In Swift, a protocol composition such as Readable & Loggable means that a type must conform to both protocols. Use & to combine requirements inline in a type annotation or generic constraint; use a named protocol when the combined contract should have a reusable name. The composition sets the requirements—it does not create a new type or make a type conform.
What does protocol composition mean in Swift?
P & Q describes a type that conforms to every protocol in the composition. For example, a value constrained to Encodable & CustomStringConvertible must satisfy both protocols. This is useful when one position in your code needs multiple capabilities but a separate protocol declaration would add no useful name or API contract.
The Swift language reference describes protocol composition types in Types. They express requirements for the underlying type; they do not declare a nominal type or add conformance.
Where can you use `&`?
Swift permits protocol compositions in type annotations, generic parameter clauses, and generic where clauses. A generic constraint is a common use:
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func log<T: Encodable & CustomStringConvertible>(_ value: T) {
print(value.description)
}Here, T is one concrete type chosen for a call, and it must conform to both Encodable and CustomStringConvertible. The function can use the requirements of both protocols. The Generics guide covers generic constraints and where clauses; the language reference also documents composition types in Types.
A composition can also appear in a type annotation when a value is intended to meet several protocol requirements. Whether that annotation uses a boxed existential or another form depends on the surrounding syntax, including whether it uses any or some.
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What can a protocol composition contain?
- Multiple protocols: every listed protocol is a requirement.
- At most one class type: a composition may combine one class type with protocols, but it cannot specify more than one class.
- Type aliases: an alias can stand for a protocol, class, or protocol composition. If aliases cause the same protocol to occur more than once, duplicate protocols are ignored.
These constraints are described in the Swift Types reference. In practice, aliases can make a composition easier to reuse, but they do not change its underlying requirements.
When should you use a composition instead of a combined protocol?
A protocol declaration can inherit from multiple protocols, aggregating their requirements under a new name. Swift describes protocol inheritance in Declarations.
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| Approach | Use it when | What it expresses |
|---|---|---|
P & Q |
A particular annotation or constraint needs both requirements, and a new name is unnecessary. | An inline requirement that the type satisfy each listed protocol. |
A protocol inheriting from P and Q |
The combined contract should have a reusable name or belong in a protocol hierarchy. | A named protocol whose conforming types inherit the requirements of both parent protocols. |
For example, a library might define a named protocol for a capability used throughout its API. For a one-off generic constraint, writing both protocol names at the use site can be clearer and avoids adding a declaration solely to bundle them.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.How do `any` and `some` relate to `P & Q`?
The composition says which constraints apply; any and some describe how a type is presented at an interface boundary. The distinction matters because a composition does not by itself tell you whether a value’s concrete type can vary.
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| Form | What it means for P & Q |
Type identity |
|---|---|---|
any P & Q |
A boxed protocol value may hold a value of any concrete type satisfying both protocols. | The held concrete type can vary at runtime. |
some P & Q |
An opaque type hides the concrete type while preserving its identity. | A function returning an opaque type must return one underlying concrete type. |
T: P & Q |
A generic type parameter is constrained to conform to both protocols. | The generic operation works with a concrete type chosen for that use; the constraint syntax is not itself a boxed value. |
The Swift guide to Opaque and Boxed Protocol Types explains these forms. Boxing can involve indirection and runtime cost, so that potential cost belongs to boxed protocol values, not to writing P & Q as a generic constraint.
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Common mistakes to avoid
- Reading
&as “either”: the type must satisfy every protocol in the composition, not just one. - Assuming the composition creates conformance: a concrete type must already meet the requirements or declare conformances to the protocols.
- Adding multiple class types: a composition can contain no more than one class type.
- Equating a composition with an existential:
P & Qnames the combined constraints;anyandsomedetermine how a value or interface presents its type. - Attributing boxing cost to all compositions: generic constraints using
&are distinct from boxedanyvalues.
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