Loading a file into a C++ object from an iOS App / issues with <iostream> on iOS - c++

Question

I'm working on integrating a C++ library (the GRT, a machine learning toolkit, to be specific) inside of an iOS app.
I've built the GRT as a framework, including using some Objective-C++ wrapper functions to call between my app and the framework.
At the moment, I'm trying to troubleshoot something involving file loading. Specifically, I'm trying to load a file from my app bundle into a GRT module.
Here's where I get the file I want access to, and initialize the GRT wrapper:
func loadTrainingData(){
let documentsUrl = FileManager.default.urls(for: .documentDirectory, in: .userDomainMask)[0]
let fileUrl = documentsUrl.appendingPathComponent("train.grt")
let pipeline = GestureRecognitionPipeline()
let test:Bool = pipeline.load(fileUrl.path)
print(test)
}
And here's the Obj-C++ wrapper code that's called when pipeline.load is called:
- (BOOL)load:(NSString *) path
{
BOOL result = self.instance->load(std::string(path.UTF8String));
if (result) {
std::cout << "GRT config";
std::cout << self.instance->getModelAsString();
std::cout << "GRT info: " << self.instance->getInfo();
}
return result;
}
Finally, here's the actual C++ code that's part of the GRT library, where file loading is handled:
bool GestureRecognitionPipeline::load(const std::string &filename){
std::fstream file;
//Clear any previous setup
clear();
file.open(filename.c_str(), std::iostream::in );
if( !file.is_open() ){
errorLog << __GRT_LOG__ << " Failed to open file with filename: " << filename << std::endl;
return false;
}
...
}
Currently, I'm always failing to have my pipeline object successfully import a file. I don't think it's necessarily something to do with the way I'm accessing the file on the iOS side (though, I could be wrong). Does anyone have any insight? Any appreciated - thanks!
EDIT: I was able to verify that I am loading my file is being loaded properly by this check:
let path = Bundle.main.path(forResource: "acc-orientation", ofType: "grt")
print(path as Any!)
But, I'm still getting the same issues as before.
EDIT 2 I verified that the path is being loaded correctly in the the Obj-C++ wrapper too; which leads me to think it may be something related to the way that is handled in iOS....totally lost here now...
EDIT 3 As suggested by a colleague, I tried using the absoluteString of the file url to pass to my wrapper and the underlying C++ code, since the C++ doesn't have access to the sandboxed environment of iOS. Still the same result:
let documentsUrl = FileManager.default.urls(for: .documentDirectory, in: .userDomainMask)[0]
let fileUrl = documentsUrl.appendingPathComponent("acc-orientation.grt")
let pipeline = GestureRecognitionPipeline()
let test:Bool = pipeline.load(fileUrl.absoluteString)
EDIT 4 As suggested in the comments, I tried using fileSystemRepresentation, but this also didn't bring success.
- (BOOL)load:(NSURL *) url {
BOOL result = self.instance->load(std::string([url fileSystemRepresentation]));
...
}
EDIT 5: I made a simple test project that attempts to only access the file and load it using Swift->Obj-C++->C++ (no framework files, in other words). Here's a link where it can be downloaded. Any help appreciated!

Answer 1

Well, you are almost there. I have downloaded your sample project and got it working. Your problem has to do with the actual location of the file you want to open. At the moment you are trying to open the file from the Documents folder but you never actually copy the file from the App Bundle to the Documents folder. So there are two solutions:
Solution 1: App Bundle
Alter the loadTrainingData method in ViewController.swift to access the file from the App Bundle:
func loadTrainingData(){
let path = Bundle.main.url(forResource: "acc-orientation", withExtension: "grt")
let wrapper = Wrapper()
let test:Bool = wrapper.load(path)
print(test)
}
Solution 2: Documents folder
Copy the file from your App Bundle to your Documents folder right after the first launch. Therefore, copy the following code snippet to your AppDelegate.swift:
func application(_ application: UIApplication, didFinishLaunchingWithOptions launchOptions: [UIApplicationLaunchOptionsKey: Any]?) -> Bool {
do {
let url = FileManager.default.urls(for: .documentDirectory, in: .userDomainMask).first!.appendingPathComponent("acc-orientation.grt")
let bundleURL = Bundle.main.url(forResource: "acc-orientation", withExtension: "grt")
try FileManager.default.copyItem(at: bundleURL!, to: url)
} catch {
print("File already exists")
}
return true
}
With either of these solutions your FileLoader::load method will return true.
Hope that helps.