What's New

Compile on 32-bit Systems


This release fixes compilation on 32-bit systems.

The upper and lower bounds for valid integer coding keys where given as string literals, and required 64-bit wide integers. The bounds now adapt to the system the library compiles on.


Tests Docs

This package provides convenient encoding and decoding to/from binary data for all Swift Codable types. It also provides limited cross-compatibility to Google Protocol Buffers.

Use cases

There are only few encoders and decoders available for Swift's Codable format, and Apple provides a JSONEncoder and a PropertyListEncoder for basic encoding. While these can cover some use cases (especially when interacting with Web Content through JSON), they lack encoding efficiency when designing APIs within an ecosystem. JSON, for example, is notoriously inefficient when it comes to binary data.

One very popular alternative for binary data are Google's Protocol Buffers, which offer broad support across different platforms and programming languages. But they don't support Swift's Codable protocol, and thus require manual message definitions, the Protobuf compiler, and a lot of copying between data structures during encoding and decoding.

So if you're looking for a decently efficient binary encoder in a pure Swift project, then BinaryCodable may be right for you. Simply make your structs (or classes!) conform to Codable, and BinaryCodable does the rest!

The message format is similar to that of Protocol Buffers (with some additions to support more types). It is possible to create limited compatibility between the two formats to exchange data with systems that don't support Swift.


Swift Package Manager

Simply include in your Package.swift:

dependencies: [
        name: "BinaryCodable", 
        url: "https://github.com/christophhagen/BinaryCodable", 
        from: "1.0.0")
targets: [
    .target(name: "MyTarget", dependencies: [
        .product(name: "BinaryCodable", package: "BinaryCodable")

Xcode project

Select your Project, navigate to the Package Dependencies tab, and add https://github.com/christophhagen/BinaryCodable using the + button.


Let's assume a message definition:

struct Message: Codable {

    var sender: String
    var isRead: Bool
    var unreadCount: Int

Simply import the module where you need to encode or decode a message:

import BinaryCodable


Construct an encoder when converting instances to binary data, and feed the message(s) into it:

let message: Message = ...

let encoder = BinaryEncoder()
let data = try encoder.encode(message)

It's also possible to encode single values, arrays, optionals, sets, enums, dictionaries, and more, so long as they conform to Codable.


Decoding instances from binary data works much the same way:

let decoder = BinaryDecoder()
let message = try decoder.decode(Message.self, from: data)

Alternatively, the type can be inferred:

let message: Message = try decoder.decode(from: data)


It is possible for both encoding and decoding to fail. All possible errors occuring during encoding produce BinaryEncodingError errors, while unsuccessful decoding produces BinaryDecodingErrors. Both are enums with several cases describing the nature of the error. See the documentation of the types to learn more about the different error conditions.

Coding Keys

The Codable protocol uses CodingKey definitions to identify properties of instances. By default, coding keys are generated using the string values of the property names.

Similar to JSON encoding, BinaryCodable can embed the property names in the encoded data.

Unlike JSON (which is human-readable), the binary representation produced by BinaryCodable is intended for cases when efficient encoding is important. Codable allows the use of integer keys for each property, which significantly increases encoding efficiency. You can specify integer keys by adding an Int enum conforming to the CodingKey protocol to the Codable type:

struct Message: Codable {

    var sender: String
    var isRead: Bool
    var unreadCount: Int
    // Assign an integer to each property
    enum CodingKeys: Int, CodingKey {
        case sender = 1
        case isRead = 2
        case unreadCount = 3

The enum must have a raw value of either Int or String, and the cases must match the property names within the type (it is possible to omit keys for properties which should not be encoded).

Using integer keys can significantly decrease the binary size, especially for long property names. Additionally, integer keys can be useful when intending to store the binary data persistently. Changes to property names can be performed in the code without breaking the decoding of older data (although this can also be achieved with custom String keys).


  • Small, positive integer keys produce the smallest binary sizes.
  • The 0 integer key shouldn't be used, since it is also used internally when encoding super.
  • Negative values for integer keys are not recommended (but possible). Since the keys are encoded as Varint, they are very inefficient for negative numbers.
  • The allowed range for integer keys is from -576460752303423488 (-2^59, inclusive) to 576460752303423487 (2^59-1, inclusive). Values outside of these bounds will cause a fatalError crash.

Property wrappers

Fixed size integers

While varints are efficient for small numbers, their encoding introduces a storage and computation penalty when the integers are often large, e.g. for random numbers. BinaryCodable provides the FixedSize wrapper, which forces integers to be encoded using their little-endian binary representations. This means that e.g. an Int32 is always encoded as 4 byte (instead of 1-5 bytes using Varint encoding). This makes 32-bit FixedSize types more efficient than Varint if values are often larger than 2^28 (2^56 for 64-bit types).

Use the property wrapper within a Codable definition to enforce fixed-width encoding for a property:

struct MyStruct: Codable {

    /// Always encoded as 4 bytes
    var largeInteger: Int32

The FixedSize wrapper is available to all Varint types: Int, UInt, Int32, UInt32, Int64, and UInt64.

Other property wrappers

There is an additional SignedValue wrapper, which is only useful when encoding in protobuf-compatible format.


The BinaryEncoder provides the sortKeysDuringEncoding option, which forces fields in "keyed" containers, such as struct properties (and some dictionaries), to be sorted in the binary data. This sorting is done by using either the integer keys (if defined), or the property names. Dictionaries with Int or String keys are also sorted.

Sorting the binary data does not influence decoding, but introduces a computation penalty during encoding. It should therefore only be used if the binary data must be consistent across multiple invocations.

Note: The sortKeysDuringEncoding option does not guarantee deterministic binary data, and should be used with care.

Protocol Buffer compatibility

Achieving Protocol Buffer compatibility is described in ProtobufSupport.md.

Binary format

To learn more about the encoding format, see BinaryFormat.md.


The library comes with an extensive test suite, which checks that encoding works correctly for many cases. These tests can be executed using swift test from the package root, or when opening the package using Xcode.


MIT. See License.md


Generate protobuf definitions

It should be possible to generate a string containing a working Protobuf definition for any type that is determined to be Protobuf compatible.


Increasing the speed of the encoding and decoding process is not a huge priority at the moment. If you have any pointers on how to improve the performance further, feel free to contribute.


Users of the library are encouraged to contribute to this repository.

Feature suggestions

Please file an issue with a description of the feature you're missing. Check other open and closed issues for similar suggestions and comment on them before creating a new issue.

Bug reporting

File an issue with a clear description of the problem. Please include message definitions and other data where possible so that the error can be reproduced.


If you would like to extend the documentation of this library, or translate the documentation into other languages, please also open an issue, and I'll contact you for further discussions.


  • Swift Tools 5.6.0
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Last updated: Thu Sep 28 2023 22:01:04 GMT-0900 (Hawaii-Aleutian Daylight Time)