swift-mirror/docs/CppInteroperability/CppInteroperabilityStatus.md

#  C++ Interoperability Status

Swift has some experimental ability to interoperate with C++.
This document provides an overview of the status of the Swift and C++ interoperability support.

## C++ to Swift Interoperability Status

Swift has the experimental ability to import a large subset of C++.
This section of the document describes which C++ language and standard library features can be imported and used from Swift in an experimental manner.

### Example
The following example demonstrates several interop features. It compiles and runs on main.

```C++
// cxx-types.h (mapped to CxxTypes module in module.modulemap)
#include <algorithm>
#include <vector>

using V = std::vector<long>;
```

```Swift
// main.swift
import CxxTypes
import std.vector
import std.algorithm

// We can extend C++ types in Swift.
extension V : RandomAccessCollection {
  public var startIndex: Int { 0 }
  public var endIndex: Int { size() }
}

// Create a vector with some data.
var numbers = V(4)
std.fill(numbers.beginMutating(), numbers.endMutating(), 41)

// Transform it using C++.
std.transform(numbers.beginMutating(), numbers.endMutating(),
              numbers.beginMutating()) { (element: Int) in
  return element + 1
}

// Loop over it in Swift.
for (index, element) in numbers.enumerated() {
  print("v[\(index)] = \(element)")
}

// We can also use anything in RandomAccessCollection, such as map and zip.
let strings = numbers.map { "\($0)" }
for (s, n) in zip(strings, numbers) {
  print("\(s) = \(n)")
}
```

### Importing C++

There are currently two experimental ways to import C++ into Swift:
- **Clang modules**: can be imported into Swift. This requires a module map.
- **Bridging header**: can be imported into Swift. Headers included in the bridging header will be imported.
Please note that support for importing C++ 20 modules isn’t implemented.

Both CMake and the Swift package manager can be configured to invoke Swift with the correct arguments to import C++ headers.

**Note**: C++ code is imported using the Objective-C++ language mode on Apple platforms.

### Experimental C++ Language Support

This status table describes which of the following C++ language features can be used in Swift: 

| **C++ Language Feature**                    | **Implemented Experimental Support For Using It In Swift**    |
|---------------------------------------------|---------------------------------------------------------------|
| Top-level functions                         | Yes    |
| Enumerations                                | Yes. That includes `enum class`  |
| Struct / Class types                        | Yes - as value types, except for types without a copy constructor. Partial experimental support for importing a C++ struct/class as a reference type |
| Typedefs / Type aliases                     | Yes    |
| Global Variables                            | Yes    |
| Namespaces                                  | Yes    |
| Inline Namespaces                           | Yes, with some known issues ([#58217](https://github.com/apple/swift/issues/58217)) |
| Exceptions                                  | No  |
| Fields                                      | Yes |
| Member functions                            | Yes. Some value category overloads aren't imported |
| Virtual Member Functions                    | No |
| Operators                                   | Yes, with some known issues    |
| Subscript Operators                         | Yes |
| Constructors                                | Yes. That includes implicit constructors    |
| Destructor                                  | Yes. C++ destructors are invoked automatically when the value is no longer used in Swift |
| Copy constructor / copy assignment operator | Yes. Swift invokes the underlying copy constructor when copying a C++ value |
| Move constructor / move assignment operator | No    |
| Base class member functions / operators     | Yes, with some known issues |
| Function templates                          | Yes    |
| Class templates                             | Yes    |
| Dependent types                             | Partially: imported as Any |
| Availability Attributes                     | Yes    |


The following C++ code patterns or language features have specific mappings to Swift language features when imported in Swift:


| **C++ Language Feature**                             | **Imported Into Swift**                                                                  |
|------------------------------------------------------|------------------------------------------------------------------------------------------|
| `get`/`set` member functions                         | Imported as computed property (starting from Swift-5.7)                                          |
| `const`/non-`const` member function overload set     | Both overloads are imported as a method, with non-`const` method being renamed to `mutating…` (starting from Swift-5.7). The renaming logic will change in a future version of Swift, and non-`const` methods won't be renamed |


Unless stated otherwise (i.e., imported reference types) all Swift features work with imported types. For example: use in generic contexts, protocol conformance, extensions, etc.


### C++ Standard Library Support

Parts of libc++ can be imported and used from Swift. C++ standard library types are bridged directly to Swift, and there is not automatic bridging to native Swift types. This means that if an imported C++ API returns `std::string`, you will get a `std::string` value in Swift as well, and not Swift's `String`.

This status table describes which of the following C++ standard library features have some experimental support for using them in Swift. Please note that this is not a comprehensive list and other libc++ APIs that use the above supported C++ language features could be imported into Swift.

| **C++ Standard Library Feature**   | **Can Be Used From Swift**                   |
|------------------------------------|----------------------------------------------|
| `std::string`                      | Yes                                          |
| `std::vector`                      | Yes                                          |

## Known Issues

### Inline Namespaces
- [#58217](https://github.com/apple/swift/issues/58217): Swift's typechecker currently doesn't allow calling a function from an inline namespace when it's referenced through the parent namespace. Example of a test that fails: https://github.com/apple/swift/blob/main/test/Interop/Cxx/namespace/inline-namespace-function-call-broken.swift


## Swift to C++ Interoperability Status

This section of the document describes which Swift language and standard library features can be imported and used from C++. 

### Importing Swift

Swift has some experimental support for generating a header that can be imported by C++.

### Swift Language Support

This status table describes which of the following Swift language features have some experimental support for using them in C++.

**Functions**

| **Swift Language Feature**     | **Implemented Experimental Support For Using It In C++** |
|--------------------------------|----------------------------------------------------------|
| Top-level `@_cdecl` functions  | Yes                                                      |
| Top-level Swift functions      | Partially, only with primitive and Swift struct and enum types. Class, protocol, and SIMD types are not supported.    |
| `inout` parameters             | Yes                                                      |
| Variadic parameters            | No                                                       |
| Multiple return values         | No                                                       |

**Structs**

| **Swift Language Feature**     | **Implemented Experimental Support For Using It In C++** |
|--------------------------------|----------------------------------------------------------|
| Fixed layout structs           | Yes                                                      |
| Resilient / opaque structs     | Yes                                                      |
| Copy and destroy semantics     | Yes                                                      |
| Initializers                   | Partially, as static `init` methods. No failable support |

**Class types**

| **Swift Language Feature**     | **Implemented Experimental Support For Using It In C++** |
|--------------------------------|----------------------------------------------------------|
| Class reference values         | Yes                                                      |
| ARC semantics                  | Yes (C++ copy constructor,assignment operator, destructor perform ARC operations)  |
| Initializers                   | No |

**Methods**

| **Swift Language Feature**     | **Implemented Experimental Support For Using It In C++** |
|--------------------------------|----------------------------------------------------------|
| Instance methods               | Yes, for structs and classes only                                    |
| Static methods                 | No                                                       |

**Properties**

| **Swift Language Feature**     | **Implemented Experimental Support For Using It In C++** |
|--------------------------------|----------------------------------------------------------|
| Getter accessors               | Yes, via `get<name>`. Boolean properties that start with `is` or `has` are remapped directly to a getter method using their original name. For structs and classes only                   |
| Setter accessors               | Yes, via `set<name>`. For structs and classes only                   |
| Mutation accessors             | No                                                       |