How to use unique_ptr with forward declared type?

Question

It is quite common to have forward declarations in order to avoid cyclic header file dependencies, or to implement pimpl, then one might end up with code like:

class A;

class B
{
  A * a;
};

When one tries to replace this with a unique_ptr:

class A;

class B
{
  std::unique_ptr<A> a;
};

the compiler complains. What is the best practice to get this working?

Did you read http://stackoverflow.com/questions/6012157/is-stdunique-ptrt-required-to-know-the-full-definition-of-t ? — m.s., Oct 19 '15 at 11:04
Ensure `A` is defined (not just forward declared) before declaring a `std::unique_ptr`. By default, `unique_ptr` uses operator `delete` (or a variant) to release the object, and that requires using either a basic non-class type (`int`, enums, unions, etc) or invoking the destructor of a `struct`/`class` type (which will not be declared with only a forward declaration of the class). — Peter, Oct 19 '15 at 11:06
Just define everything that needs to know the complete type when it is known. http://coliru.stacked-crooked.com/a/92407ffd3e333a97 — Baum mit Augen, Oct 19 '15 at 11:34
@m.s. well that question is a bit convoluted. I am looking for something straightforward — g24l, Oct 19 '15 at 12:36

score 2 · Accepted Answer · answered Oct 19 '15 at 12:42

One needs to declare the destructor of the enclosing class B, and set this to default in the source file, such that the unique_ptr does not forcibly inline its own default deleter.

class B
{
  public:
  ~B();
  private:
  std::unique_ptr<A> a;
};

and in the source file

#include <A.h>
... do stuff with ( a )
B::~B() = default;

Should do the job.

How to use unique_ptr with forward declared type?

1 Answers1