C++ input operator overloading for class and vector

Question

I have a class with a constructor that takes a vector as an argument and some member functions that operate on the vector:

class myclass{
vector<double> myvec;
public:
    myclass(){ cout << "Constructor 1 " << endl; }

    myclass(const vector <double> &v){ "Constructor 2 " << endl; }

    ostream& print(ostream &s) const { //print function used in overloading output operator }

    double minimum(){ //return min value of vector }

    double maximum(){ //return max value of vector }
};

I have overloaded the input operator to take the values of a vector in a specific format: "<5: 1 2 3 4 5>" :

istream& operator>>(istream &s, myclass &mc) {
int size;
double item;
vector<double> tempvec;
char lpar, colon, rpar;
if (s >> lpar) {
    if ((s >> size >> colon) && (lpar == '<' && colon == ':')){
        tempvec[size];
        while(s >> item && rpar != '>'){
            tempvec.push_back(item);
        }
        mc = myclass(tempvec);
        s >> rpar;
    }else{
        s.setstate(ios::badbit);
    }
}
return s; 
}

I go to test my code:

int main(){
myclass mc;
cout << "Main Start" << endl;
while (cin >> mc)
    cout << mc << endl << mc.minimum() << endl << mc.maximum() << endl;
if (cin.bad()) cerr << "\nBad input\n\n";
cout << "Main Start" << endl;
return (0);
}

I run the code input the values in the format "<5: 1 2 3 4 5>", but instead of getting the min and max value printed out all I get is:

Constructor 1
Main Start
<5: 1 2 3 4 5>
Constructor 2
Main End

If I change the while loop in the operator overloading to:

while(s >> item >> rpar && rpar != '>'){
            tempvec.push_back(item);
        }
        mc = myclass(myvec);

I get the min and max when I test the code in main but only get half the inputs:

Constructor 1
Main Start
<5: 1 2 3 4 5>
Constructor 2
<2: 1 3>
1
3

I understand why this is: item = 1, rpar = 2, rpar is not equal to '>', myvec.pushback(1) and so on...

So I think the error might to do with the while loop, but I'm not sure where I'm going wrong.

Edit:

So I included a count to compare the number of inputs to the size:

while(s >> item && count < size && rpar != '>'){
        tempvec.push_back(item);
        ++count; // count++ doesnt change anything
    }
    mc = myclass(tempvec);
    s >> rpar;

I now get:

Constructor 1
Main Start
<5: 1 2 3 4 5>
Constructor 2
<4: 1 2 3 4>
1
4
Main End

The last value is not being included.

Sam Varshavchik · Accepted Answer · 2016-12-03 01:35:13Z

There are two bugs in the same line in your overloaded >> operator.

    while(s >> item && rpar != '>'){

rpar is not initialized before its value is compared here. This is undefined behavior.
The likely result of this undefined behavior is that rpar will be some garbage, and there's only 1 in 256 chance that it will be the '>' character. Otherwise this comparison will always be true, and this while loop is logically equivalent to:
```
while (s >> item)
```

This means that this while loop will continue to run until the input stream either goes into failed state, or until it reaches the end of the file.

This means that when your >> operator overload returns, the stream is guaranteed to be in an error or in a failed state. This means that in your main function:

while (cin >> mc)

will never be true, and that's why you do not get your minimum and maximum values shown.

You need to:

Definitely fix the undefined behavior.
Probably fix the logic in your >> operator.

Anjoys · Accepted Answer · 2016-12-03 14:10:28Z

0

Ok I think I may have fixed my problem I changed the logic and so I'm now using a for-loop, I could have still used a while-loop but I think a for-loop is better:

for (int i = 1; i <= size; ++i){
            s >> item;
            tempvec.push_back(item);
        }
        if (s >> rpar && rpar == '>'){
            mc = myclass(tempvec);
        }

Hopefully this is correct.

edited Dec 3, 2016 at 14:10

answered Dec 3, 2016 at 12:08

Anjoys

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