Correct way to initialize an object with exception throwing constructor

Question

This seems to be a trivial question but I got hung on it for a few hours now (maybe too much Java killed my C++ braincells).

I have created a class that has the following constructor (i.e. no default constructor)

VACaptureSource::VACaptureSource( std::string inputType, std::string inputLocation ) {
    if( type == "" || location == "" ) {
    throw std::invalid_argument("Empty type or location in VACaptureSource()");
}
type = inputType;
location = inputLocation;

// Open the given media source using the appropriate OpenCV function.
if( type.compare("image") ) {
    frame = cvLoadImage( location.c_str() );
    if( !frame ) {
        throw std::runtime_error("error opening file");
    }
}
else {
    throw std::invalid_argument("Unknown input type in VACaptureSource()");
}

}

When I want to create an instance, I use

    // Create input data object
try {
    VACaptureSource input = VACaptureSource("image", "/home/cuneyt/workspace/testmedia/face_images/jhumpa_1.jpg");
}
catch( invalid_argument& ia ) {
    cerr << "FD Error: " << ia.what() << endl;
    usage(argv[0]);
}
catch( runtime_error& re ) {
    cerr << "FD Error: " << re.what() << endl;
    usage(argv[0]);
}

However, in this case the instance is local to this block and I can't refer to it anywhere else. On the other hand, I can't say

VACAptureSource input;

at the beginning of the program since there's no default constructor.

What is the correct way to do this?

Thanks!

Answer 1

why do you need to refer to it outside the try block?

Instead of

try {
  VACaptureSource input = VACaptureSource("image", "/home/cuneyt/workspace/testmedia/face_images/jhumpa_1.jpg");
}
//catch....

//do stuff with input

you could move everything into the try block:

try {
  VACaptureSource input = VACaptureSource("image", "/home/cuneyt/workspace/testmedia/face_images/jhumpa_1.jpg");
  //do stuff with input
}
//catch....

or you could factor it out into a separate function, which is called from the try block:

void doStuff(VACaptureSource& input){
  //do stuff with input
}

try {
  VACaptureSource input = VACaptureSource("image", "/home/cuneyt/workspace/testmedia/face_images/jhumpa_1.jpg");
  doStuff(input);
}
//catch....

The last one even gives you the nice bonus of separating the construction from use, which places nicely into unit tests where you might want the function to work on a mock object instead.