I have a header file that is included by *.c and by *.m sources. When a C source includes the file, I'd like to skip some lines that contain Objective C stuff. Thus, I was wondering if gcc has a builtin compiler define that is active whenever it is compiling an Objective C file, something similar to the __cplusplus define that indicates that the compiler is currently compiling a C++ source. Is there an Objective C equivalent?
Of course, I could just #define my own preprocessor symbol but I'd like to know whether gcc already has an inbuilt definition for this.
To answer my own question, gcc defines the __OBJC__ macro whenever it is compiling an Objective C source.
I just changed my .m files to .mm and use C++. Is there a way to do the same with Swift?
The confusion may come from the assumption that merely changing a file extension from .m to .mm is all you need to bridge the languages, when, in reality, it does nothing of that sort. It is not the .mm that causes friction with .cpp, it is the .h header which must positively not be a C++ header.
Same project: Yes.
In the same project, you can happily mix C, C++, Objective-C, Objective C++, Swift, and even Assembly.
...Bridging-Header.h: you expose C, Objective-C and Objective-C++ to Swift using this bridge
<ProductModuleName>-Swift.h: exposes automatically your Swift classes marked with #objc to Objective-C
.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.
Same file: No.
In the same file, you can't mix all 5. In the same source file:
.swift: you can't mix Swift with anything
.m: you can mix Objective-C with C. (#Vinzzz)
.mm: you can mix Objective-C with C++. This bridge is Objective-C++. (#Vinzzz).
.c: pure C
.cpp: you can mix C++ & Assembly (#Vality)
.h: ubiquitous and ambiguous C, C++, Objective-C or Objective-C++, so the answer is it depends.
References
Invoke Assembly from C++ (Brett Hale)
Invoke Swift from Objective-C (Svitlana)
Invoke C, C++, Obj-C, Obj-C++ from Swift (SwiftArchitect, self)
To download a full iOS 9, Xcode 7 project, search for SO-32541268 in Swift Recipes.
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.
I wrote a simple Xcode 6 project that show how to mix C++, Objective C and Swift code:
https://github.com/romitagl/shared/tree/master/C-ObjC-Swift/Performance_Console
In particular the example call an Objective C and a C++ function from the Swift.
The key is to create a shared header Project-Bridging-Header.h and put the Objective C headers there.
Please download the project as a complete example.
You can also skip the Objective-C file in between. Just add a C header file with a .cpp source file. Have only C declarations in the header file and include any C++ code in the source file. Then include the C header file in the **-Bridging-Header.h.
The following example returns a pointer to a C++ object (struct Foo) so Swift can store in a COpaquePointer instead of having struct Foo defined in the global space.
Foo.h file (seen by Swift - included in the bridging file)
#ifndef FOO_H
#define FOO_H
// Strictly C code here.
// 'struct Foo' is opaque (the compiler has no info about it except that
// it's a struct we store addresses (pointers) to it.
struct Foo* foo_create();
void foo_destroy(struct Foo* foo);
#endif
Inside source file Foo.cpp (not seen by Swift):
extern "C"
{
#include "Foo.h"
}
#include <vector>
using namespace std;
// C++ code is fine here. Can add methods, constructors, destructors, C++ data members, etc.
struct Foo
{
vector<int> data;
};
struct Foo* foo_create()
{
return new Foo;
}
void foo_destroy(struct Foo* foo)
{
delete foo;
}
I have just made a little example project using Swift, Objective-C and C++. It's a demo of how to use OpenCV stitching in iOS. The OpenCV API is C++ so we can't talk to it directly from Swift. I use a small wrapper class who's implementation file is Objective-C++. The Header file is clean Objective-C, so Swift can talk to this directly. You have to take care not to indirectly import any C++-ish files into the the headers that Swift interacts with.
The project is here: https://github.com/foundry/OpenCVSwiftStitch
Here's my attempt at a clang tool to automate C++/swift communication. You can instanciate C++ classes from swift, inherit from C++ class and even override virtual methods in swift.
It will parse the C++ class you want to export to swift and generate the Objective-C/Objective-C++ bridge automatically.
https://github.com/sandym/swiftpp
Swift is not directly compatible with C++. You can work around the issue by wrapping your C++ code with Objective-C, and using the Objective C wrapper in Swift.
I also have a demo program for swift combining opencv.
You can download it from https://github.com/russj/swift_opencv3_demo.
More information about the demo http://flopalm.com/opencv-with-swift/.
No, not in a single file.
However, you may use C++ in Swift Projects without needing a static library or framework. Like others have said, the key is to make an Objective-C bridging header that #includes C-compatible C++ headers that are marked as C compatible with the extern "C" {} trick.
Video tutorial: https://www.youtube.com/watch?v=0x6JbiphNS4
Other answers are slightly inaccurate. You can actually mix both Swift and [Objective-]C[++] in the same file, though not quite the way you would expect.
This file (c.swift) compiles to a valid executable with both swiftc c.swift and clang -x objective-c c.swift
/* /* */
#if 0
// */
import Foundation
print("Hello from Swift!")
/* /* */
#endif
#include <stdio.h>
int main()
{
puts("Hello from C!");
return 0;
}
// */
One trick (of many) is that
You need a separate header for your bridging obj-c++ file...
You can't just throw #interface and #implementation in the same .mm file as one often does normally.
So in your bridging header file you have
#import "Linkage.hpp"
Linkage.hpp has the #interface for Linkage and Linkage.mm has the #implementation for .mm
And then
...you actually don't #include "yourCpp.hpp" in Linkage.hpp.
You only put #include "yourCpp.hpp" in the Linkage.mm file, not in the Linkage.hpp file.
In many online examples/tutorials, the writer simply puts the #interface and #implementation in the same .mm file, as one often does.
That will work in very simple cpp bridging examples, but,
The problem is:
if your yourCpp.hpp has any c++ features at all which it surely will (like, the first line #include <something>), then the process will fail.
But if you just don't have the #include "yourCpp.hpp" in the Linkage header file (it's fine to have it in the .mm file, obviously you'll need to) - it works.
Again this is unfortunately just one tip in the whole process.
In case this is helpful to anyone, I also have a brief tutorial on calling a simple C++ static library from a trivial Swift command line utility. This is a really bare-bones proof of concept piece of code.
No Objective-C involved, just Swift and C++. Code in a C++ library is called by a C++ wrapper that implements a function with extern "C" linkage. That function is then referenced in the bridging header and called from Swift.
See http://www.swiftprogrammer.info/swift_call_cpp.html
I am providing a link to SE-0038 in the official resource, described as
This maintains proposals for changes and user-visible enhancements to the Swift Programming Language.
The status as of today is that this is the feature request which has been accepted but not yet scheduled.
This link is intended to steer anyone looking for this feature in the right direction
For instance in this big project I'm working on there is a "global" header file with #Defines to URLs. I cant use those in swift, but for clarity I want to have the swift alternative in the same .h file so if one is ever changed it should be obvious to change the other.
I was hoping with some compiler magic, this might be possible.
Don't use #define -- instead use constants.
Constants in Objective-C
You can access Objective-C constants from Swift (using a bridging header)
You cannot. You can use them in the same project though.
Using Swift with Cocoa and Objective-C
I have the source code for a video decoder which is written in C.
The code was successfully compiled and executed on MAC terminal (which uses GCC compiler). Now I'm trying to create an application on Xcode with the same source code. Only the GUI for the application is written in Objective-C.
When I tried to execute on Xcode (which uses LLVM compiler), I'm getting a lot of errors in the C code.
The only other compiler option that Xcode provides is LLVM GCC 4.2. I compiled using that and found that it is not able to recognize the code written in Objective-C.
Is there any compiler that can be used for both Objective-C and C?
Can GCC 4.2 compiler be used for compiling Objective-C code?
How to tell Xcode to compile using GCC 4.2 compiler?
Kindly help. Thanks in advance!
Both LLVM and GCC can be used to compile C, C++ and Objective-C code.
My guess is that some compiler flags are set incorrectly.
Objective-C is a strict superset of ANSI C, so any compiler that can compile Objective-C code can also compile (ANSI) C code. If your library does not use standard C, then you might run into problems.
The same is true for Objective-C++, which is a strict superset of C++.
If, like in your case, you have a conflicting typedef in your header file, you can use conditional compilation to hide the typedef when compiling Objective-C code:
#ifndef __OBJC__
typedef unsigned char BOOL;
#endif
Is it possible to generate a C source file (.c) from an Objective-C source file (.m), using, maybe, GCC or Clang (or another tool)?
No, but you may be interested in the C++ rewriter. Given an objective-C file, let's say:
#import <stdio.h>
int main() {
void (^blk)(void) = ^{
printf("hi");
};
blk();
return 0;
}
You can get a C++ version with
clang -rewrite-objc main.c
which is great to see how objective-C is implemented. Most of the file will be just C.
Some of the resulting expressions are dense. You'll need patience, C knowledge, and the Objective-C Runtime Reference. http://cdecl.org/ helps with small pieces.