Incomplete types and shared_ptr / unique_ptr

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 neccessary 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();
    // ...
};

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 members of shared_ptr and unique_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". I've also added a column for noexcept (True or False) for each operation.

Complete type rquirements for unique_ptr and shared_ptr

	unique_ptr		shared_ptr
	I/C	noexcept	I/C	noexcept
P() default constructor	I	T	I	T
P(const P&) copy constructor	N/A	N/A	I	T
P(P&&) move constructor	I	T	I	T
~P() destructor	C	T	I	T
P(A*)	I	T	C	F
operator=(const P&) copy assignment	N/A	N/A	I	T
operator=(P&&) move assignment	C	T	I	T
reset()	C	T	I	T
reset(A*)	C	T	C	F

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