Question

I have some code in a header that looks like this:

#include <memory>

class Thing;

class MyClass
{
    std::unique_ptr< Thing > my_thing;
};

If I include this header in a cpp that does not include the Thing type definition, then this does not compile under VS2010-SP1:

1>C:\Program Files (x86)\Microsoft
Visual Studio
10.0\VC\include\memory(2067): error C2027: use of undefined type ‘Thing’

Replace std::unique_ptr by std::shared_ptr and it compiles.

So, I’m guessing that it’s the current VS2010 std::unique_ptr‘s implementation that requires the full definition and it’s totally implementation-dependant.

Or is it? Is there something in it’s standard requirements that makes impossible for std::unique_ptr‘s implementation to work with a forward declaration only? It feels strange as it should only hold a pointer to Thing, shouldn’t it?

Answer 1

Adopted from here.

Most templates in the C++ standard library require that they be instantiated with complete types. However shared_ptr and unique_ptr are partial exceptions. Some, but not all of their members can be instantiated with incomplete types. The motivation for this is to support idioms such as pimpl using smart pointers, and without risking undefined behavior.

Undefined behavior can occur when you have an incomplete type and you call delete on it:

class A;
A* a = ...;
delete a;

The above is legal code. It will compile. Your compiler may or may not emit a warning for above code like the above. When it executes, bad things will probably happen. If you’re very lucky your program will crash. However a more probable outcome is that your program will silently leak memory as ~A() won’t be called.

Using auto_ptr<A> in the above example doesn’t help. You still get the same undefined behavior as if you had used a raw pointer.

Nevertheless, using incomplete classes in certain places is very useful! This is where shared_ptr and unique_ptr help. Use of one of these smart pointers will let you get away with an incomplete type, except where it is necessary to have a complete type. And most importantly, when it is necessary to have a complete type, you get a compile-time error if you try to use the smart pointer with an incomplete type at that point.

No more undefined behavior

If your code compiles, then you’ve used a complete type everywhere you need to.

class A
{
    class impl;
    std::unique_ptr<impl> ptr_;  // ok!

public:
    A();
    ~A();
    // ...
};

Type completeness requirements for unique_ptr and shared_ptr

shared_ptr and unique_ptr require a complete type in different places. The reasons are obscure, having to do with a dynamic deleter vs a static deleter. The precise reasons aren’t important. In fact, in most code it isn’t really important for you to know exactly where a complete type is required. Just code, and if you get it wrong, the compiler will tell you.

However, in case it is helpful to you, here is a table which documents several operations of shared_ptr and unique_ptr with respect to completeness requirements.

Operation	unique_ptr	shared_ptr
P() (default constructor)	incomplete	incomplete
P(const P&) (copy constructor)	—	incomplete
P(P&&) (move constructor)	incomplete	incomplete
~P() (destructor)	complete	incomplete
P(A*) (constructor from ptr)	incomplete	complete
operator=(const P&) (copy assignment)	—	incomplete
operator=(P&&) (move assignment)	complete	incomplete
reset()	complete	incomplete
reset(A*)	complete	complete

Any operations requiring pointer conversions require complete types for both unique_ptr and shared_ptr.

The unique_ptr<A>{A*} constructor can get away with an incomplete A only if the compiler is not required to set up a call to ~unique_ptr<A>(). For example if you put the unique_ptr on the heap, you can get away with an incomplete A. More details on this point can be found in BarryTheHatchet’s answer here.