Thread: File handling with Array filled with a class

If you want to do it the fancy way, you would declare a operator << for the class, that in itself reads the hightemp, lowtemp, rain, wind, etc.

But that may be a bit advanced if you are a beginner, in which case you loop around reading in each element until end of file (unsuccessful reading of value indicates end of file) or full array.

The syntax would be something like:

Code:

if_stream >> report[i].hightemp;

--
Mats

Code:

istream& operator>> (istream& ins, WeatherReport& report)
{
ifstream in_stream1;

		in_stream1>>report.DayOfMonth>>report.HighTemp>>report.LowTemp
		>>report.AmountRain>>report.AmountSnow;
}

You should use "ins", not a local "in_stream1" variable.

--
Mats

Similarly you should use (and return) the passed stream in operator<<. Now it would compile and even be able to write WhetherReport to file.

Code:

ostream& operator<< (ostream& outs,const WeatherReport& report)
{
    outs<<report.DayOfMonth<<" ";
    ...
    outs<<endl;
    return outs;
}

Code:

istream& operator>> (istream& ins, WeatherReport& report)
{
ifstream in_stream1;

		in_stream1>>report.DayOfMonth>>report.HighTemp>>report.LowTemp
		>>report.AmountRain>>report.AmountSnow;
}

Actually, let me ask...
WHY are you using a local variable, the in_stream1? What is the purpose of doing so?

You are passing ONE element of report to the operator >>, not an array. If you want another operator to take an array, then you need to pass an array of the class. But I would say that your loop should be OUTSIDE.

Also, you should not open the file in operator >> - it should be already existing and opened by the code outside of the operator.

--
Mats

The easiest way is to just make your array a std::vector and pass that by reference.
Otherwise you can pass a pointer.

Originally Posted by MarlonDean

Ok sorry but i'm slow today - I need to pass an array to the << operator, not just a single element... how will i do that?

Code:

istream& operator>> (ifstream& ins, WeatherReport& report[i])

or is it like when I pass an array to a function?

It is like when you pass an array to a function, but in this case, my suggestion is to make a loop OUTSIDE the operator >>, and use your existing "read on report" operator >> to read the required number of elements into the array.

--
Mats

How would you write a vector of int's to a file? Writing WhetherReports to a file should not be different at all.

When overloading operators the general guideline is to make the behaviour similar to built-in (or already existing) types.

Code:

int i;
ofstream fin;
fin >> i;

Does this work? No, because no file is opened. Therefore operator>> overloads shouldn't open or close any files either.

If the behaviour you want from these operators is too different, you should consider a named function instead - operator overloading is just syntactic sugar, not something you absolutely must do. It is sweet only as long as the behaviour of the operator is very similar to other types: the user of your class already knows how to use operator>>, so they should be able to use it right away without having to study references on how the function is exactly supposed to be used.

For what code? Are you passing a non-array by reference, as the OP?