

Encouraged by @xtofl in How do I use unique_ptr for pimpl? here I'd like to raise a question:

Why I cannot implement PImpl idiom using std::unique_ptr without user defined constructor?

Consider following code:

#include <memory>

class X {
public:
    // X();
    ~X();

private:
    struct Impl;
    std::unique_ptr<Impl> impl_;
};

int main() {
    X x;
}

g++ fails with following error

In file included from C:/MinGW64/mingw64/lib/gcc/x86_64-w64-mingw32/8.1.0/include/c++/memory:80,
                 from pimpl.cpp:1:
.../x86_64-w64-mingw32/8.1.0/include/c++/bits/unique_ptr.h: In instantiation of 'void std::default_delete<_Tp>::operator()(_Tp*) const [with _Tp = X::Impl]':
.../x86_64-w64-mingw32/8.1.0/include/c++/bits/unique_ptr.h:274:17:   required from 'std::unique_ptr<_Tp, _Dp>::~unique_ptr() [with _Tp = X::Impl; _Dp = std::default_delete<X::Impl>]'
pimpl.cpp:3:7:   required from here
.../x86_64-w64-mingw32/8.1.0/include/c++/bits/unique_ptr.h:79:16: error: invalid application of 'sizeof' to incomplete type 'X::Impl'
  static_assert(sizeof(_Tp)>0,
                ^~~~~~~~~~~

clang fails with following error:

In file included from pimpl.cpp:1:
...\14.16.27023\include\memory:2082:21: error: invalid application of 'sizeof' to an incomplete type 'X::Impl'
                static_assert(0 < sizeof (_Ty),
                                  ^~~~~~~~~~~~
...\14.16.27023\include\memory:2296:4: note: in instantiation of member function 'std::default_delete<X::Impl>::operator()' requested here
                        this->get_deleter()(get());
                        ^
pimpl.cpp:3:7: note: in instantiation of member function 'std::unique_ptr<X::Impl, std::default_delete<X::Impl> >::~unique_ptr' requested here
class X {
      ^
pimpl.cpp:9:12: note: forward declaration of 'X::Impl'
    struct Impl;
           ^

Visual Studio fails with the following error:

pimpl.cpp
...\14.16.27023\include\memory(2082): error C2027: use of undefined type 'X::Impl'
pimpl.cpp(9): note: see declaration of 'X::Impl'
...14.16.27023\include\memory(2081): note: while compiling class template member function 'void std::default_delete<_Ty>::operator ()(_Ty *) noexcept const'
        with
        [
            _Ty=X::Impl
        ]
...\14.16.27023\include\memory(2296): note: see reference to function template instantiation 'void std::default_delete<_Ty>::operator ()(_Ty *) noexcept const' being compiled
        with
        [
            _Ty=X::Impl
        ]
...\14.16.27023\include\memory(2132): note: see reference to class template instantiation 'std::default_delete<_Ty>' being compiled
        with
        [
            _Ty=X::Impl
        ]
...\14.16.27023\include\memory(2179): note: see reference to class template instantiation 'std::_Unique_ptr_base<_Ty,_Dx>' being compiled
        with
        [
            _Ty=X::Impl,
            _Dx=std::default_delete<X::Impl>
        ]
pimpl.cpp(10): note: see reference to class template instantiation 'std::unique_ptr<X::Impl,std::default_delete<_Ty>>' being compiled
        with
        [
            _Ty=X::Impl
        ]
...\14.16.27023\include\memory(2082): error C2338: can't delete an incomplete type
...\14.16.27023\include\memory(2084): warning C4150: deletion of pointer to incomplete type 'X::Impl'; no destructor called
pimpl.cpp(9): note: see declaration of 'X::Impl'

The thread you linked to should yield the answer you're looking for. For the deletion to take place, `Impl` must be a known type, which might not be the case if you let the compiler generate a destructor for you. — lubgr, Apr 11 '19 at 12:33
The error mentions that the object can't be `deleted`. The illustrating code should only _construct_ the object to prove the point. (e.g. `unique_ptr createX() { return make_unique(); }`). Does that code fail to compile, too? — xtofl, Apr 11 '19 at 12:43
@lubgr: In my sample I define destructor. So it is not the case. Looks like I know why it fails to compile. If class is partially constructed and exception is generated, constructor must be able to destroy already constructed member variables. That is why constructor of the `X` calls destructor of the `std::unique_ptr` — Dmytro Ovdiienko, Apr 11 '19 at 12:44
`struct Impl;` is a forward declaration of type `Impl`. You need to define it, otherwise it's an incomplete type. — vahancho, Apr 11 '19 at 12:47
@vahancho it will be defined in another cpp file. That is the point of PImpl — Dmytro Ovdiienko, Apr 11 '19 at 12:57
Aw shame, I was writing an answer. The dupe looks appropriate however. — StoryTeller - Unslander Monica, Apr 11 '19 at 13:27
@StoryTeller I was actually surprised exceptions were the cause. TIL — Guillaume Racicot, Apr 11 '19 at 13:28
@GuillaumeRacicot - Now the anti-exception people will have another reason to say exceptions are bad :P — StoryTeller - Unslander Monica, Apr 11 '19 at 13:29
You need deleter functor `std::unique_ptr impl_;` with `struct ImplDeleter { void operator()(Impl*); };` — Eljay, Apr 11 '19 at 13:34
@GuillaumeRacicot Yeah, it does answer. I've already figured out the reason "why". Posted it as comment above. It is the same as answer in "dupe" :) Thanks — Dmytro Ovdiienko, Apr 11 '19 at 13:55
@Eljay with custom deleter size of `std::unique_ptr` is two times higher. — Dmytro Ovdiienko, Apr 11 '19 at 13:56

PImpl without user defined constructor using std::unique_ptr

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