> For the complete Mojo documentation index, see [llms.txt](/llms.txt). > Markdown versions of all pages are available by appending .md to any URL (e.g. /docs/manual/basics.md). # Intro to pointers A pointer is an indirect reference to one or more values stored in memory. The pointer is a value that holds an address to memory, and provides APIs to store and retrieve values to that memory. The value pointed to by a pointer is also known as a _pointee_. The Mojo standard library includes several types of pointers, which provide different sets of features. All of these pointer types are _generic_—they can point to any type of value, and the value type is specified as a parameter. For example, the following code creates an `OwnedPointer` that points to an `Int` value: ```mojo from std.memory import OwnedPointer var ptr: OwnedPointer[Int] ptr = OwnedPointer(100) ``` The `ptr` variable has a value of type `OwnedPointer[Int]`. The pointer _points to_ a value of type `Int`, as shown in Figure 1.
![](../images/owned-pointer-diagram.png#light) ![](../images/owned-pointer-diagram-dark.png#dark)
**Figure 1.** Pointer and pointee
Accessing the memory—to retrieve or update a value—is called _dereferencing_ the pointer. You can dereference a pointer by following the variable name with an empty pair of square brackets: ```mojo # Update an initialized value ptr[] += 10 # Access an initialized value print(ptr[]) ``` ## Pointer terminology Before we jump into the pointer types, here are a few terms you'll run across. Some of them may already be familiar to you. - **Safe pointers**: are designed to prevent memory errors. Unless you use one of the APIs that are specially designated as unsafe, you can use these pointers without worrying about memory issues like double-free or use-after-free. - **Nullable pointers**: can point to an invalid memory location (typically 0, or a "null pointer"). None of the standard library pointer types are nullable. To model a nullable pointer, use `Optional[UnsafePointer]`. - **Smart pointers**: own their pointees, which means that the value they point to may be deallocated when the pointer itself is destroyed. Non-owning pointers may point to values owned elsewhere, or may require some manual management of the value lifecycle. - **Memory allocation**: some pointer types can allocate memory to store their pointees, while other pointers can only point to pre-existing values. Memory allocation can either be implicit (that is, performed automatically when initializing a pointer with a value) or explicit. - **Uninitialized memory**: refers to memory locations that haven't been initialized with a value, which may therefore contain random data. Newly-allocated memory is uninitialized. The safe pointer types don't allow users to access memory that's uninitialized. Unsafe pointers can allocate a block of uninitialized memory locations and then initialize them one at a time. Being able to access uninitialized memory is unsafe by definition. - **Copyable types**: can be copied implicitly (for example, by assigning a value to a variable). Also called _implicitly copyable types_. ```mojo copied_ptr = ptr ``` _Explicitly copyable_ types require the user to request a copy, using a constructor with a keyword argument: ```mojo copied_owned_ptr = OwnedPointer(other=owned_ptr) ``` ## Pointer types The Mojo standard library includes several pointer types with different characteristics: - [`Pointer`](/docs/std/memory/pointer/Pointer/) is a safe pointer that points to a single value that it doesn't own. - [`OwnedPointer`](/docs/std/memory/owned_pointer/OwnedPointer/) is a smart pointer that points to a single value, and maintains exclusive ownership of that value. - [`ArcPointer`](/docs/std/memory/arc_pointer/ArcPointer/) is a reference-counted smart pointer that points to an owned value with ownership potentially shared with other instances of `ArcPointer`. - [`UnsafePointer`](/docs/std/memory/unsafe_pointer/UnsafePointer/) points to one or more consecutive memory locations, and can refer to uninitialized memory. Table 1 summarizes the different types of pointers:
| | `Pointer` | `OwnedPointer` | `ArcPointer` | `UnsafePointer` | |--------------------------------------------------|-----------|-------------------------|-------------------------|-----------------| | Safe | Yes | Yes | Yes | No | | Allocates memory | No | Implicitly 1 | Implicitly 1 | Explicitly | | Owns pointee(s) | No | Yes | Yes | No 2 | | Copyable | Yes | No 3 | Yes | Yes | | Nullable | No | No | No | No | | Can point to uninitialized memory | No | No | No | Yes | | Can point to multiple values (array-like access) | No | No | No | Yes |
**Table 1.** Pointer types
1 `OwnedPointer` and `ArcPointer` implicitly allocate memory when you initialize the pointer with a value. 2 `UnsafePointer` provides unsafe methods for initializing and destroying instances of the stored type. The user is responsible for managing the lifecycle of stored values. 3 `OwnedPointer` is explicitly copyable, but explicitly copying an `OwnedPointer` copies the _stored value_ into a new `OwnedPointer`. The following sections provide more details on each pointer type. ## `Pointer` The [`Pointer`](/docs/std/memory/pointer/Pointer/) type is a safe pointer that points to an initialized value that it doesn't own. Some example use cases for a `Pointer` include: - Storing a reference to a related type. For example, a list's iterator object might hold a `Pointer` back to the original list. - Passing the memory location for a single value to external code via `external_call()`. - Where you need an API to return a long-lived reference to a value. (Currently the iterators for standard library collection types like `List` return pointers.) You can construct a `Pointer` to an existing value by calling the constructor with the `to` keyword argument: ```mojo from std.memory import Pointer ptr = Pointer(to=some_value) ``` You can also create a `Pointer` by copying an existing `Pointer`. A `Pointer` carries an [`origin`](/docs/manual/values/lifetimes/) for the stored value, so Mojo can track the lifetime of the referenced value. ## `OwnedPointer` The [`OwnedPointer`](/docs/std/memory/owned_pointer/OwnedPointer/) type is a smart pointer designed for cases where there is single ownership of the underlying data. An `OwnedPointer` points to a single item, which is passed in when you initialize the `OwnedPointer`. The `OwnedPointer` allocates memory and moves or copies the value into the reserved memory. ```mojo from std.memory import OwnedPointer o_ptr = OwnedPointer(some_big_struct) ``` An owned pointer can hold almost any type of item, but when constructing an `OwnedPointer`, the stored item must be either `Movable` or `Copyable`. Since an `OwnedPointer` is designed to enforce single ownership, the pointer itself can be moved, but not copied. Note: Currently, you can't create an `Optional[OwnedPointer[T]]` because the `Optional` type only works with types that are both movable and copyable. This restricts some use cases that would otherwise be a natural fit for `OwnedPointer`, including self-referential data structures like linked lists and trees. (Until this use case is supported for `OwnedPointer`, it's possible to use `ArcPointer` where you need a smart pointer that can be `Optional`.) ## `ArcPointer` An [`ArcPointer`](/docs/std/memory/arc_pointer/ArcPointer/) is a reference-counted smart pointer, ideal for shared resources where the last owner for a given value may not be clear. Like an `OwnedPointer`, it points to a single value, and it allocates memory when you initialize the `ArcPointer` with a value: ```mojo from std.memory import ArcPointer attributesDict: Dict[String, String] = {} attributes = ArcPointer(attributesDict) ``` Unlike an `OwnedPointer`, an `ArcPointer` can be freely copied. All instances of a given `ArcPointer` share a reference count, which is incremented whenever the `ArcPointer` is copied and decremented whenever an instance is destroyed. When the reference count reaches zero, the stored value is destroyed and the allocated memory is freed. You can use `ArcPointer` to implement safe reference-semantic types. For example, in the following code snippet `SharedDict` uses an `ArcPointer` to store a dictionary. Copying an instance of `SharedDict` only copies the `ArcPointer`, not the dictionary, which is shared between all of the copies. ```mojo from std.memory import ArcPointer struct SharedDict(Copyable): var attributes: ArcPointer[Dict[String, String]] def __init__(out self): attributesDict: Dict[String, String] = {} self.attributes = ArcPointer(attributesDict) def __init__(out self, *, copy: Self): self.attributes = copy.attributes def __setitem__(mut self, key: String, value: String): self.attributes[][key] = value def __getitem__(self, key: String) -> String: return self.attributes[].get(key, default="") def main(): thing1 = SharedDict() thing2 = thing1 thing1["Flip"] = "Flop" print(thing2["Flip"]) ``` Note: The reference count is stored using an [`Atomic`](/docs/std/atomic/atomic/Atomic/) value to ensure that updates to the reference count are thread-safe. However, Mojo doesn't currently enforce exclusive access across thread boundaries, so it's possible to form race conditions. ## UnsafePointer [`UnsafePointer`](/docs/std/memory/unsafe_pointer/UnsafePointer/) is a low-level pointer that can access a block of contiguous memory locations, which might be uninitialized. It's analogous to a raw pointer in the C and C++ programming languages. `UnsafePointer` provides unsafe methods for initializing and destroying stored values, as well as for accessing the values once they're initialized. As the name suggests, `UnsafePointer` doesn't provide any memory safety guarantees, so you should reserve it for cases when none of the other pointer types will do the job. Here are some use cases where you might want to use an `UnsafePointer`: - Building a high-performance array-like structure, such as `List` or `Tensor`. A single `UnsafePointer` can access many values, and gives you a lot of control over how you allocate, use, and deallocate memory. Being able to access uninitialized memory means that you can preallocate a block of memory, and initialize values incrementally as they are added to the collection. - Interacting with external libraries including C++ and Python. You can use`UnsafePointer` to pass a buffer full of data to or from an external library. For more information, see [Unsafe pointers](/docs/manual/pointers/unsafe-pointers).