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Rich-Typed Pointers

A smart pointer library that relies on the type system.

Copyright 2013 Julian Gonggrijp

Version 0, alpha 1.

For the impatient: compiling the examples

Assuming your working directory is the one where this Readme and rich_typed_ptr.hpp live:

g++ -std=c++11 -I. example/basic_usage.cpp

or

clang++ -std=c++11 -stdlib=libc++ -I. example/basic_usage.cpp

As the current implementation strongly relies on C++11 features, it is likely not to compile with other compilers.

Rationale

The purpose of smart pointers is to offer the same flexible access to heap storage as traditional (raw) pointers, while leveraging modern C++ idioms to take away the pitfalls associated with manual resource management. In particular, smart pointers remove the need to explicitly deallocate the associated object when it isn't needed anymore.

As of C++11, the standard library offers two different smart pointer implementations, each with its own virtues. shared_ptr allows the same object to be accessed from multiple places and employs reference counting to ensure that allocated objects stay live as long as they are needed. However, it does introduce its own pitfall where cyclic references may still cause memory leaks, and the reference counting adds overhead at runtime. unique_ptr solves these problems by restricting ownership to a single smart pointer instance. However, since it can't be copied it does not allow access from multiple places, unless you break the safety by retrieving the underlying raw pointer through the get function.

One may wonder why we can't just have a smart pointer that does everything right. Why can't we have a pointer that manages ownership and deallocation, allows access from multiple places, and does all of that without adding runtime overhead? Indeed, there's nothing that stops us from having such a pointer! All we need to do is rely on the type system.

How it works

The system revolves around two interrelated types, owner_ptr and weak_ptr.

owner_ptr is a bit like std::unique_ptr as it takes unique ownership of the object on the heap and it cannot be copied. However, it is even stricter: it can only be move-constructed from another owner_ptr. It cannot be default-constructed, it cannot be constructed from a raw pointer (including nullptr) and it cannot be assigned. The only way to attach a newly allocated object to a newly constructed owner_ptr is through the auxiliary factory function make. It doesn't provide any backdoor to access the underlying raw pointer, and under normal use it is guaranteed to reference a live object for its entire lifetime.

"Normal usage" means that the user doesn't explicitly move owner_ptrs around.

weak_ptr serves a role similar to what std::weak_ptr does for std::shared_ptr: it provides access without taking ownership. It can be constructed and assigned either from an lvalue owner_ptr or from another weak_ptr (and nothing else) and can be freely copied. Under normal use it cannot outlive the referenced object. Like owner_ptr, it doesn't provide any backdoor; it can safely be assumed to reference a live object.

Within function scope, users normally need not worry about the distinction between these types; they can use auto type deduction for all pointers received from other functions and dereference or compare pointers regardless of their type. Functions generally take weak_ptrs as parameters. owner_ptr automatically casts to weak_ptr when passed to such a parameter. Functions return weak_ptr if the pointer was previously taken as an argument, or owner_ptr if it was allocated within the function body.

This sums up the basic mechanics of rich-typed pointers. For an illustration, see example/basic_usage.cpp.

Data structures

Those who paid close attention while reading the description above may think that owner_ptr is too restrictive for the implementation of data structures. Indeed, data structures generally require that pointers can be zero-initialized and owner_ptr does not allow this. Fortunately there is a solution.

For the specific purpose of implementing data structures owner_ptr has a slightly more permissive sibling, data_ptr. data_ptr is almost identical to owner_ptr, including the ability to initialize from make (or make_dynamic, see below) and to be cast to weak_ptr. In addition it can be explicitly initialized from nullptr and allows move assignment.

Note that the added freedom that data_ptr provides should really only be needed for the implementation of data structures. Using it for any other purpose indicates a design fault.

A fairly elaborate illustration of the usage of data_ptr is provided in example/linked_list.cpp.

Dynamic binding

For the purpose of object-oriented programming the rich-typed pointer library implements an alternative factory function, make_dynamic. Given two types base and derived, this function will create a pointer to base that references an object on the heap of type derived. make_dynamic ensures that derived is truly a subclass of base and that base has a virtual destructor. For an illustration, see example/shapes.cpp.

As of yet there is no way to downcast a rich-typed pointer created with make_dynamic. However, such a feature can be added if there is sufficient demand.

Arrays

All templates in rich_typed_ptr.hpp will be specialized for array types. This is not implemented yet.

Custom allocators

The rich-typed pointer library will support custom allocators. This is not implemented yet.

Storage in containers

owner_ptr, data_ptr and weak_ptr can all be safely stored in containers. In the former two cases the container owns all objects referenced by the pointers it stores, in the latter case it does not.

Cyclic references

Under normal usage, cyclic ownership is impossible so it cannot lead to memory leaks. Cyclic referencing through weak_ptrs is possible but unproblematic.

Cyclic ownership using data_ptr and move is possible with some effort, but cannot happen by accident. The data structure designer is forced to consider what owns what, and naive mistakes that would lead to cyclic ownership when using std::shared_ptr will not compile when using rich typed pointers. When cyclic ownership is created by design, it can also be undone. Cyclic ownership by design is illustrated in example/linked_ring.cpp.

Note that rich typed pointers can never be part of multiple ownership cycles at the same time. This is a true impossibility because at any time exactly one pointer will own a given object.

Thread safety

Dangling pointers and null pointers are guaranteed not to occur under normal usage if one of the following strategies is adopted:

  1. A pointer is owned by the common ancestor of all threads that share access to it, at or above the scope where all sharing threads are launched and joined.
  2. Threads transfer ownership in order to access a pointer, and non-owning threads do not keep any weak_ptr to the referenced object.

As with all smart pointers, it is the responsibility of the user to prevent race conditions on the referenced object. The inherent synchronization issue associated with reference counting does not apply to rich typed pointers.

Exception safety

make and make_dynamic will usually offer some degree of exceptions safety, depending on the allocator and the constructor of the object being allocated. Move and copy operations, the comparison operators and weak offer the no-throw guarantee. All other operations offer the basic no-leak guarantee on the condition that the referenced object has a non-throwing destructor. None of the functions in the library will throw exceptions out of themselves.

How it could be even better

The C++ type system has two shortcomings that prevent rich-typed pointers from being the ultimate safe solution to nearly all use cases. Solving the first shortcoming would remove the need for explicit typecasts to weak_ptr. Solving the second shortcoming would remove the need for the runtime assertions that are currently included in the code, by catching the corresponding errors at compile time.

Better type deduction

The auto keyword can be used when an owner_ptr is created:

auto foo = make<int>();

The compiler determines that the type of the expression make<int>() is owner_ptr<int> and correctly assigns this type to foo. However, the following fails to compile:

auto bar = foo;

The C++ standard dictates that the type of bar should be deduced by inferring the type of the right-hand side of the assignment, which is owner_ptr<int>. This makes the entire statement a copy assignment of owner_ptr<int> to owner_ptr<int>, which is not allowed. However, any human reader with knowledge of the rich-typed pointer library can see that the type of bar should be weak_ptr<int>. What the auto keyword really should be doing is to take the entire statement into account, by looking for a type that can substitute the question mark in the following constructor prototype:

? (const owner_ptr<int> &);

and then it would unambiguously find weak_ptr<int>. Since this is not what auto does, we have to write this workaround:

auto bar = weak(foo);

The same problem occurs in template parameter type resolution. Consider the following code that does not compile:

template <class Ptr>
void baz (Ptr arg) { std::cout << *arg << std::endl; }

baz(foo);  // foo is still of type owner_ptr<int>

The standard again dictates that Ptr should simply be resolved to the type of foo, even though the compiler could use the suggestion that baz takes it argument by value and the fact that it knows a type which can be constructed from an lvalue owner_ptr<int>. Because it doesn't we have to use weak again:

baz(weak(foo));

Dependent typing

Dependent typing should not be confused with dynamic typing. In dynamic typing the type of an object is enforced at runtime. In dependent typing, the type of an object can change depending on the operations performed on it but it may still be enforced at compile time.

In C++, once an object is declared it cannot change type. Consequently, if we want to allow certain operations on the object during certain parts of its lifetime, we have to anticipate this by giving it a type that will allow those operations for its entire lifetime -- even if those operations would be semantically invalid for most of its lifetime. This leaves the possibility of problematic code like the following:

template <class T> void baz (weak_ptr<T>);

auto foo = data_ptr<int>(nullptr);
*foo;             // invalid, but will only be detected at runtime
baz(weak(foo));   // compiles, baz cannot safely assume that its
                  // argument is not-null

foo = make<int>(0);
*foo;                   // still compiles, now valid
baz(weak(foo));         // compiler detects no difference

auto bar = move(foo);   // foo becomes null again
*foo;                   // invalid, still compiles
auto ooz = weak(foo);   // compiles even though risky
*ooz;                   // also compiles, but invalid!

If we had dependent types in C++ we could eliminate data_ptr, remove all runtime assertions and have this instead:

template <class T> void baz (weak_ptr<T>);

auto foo = nullptr;     // nullptr_t
*foo;                   // does not compile
baz(weak(foo));         // error, no conversion known

foo = make<int>(0);     // becomes owner_ptr<int>
*foo;                   // compiles
baz(weak(foo));         // fine

auto bar = move(foo);   // foo becomes nullptr_t again
*foo;                   // compiler error
auto ooz = weak(foo);   // error, no conversion known
auto ooz = foo;         // ooz is also nullptr_t
*ooz;                   // compiler error

Dependent typing is not trivial to implement, but as long as the possibility for an object to change type is restricted to other types that have the same underlying binary representation, it should be doable. Doing just that would make C++ an even more powerful language than it is today.

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Smart pointers that rely on the type system

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