RAII way to choose between two constructors

Question

I have a class that holds a large table of data, with a constructor that takes all of the parameters needed to calculate that data. However, it takes a long time to run, so I've added a constructor that takes a stream, and reads the data in from that stream. I'm having trouble coming up with a RAII way of designing this class though, since I have two constructors, and at run time I need to choose between them. This is what I've come up with:

std::string filename; // Populated by command line arguments
DataTable table; // Empty constructor, no resource acquisition or initialization

if( filename.empty() ) {
    table = DataTable(/*various parameters*/);
} else {
    std::ifstream filestream(filename);

    table = DataTable(filestream); // Reads from file
}

That looks pretty fragile to me. The default constructor will leave the object in a valid state, but a useless one. The only use of it is to create a "temporary" object in the outer scope, to be assigned to in one of the branches of the if statement. Additionally, there's a flag "inited" behind the scenes to manage if the object was default-constructed or fully initialized. Is there a better way to design this class?

Answer 1

Maybe like this:

DataTable foo = filename.empty()
              ? DataTable(x, y, z)
              : DataTable(std::ifstream(filename));