从C ++在Mac OSX上启动一个打开的对话框

Question

I have a header file that contains the code

#ifndef PLATFORM_DEPENDENCE
#define PLATFORM_DEPENDENCE

#define USE_MAC
#include "WindowsFunctions.h"
#include "MacFunctions.h

#endif

WindowsFunctions.h:

#ifndef WINTRANSFERINCLUDE
#define WINTRANSFERINCLUDE
#ifdef USE_WINDOWS

#include <string>
#include <boost/shared_ptr.hpp>

using namespace std;

#include <windows.h>
#include <Shlobj.h>

boost::shared_ptr<wstring> browseFolder();
boost::shared_ptr<vector<wstring>> getFilesInDirRecursiveImplementation(boost::shared_ptr<vector<wstring>> dirs, boost::shared_ptr<vector<wstring>> files);

#endif
#endif

and MacFunctions.h:

#ifndef MACTRANSFERINCLUDE
#define WACTRANSFERINCLUDE
#ifdef USE_MAC

#include <string>
#include <boost/shared_ptr.hpp>

using namespace std;

boost::shared_ptr<wstring> browseFolder();
boost::shared_ptr<vector<wstring>> getFilesInDirRecursiveImplementation(boost::shared_ptr<vector<wstring>> dirs, boost::shared_ptr<vector<wstring>> files);

#endif
#endif

The WindowsFunctions.cpp is already implemented, and works. How would I do it for Mac? I have written a method that opens up an open dialog, but I have no idea how to return the data to where it was called in C++.

This is the code that opens the dialog:

#import <Cocoa/Cocoa.h>
#include <string>

std::string* path() {
    NSOpenPanel *op = [NSOpenPanel openPanel];
    if ([op runModal] == NSOKButton) {
            NSURL *nsurl = [[op URLs] objectAtIndex:0];
            // nsurl.path contains the NSString I want to return as std::string
    }
    // ???
    return something;
}

How would I do this, or am I approaching the problem in a wrong way?

Answer 1

It's a bit long-winded, but here it is:

std::string([[nsurl path] UTF8String]);

Edit: Ok, so maybe not that long-winded.

Answer 2

You have several problems to solve:

1) Pick one string type for your C++ API. Your example of the Windows version uses wstring, which uses wchar_t, which is 32-bit in size on Mac and many Unixes, but 16 bits long on Windows. Also, wstring is intended for fixed-size string encodings, even though Windows abuses it to store UTF16 (which is a variable-length encoding). Your Mac code uses string, which uses char, which is 1 byte on all platforms. You could use UTF8 on all platforms, then you can use string everywhere. It's still not fixed-length (ie some characters can be encoded in up to 4 bytes), but it's supported on many platforms and all of them can convert their strings to UTF8. You can use different string types on different platforms, but then you'll have to litter your code with #if and #endif statements to do the right thing on each platform.

2) get the path out of the object. As your comment in the example says, you do this by calling [nsurl path] first, or to be safe [[nsurl absoluteURL] path] in case the URL was a relative URL. That way you get the full path, and not just the relative part. That gives you an NSString object, which you can then ask for a C string representation. There are several methods: cString only works for ASCII strings, so don't use that. If you want UTF8, use UTF8String. If you want any other encoding (like UTF16 or UTF32), you can use dataUsingEncoding: to get an NSData object, then look at its bytes and length to create a string.

3) Be careful wherever you use file paths. The file system has a special 'canonical' way of encoding a file name. That's needed, because Unicode has several representations for the same character. Eg 'ä' or '¨a'. (a special diaeresis character that gets glued onto a normally, not the regular one I'm using here). So you need to make sure you transform it to that canonical decomposition. That's what NSString's -fileSystemRepresentation is for if you need a C string to pass into POSIX API that takes a file path (if you use Mac API that takes an NSString, it will do that for you transparently under the hood).