Boost C++ Libraries of the most highly regarded and expertly designed C++ library projects in the world. Herb Sutter and Andrei Alexandrescu, C++ Coding Standards


The Interface

Since the purpose of optional is to allow us to use objects with a formal uninitialized additional state, the interface could try to follow the interface of the underlying T type as much as possible. In order to choose the proper degree of adoption of the native T interface, the following must be noted: Even if all the operations supported by an instance of type T are defined for the entire range of values for such a type, an optional<T> extends such a set of values with a new value for which most (otherwise valid) operations are not defined in terms of T.

Furthermore, since optional<T> itself is merely a T wrapper (modeling a T supertype), any attempt to define such operations upon uninitialized optionals will be totally artificial w.r.t. T.

This library chooses an interface which follows from T's interface only for those operations which are well defined (w.r.t the type T) even if any of the operands are uninitialized. These operations include: construction, copy-construction, assignment, swap and relational operations.

For the value access operations, which are undefined (w.r.t the type T) when the operand is uninitialized, a different interface is chosen (which will be explained next).

Also, the presence of the possibly uninitialized state requires additional operations not provided by T itself which are supported by a special interface.

Lexically-hinted Value Access in the presence of possibly uninitialized optional objects: The operators * and ->

A relevant feature of a pointer is that it can have a null pointer value. This is a special value which is used to indicate that the pointer is not referring to any object at all. In other words, null pointer values convey the notion of nonexistent objects.

This meaning of the null pointer value allowed pointers to became a de facto standard for handling optional objects because all you have to do to refer to a value which you don't really have is to use a null pointer value of the appropriate type. Pointers have been used for decades—from the days of C APIs to modern C++ libraries—to refer to optional (that is, possibly nonexistent) objects; particularly as optional arguments to a function, but also quite often as optional data members.

The possible presence of a null pointer value makes the operations that access the pointee's value possibly undefined, therefore, expressions which use dereference and access operators, such as: ( *p = 2 ) and ( p->foo() ), implicitly convey the notion of optionality, and this information is tied to the syntax of the expressions. That is, the presence of operators * and -> tell by themselves —without any additional context— that the expression will be undefined unless the implied pointee actually exist.

Such a de facto idiom for referring to optional objects can be formalized in the form of a concept: the OptionalPointee concept. This concept captures the syntactic usage of operators *, -> and contextual conversion to bool to convey the notion of optionality.

However, pointers are good to refer to optional objects, but not particularly good to handle the optional objects in all other respects, such as initializing or moving/copying them. The problem resides in the shallow-copy of pointer semantics: if you need to effectively move or copy the object, pointers alone are not enough. The problem is that copies of pointers do not imply copies of pointees. For example, as was discussed in the motivation, pointers alone cannot be used to return optional objects from a function because the object must move outside from the function and into the caller's context.

A solution to the shallow-copy problem that is often used is to resort to dynamic allocation and use a smart pointer to automatically handle the details of this. For example, if a function is to optionally return an object X, it can use shared_ptr<X> as the return value. However, this requires dynamic allocation of X. If X is a built-in or small POD, this technique is very poor in terms of required resources. Optional objects are essentially values so it is very convenient to be able to use automatic storage and deep-copy semantics to manipulate optional values just as we do with ordinary values. Pointers do not have this semantics, so are inappropriate for the initialization and transport of optional values, yet are quite convenient for handling the access to the possible undefined value because of the idiomatic aid present in the OptionalPointee concept incarnated by pointers.

Optional<T> as a model of OptionalPointee

For value access operations optional<> uses operators * and -> to lexically warn about the possibly uninitialized state appealing to the familiar pointer semantics w.r.t. to null pointers.

[Caution] Caution

However, it is particularly important to note that optional<> objects are not pointers. optional<> is not, and does not model, a pointer.

For instance, optional<> does not have shallow-copy so does not alias: two different optionals never refer to the same value unless T itself is a reference (but may have equivalent values). The difference between an optional<T> and a pointer must be kept in mind, particularly because the semantics of relational operators are different: since optional<T> is a value-wrapper, relational operators are deep: they compare optional values; but relational operators for pointers are shallow: they do not compare pointee values. As a result, you might be able to replace optional<T> by T* on some situations but not always. Specifically, on generic code written for both, you cannot use relational operators directly, and must use the template functions equal_pointees() and less_pointees() instead.