PIMPL: Exporting classes with single STL member (std::unique_ptr)

Question

Suppose I have a class X with private implementation Ximpl:

//Proj.h

#ifdef PROJ_EXPORTS
#define PROJ_API __declspec(dllexport)
#else
#define PROJ_API __declspec(dllimport)
#endif


//X.h

class PROJ_API X
{
  void foo();
  //..
private:
  class Ximpl;
  std::unique_ptr<Ximpl> x_impl;
}

This works perfectly, but generates the warning:

Compiler Warning (level 1) C4251:

warning C4251: 'X::Ximpl' : class 'std::unique_ptr>' needs to have dll-interface to be used by clients of class 'X::Ximpl'

I have tried to follow the suggested MSDN article, but 1). it didn't work. 2) I don't clearly understand the solution(workaround)

Any advice/explanation on how to get rid of these warnings (#pragma disable's are not an option :) ).

Why is it Level 1 warning?

NOTE: It turns out that if Ximpl is not declared in X scope, then it's possible to export a unique_ptr<Ximpl> after forward-declaration of Ximpl, but if it's in Xs scope, then it's a nested class and therefore can't be forward-declared..

However, for templated class even above solution (have non-nested impl class), seems to fail:

//X.h

template<typename T>
class PROJ_API X
{
  T foo();
  //..
private:
  class Ximpl;
  std::unique_ptr<Ximpl> x_impl;
}

I assume there is either a direct or macro `__declspec(dllexport)` hanging out after `class` and before `X` as well; correct ? — WhozCraig, Feb 14 '15 at 03:03
@WhozCraig - right, edited. And declaraion/definitionss of `Ximpl` in .cpp file — Oleg Shirokikh, Feb 14 '15 at 04:15
Possible solution is to remove PROJ_API specifier from class X and add it to individual X methods. — Nikerboker, Feb 14 '15 at 17:44
Possible duplicate of [std::unique\_ptr pimpl in dll generates C4251 with visual studio](https://stackoverflow.com/questions/32098001/stdunique-ptr-pimpl-in-dll-generates-c4251-with-visual-studio) — Mark Ingram, Jun 09 '17 at 08:32

score 2 · Answer 1 · answered Feb 14 '15 at 22:24

2

Make sure your class X exports and declares non-inlined destructor. Otherwise the compiler implicitly declares inline destructor, which needs to know something about Ximpl in order to call its destructor. And with inline destructor, users of X also need to know how to destroy std::unique_ptr, which leads to the warning.

You can confirm this by checking that ~Ximpl() isn't called when X gets deleted, unless you export non-inlined ~X().

answered Feb 14 '15 at 22:24

Tonttu

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- Thanks for a great advice, it makes perfect sense. However, I have non-inlined constructors implemented in each class. However, it doesn't get rid of the warning if I'm not exporting (as described in MSDN article) `unique_ptr`. In a template-class case, I can't even export the STL member (it's now 2-nested templates). Note, that it's possible to export instantiated template class, but not generic `X` – Oleg Shirokikh Feb 15 '15 at 02:13
Do you mean you have non-inlined destructors? – Tonttu Feb 15 '15 at 10:08
Yes, I meant 'destructors', not 'constructors'. However, I have both – Oleg Shirokikh Feb 15 '15 at 10:10

PIMPL: Exporting classes with single STL member (std::unique_ptr)

1 Answers1