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?
Best Answer
Adopted from here.
Most templates in the C++ standard library require that they be instantiated with complete types. However
shared_ptr
andunique_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: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
andunique_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.
shared_ptr
andunique_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 members of
shared_ptr
andunique_ptr
with respect to completeness requirements. If the member requires a complete type, then entry has a "C", otherwise the table entry is filled with "I".Any operations requiring pointer conversions require complete types for both
unique_ptr
andshared_ptr
.The
unique_ptr<A>{A*}
constructor can get away with an incompleteA
only if the compiler is not required to set up a call to~unique_ptr<A>()
. For example if you put theunique_ptr
on the heap, you can get away with an incompleteA
. More details on this point can be found in BarryTheHatchet's answer here.