how to clear the buffer of an istringstream object in c++?

Question

I am trying to validate the input in a way that the user always has to enter kg or lb, if it doesn't do it will ask the user to reenter. It works until that point but it doesn't read the new input. I know that I need to clear the buffer but iss.clear() is not working.

float readMass()
{
    string weight;
    getline(cin, weight);

    float weightValue;
    string massUnit;
    istringstream iss(weight);
    streampos pos = iss.tellg();

    iss >> weightValue >> massUnit;

    if (massUnit == "lb")
    {
        weightValue = weightValue / 2.20462;
    }
    else if (massUnit == "kg")
    {
        weightValue = weightValue;
    }
    else
    {
        cout << "Please enter a correct weight and mass indicator: \n";
        iss.clear();
        readMass();
    }

    return weightValue;
}

Also see [this](http://stackoverflow.com/questions/2525352/problem-of-using-cin-twice) and [this](http://stackoverflow.com/questions/5131647/why-would-we-call-cin-clear-and-cin-ignore-after-reading-input). — Denilson Amorim, May 13 '15 at 21:21
`clear()` is to [clear](http://www.cplusplus.com/reference/ios/ios/clear/) the error flags — Christophe, May 13 '15 at 21:27

score 3 · Answer 1 · edited May 13 '15 at 21:25

3

iss.str("");

I'd also recommend putting this inside of a while loop instead of recursively calling the function

edited May 13 '15 at 21:25

Christophe

68,716
7
72
138

answered May 13 '15 at 21:22

user3245832

101
2

score 0 · Answer 2 · answered May 13 '15 at 21:36

You need to call str("") to reset the actual buffer, and also call clear() to reset the error flags.

I would suggest you implement the function as a simple loop instead of a recursive call, and just use a new istringstream on each loop iteration:

float readMass()
{
    string weight;

    while (getline(cin, weight))
    {
        float weightValue;
        string massUnit;

        istringstream iss(weight);    
        if (iss >> weightValue >> massUnit)
        {
            if (massUnit == "lb")
                return weightValue / 2.20462;

            if (massUnit == "kg")
                return weightValue;
        }

        cout << "Please enter a correct weight and mass indicator: \n";
    }

    return 0.0f;
}

But, if you want it recursive, then you don't need to worry about resetting the istringstream since you will never be using it again, the next recursion will use a new istringstream instance:

float readMass()
{
    string weight;
    getline(cin, weight);

    float weightValue;
    string massUnit;

    istringstream iss(weight);
    if (iss >> weightValue >> massUnit)
    {
        if (massUnit == "lb")
            return weightValue / 2.20462;

        if (massUnit == "kg")
            return weightValue;
    }

    cout << "Please enter a correct weight and mass indicator: \n";
    return readMass();
}

how to clear the buffer of an istringstream object in c++?

2 Answers2