Read file fail when call C++ function in Swift code

Question

guys. I am writing an iOS app in swift, and I need to call some C++ lib. So I've build a simple example on how to bridge between C++ and Swift, and test on an iTouch. I wrapped the C++ interface with extern C. But I can't read the file when I call C++ function. Here is the code.

When I click the button on the iOS device, it needs to call the myFun():

main.swift

    @IBAction func button(sender: AnyObject) {
        myFun()
    }

myFun() is my C++ function, which just reads a local file("hi.c").

DlibFun.cpp

#include <stdlib.h>
#include <stdio.h>
#include <string>
#include "DlibFun.h"
#include <unistd.h>

void myFun(){
    char* path = (char*)"/hi.c";
    FILE* f = fopen(path, "r");
    if(f != NULL){
        printf("open it\n");
        fclose (f);
    }else{
        printf("FAIL\n");
    }
}

Wrapper the C++ code in C

DlibFun.h

#ifdef __cplusplus
extern "C" {
#endif

    int myFun();

#ifdef __cplusplus
}
#endif

photo-Bridging-Header.h

#include "DlibFun.h"

The result is that every time it prints out "FAIL". And any one give me any hint? I have tried the different path, but none of them are correct. Is it possible that my path is wrong? or there is any thicky thing that I don't know?

File folder

Answer 1

As you said, the code in the question is a simple example. I don't think the problem you are asking about, namely the fact that "FAIL" is output, is related to the real difficulties of bridging between C++ and Swift. The C++ function is called correctly, but the file can't be opened, most likely because it isn't there or isn't readable. In fact, I reproduced your example in Xcode, and got the output "open it" as long as the file was available; otherwise it would be "FAIL," as in your case.

Because DlibFun.cpp includes DlibFun.h, where myFun() is declared extern "C", the C++ compiler will compile myFun() to have C linkage, meaning it can be called from C and Swift. When Swift sees myFun() through the bridging header, it just treats it as a C function and calls it as such.

In a real-world situation, myFun() would be implemented in some C++ library and compiled using a C++ compiler, giving it C++ linkage, so just creating a header in Xcode, declaring myFun() extern "C", and then including the header in the bridge won't help. The build will fail with a link error.

To call the C++ library function myFun() you can write a wrapper as follows:

///////////////////
// File DlibFunW.h:

#ifndef DlibFunW_h
#define DlibFunW_h

// Because this is a C file for use by Swift code, via
// the bridge header, we don't need #ifdef __cplusplus.
// And because myFunW() was marked extern "C" in our C++
// wrapper, it's just a C function callable from Swift.
void myFunW();

#endif /* DlibFunW_h */

////////////////////
// File DlibFun.cpp:

#include "DlibFun.h"

// This file is C++ because it calls myFun(), which is
// a function with C++ linkage.

// This code is visible only to the C++ compiler, so 
// we don't need #ifdef __cplusplus
extern "C" void myFunW() { myFun(); }

Now we don't need extern "C" in DlibFun.h, since myFun() has C++ linkage, as a real-world C++ library function would. The bridging header is now just

#include "DlibFunW.h"

and Swift calls myFunW() instead of myFun() .

Of course, this is a very simple example dealing only with the C vs. C++ linkage problem. A real-world C++ function would take parameters and return values, often of pointer, struct, and class types, and dealing with those is a completely different can of worms. Here on StackOverflow you'll find plenty of info on that. Some questions I'd recommend:

Swift converts C's uint64_t different than it uses its own UInt64 type

How do I get a specific bit from an Integer in Swift?

Converting inout values to UnsafeMutablePointer<Unmanaged<TYPE>?>

Is it possible to convert a Swift class into C void* pointer?

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

Hope you find useful info there, all the best!