How to Mix Swift With C++? Like the Objective-C .Mm Files

Can I mix Swift with C++? Like the Objective-C .mm files

No. When you switch from .m to .mm you are actually switching from Objective-C to a different language (which has many subtle differences) called Objective-C++. So you're not really using C++; you're using Objective-C++ which accepts most C++ as input (in the same way that C++ accepts most but not all C as input). When I say it's not quite C++, consider a C++ file that includes a variable named nil (which is legal C++) and then try to compile that as Objective-C++.

Swift doesn't have the same relationship. It is not a superset of C or C++, and you can't directly use either in a .swift file.

"Using Swift with Cocoa and Objective-C" also tells us:

You cannot import C++ code directly into Swift. Instead, create an Objective-C or C wrapper for C++ code.

Invoking swift in objective-C++ (.mm file)

I just succeeded in compiling a .mm file invoking swift by adding -fcxx-modules flag. The <Module>-Swift.h file generated by Xcode includes @import and checks for modules support. Found out the flag from this answer: https://stackoverflow.com/a/20708695/2570853.

Can I have Swift, Objective-C, C and C++ files in the same Xcode project?

YES.

You can mix Swift, C, C++, Objective-C & Objective-C++ files in the same Xcode project.

C

// Declaration: C.h
#ifndef C_h
#define C_h
#ifdef __cplusplus
extern "C" {
#endif
    void hello_c(const char * name);
#ifdef __cplusplus
}
#endif
#endif /* C_h */

// Definition: C.c
#include "C.h"
#include <stdio.h>
void hello_c(const char * name) {
    printf("Hello %s in C\n", name);
}

C++

// Declaration: CPP.hpp
#pragma once
#include <string>
class CPP {
public:
    void hello_cpp(const std::string& name);
};

// Definition: CPP.cpp
#include "CPP.hpp"
#include <iostream>
using namespace std;
void CPP::hello_cpp(const std::string& name) {
    cout << "Hello " << name << " in C++" << endl;
}

Objective-C wrapper for C++

// Declaration: CPP-Wrapper.h
#import <Foundation/Foundation.h>
@interface CPP_Wrapper : NSObject
- (void)hello_cpp_wrapped:(NSString *)name;
@end

// Definition: CPP-Wrapper.mm
#import "CPP-Wrapper.h"
#include "CPP.hpp"
@implementation CPP_Wrapper
- (void)hello_cpp_wrapped:(NSString *)name {
    CPP cpp;
    cpp.hello_cpp([name cStringUsingEncoding:NSUTF8StringEncoding]);
}
@end

Objective-C

// Declaration: Objective-C.h
#import <Foundation/Foundation.h>
@interface Objective_C : NSObject
- (void)hello_objectiveC:(NSString *)name;
@end

// Definition: Objective-C.m
#import "Objective-C.h"
@implementation Objective_C
- (void)hello_objectiveC:(NSString*)name {
    printf("Hello %s in Objective-C\n", [name cStringUsingEncoding:NSUTF8StringEncoding]);
}
@end

Objective-C++

// Declaration: Objective-CPP.h
#import <Foundation/Foundation.h>
@interface Objective_CPP : NSObject
- (void)hello_objectiveCpp:(NSString *)name;
@end

// Definition: Objective-CPP.mm
#include <iostream>
#import "Objective-CPP.h"
using namespace std;
@implementation Objective_CPP
- (void)hello_objectiveCpp:(NSString *)name {
    cout << "Hello " << [name cStringUsingEncoding:NSUTF8StringEncoding] << " in Objective-C++\n";
}
@end

Swift

// Declaration & definition: Swift.swift
func hello_swift(_ name: String) {
    print("Hello \(name) in Swift")
}

Bridging-Header.h

Cannot import CPP.hpp header file, not because of it's naming convention, but because it contains the class keyword.

#import "C.h"
#import "CPP-Wrapper.h"
#import "Objective-C.h"
#import "Objective-CPP.h"

Invocation from Swift

// Invoke C
hello_c("World".cStringUsingEncoding(NSUTF8StringEncoding))

// Can't Invoke C++ without a wrapper
// CPP().hello_cpp("World".cStringUsingEncoding(NSUTF8StringEncoding))
// Invoke C++ through Objective-C
CPP_Wrapper().hello_cpp_wrapped("World")

// Invoke Objective-C
Objective_C().hello_objectiveC("World")

// Invoke Objective-C++
Objective_CPP().hello_objectiveCpp("World")

// Invoke Swift
Swift().hello_swift("World")

.h (Headers)

(See item 3 in this Stack Overflow answer)

.h: this is the tricky part, since they are ambiguously used for all flavors of C, ++ or not, Objective or not. When a .h does not contain a single C++ keyword, like class, it can be added to the ...Bridging-Header.h, and will expose whatever function the corresponding .c or .cpp functionalities it declares. Otherwise, that header must be wrapped in either a pure C or Objective-C API.

Output

Hello World in C
Hello World in C++
Hello World in Objective-C
Hello World in Objective-C++
Hello World in Swift

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Comments

Cy-4AH:

Yes. You only need wrap C++ into C or Objective-C to use in Swift.

Tommy

Indeed, I have a project that does exactly that. C++ for the thrust of the abstract cross-platform model stuff with some C parts underneath; Objective-C to wrap the C++ classes for Swift purposes, Swift to bind all that to a subclass of NSDocument, with some custom views that interrogate the C stuff.

MaddTheSane

Added the extern "C" wrapper as per your excellent suggestion. To invoke the C method void hello_c(const char * name) from C++ method hello_cpp(const std::string& name), add #include "C.h" and call hello_c(name.c_str());.

Keith Adler

The new SO-32541268: Now with parameters!

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► Find this solution on GitHub and additional details on Swift Recipes.

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