Stringstream Question

[ParkourPenguin]

Hello. This isn't really a problem, but more so I'm confused why something happens.

Basically, I'm making a program that will automatically factor any polynomial. I figured I'd want the degree of the polynomial before I try to factor it, so I'm trying to safely get an integer variable "degree" safely (so it will not, under any circumstances, give me an error, no matter what I type in- even characters). Here is my code:

#include <iostream>
#include <string>
#include <sstream>
using namespace std;

int main()
{
    int degree;
    string inputstr;
    cout << "Degree of function: ";
    getline(cin,inputstr);
    while(true)
    {
        stringstream mystream(inputstr);
        if (mystream >> degree){break;}
        cout << "\"" << inputstr << "\" is not a number.\nDegree of function: ";
        getline(cin,inputstr);
    }
    return 0;
}

(P.S.: feel free to comment on my code. I'm open for improvements and learning new things.)

In line 14, you see stringstream mystream(inputstr);. However, if I move this line down to below what is currently line 17 (getline(cin,inputstr);), it won't work anymore. I played around a bit with the code and displaying what the variables contain, and I found out that the mystream variable contains 0 when the program runs line 15 if it's placed below getline(cin,inputstr);. However, if it is placed where it is now (line 14 from the above code), it seems to contain (what I would guess to be) a hexadecimal memory address (0x28fdc4). At the end of the while() loop, does something happen to the stringstream variable that causes it to be set equal to 0?

I guess I'm just confused about what a stringstream is and how it works. Does anyone know of a good tutorial on streams? I didn't find anything in this website's tutorial page ( http://www.cplusplus.com/doc/tutorial/ ), which is what I usually go to if I don't understand or if I forget something.

Thank you for your time, and if you need any other information, please ask.

[Chervil]

It sounds as though you are confused about local variables.
Consider the string inputstr. That is defined on line 9. It exists and can be referred to at any point from there until the closing brace on line 20.

Now consider the stringstream mystream. That is declared within the scope of the while loop. It is created on line 14 and is destroyed when it goes out of scope at the closing brace on line 18.

During each repetition of the while loop, a brand-new instance of the stringstream is created on line 14, and is destroyed again at the end of that pass through the loop.

I'm guessing that you expect it to exist continuously from one iteration to the next, which is not the case.

Also, at a more general level, you cannot use a variable before it has been declared. For example, this won't work:

    cout << a;
    int a = 5;

There's nothing wrong with your approach, though this might be simpler. It doesn't use string or stringstream:

    int degree;
    cout << "Degree of function: ";

    while(!(cin >> degree))
    {
        cin.clear();
        cin.ignore(1000, '\n');
        cout << "Not a number.\nDegree of function: ";
    }

At line 4, the integer value is read from cin. If the input is not numeric, the cin stream error flags are set, and so line 6 clears the error condition and the ignore at line 7 removes the invalid characters from the input buffer.

[jlb]

There is a slight difference with your approach. If your variable is an integral type and you enter a floating point number you will leave the fractional part in the buffer which could cause problems with the next input. The code provided by the OP discards this extra data.

[ParkourPenguin]

Oh, that makes a lot more sense now, Chervil. Thank you!