Question

I realize this error is usually due to some syntax or type issues but I am not sure how to solve this problem. I think it may do with the type of findRt.

vector<triangle> findRightTriangles(unsigned long l, unsigned long h) {

<triangle> retval; // storage for return value.

triangle t;    
double what;

for(t.s1 = 3; t.s1 <= h; t.s1++) {
    for(t.s2 = t.s1; t.s2 <= h; t.s2++) {
        what = sqrt((t.s1*t.s1) + (t.s2*t.s2));
        t.s3 = static_cast<unsigned int>(what);
        if(((t.s1*t.s1)+(t.s2*t.s2)) != (t.s3*t.s3) 
            || t.s1+t.s2+t.s3 < l 
                || t.s1+t.s2+t.s3 > h) {
            continue;
        }
        else if(t.s1+t.s2+t.s3 <= h 
            && t.s1+t.s2+t.s3 >= l 
                && t.s1+t.s2 > t.s3 
                    && ((t.s1*t.s1)+(t.s2*t.s2)) == (t.s3*t.s3)) {
            retval.push_back(t);
        }
    }
}
return retval;
}

int main(){

unsigned long min, max;

cin >> min >> max;

    //Here is the problem:
cout << findRightTriangles(min, max) << endl;

return 0;
}

How to output the vector using cout?

Answer 1

Why the error?
The compiler reports an error because there is no overloaded version of << operator to handle the type vector<triangle> which your function findRightTriangles() returns.
<< is overloaded only for most of the built-in data types and not for custom classes.

How to output the vector using cout?
There are two ways:

Solution 1:
A two step procedure:

Step1: You will have to iterate through the vector and cout each contained triangle.

std::vector<triangle>::const_iterator iter= vec.begin();
for(iter; iter != vec.end(); ++iter)
{
    cout<<*iter; //This is what step 2 provides for
}

Step 2: You will have to overload << for the triangle class as well.

ostream& operator<<( ostream& os, const triangle &) {}

---

Solution 2:
One step Solution.
Alternately, You can overload << for vector type itself:

ostream& operator<<( ostream& os, const vector<triangle>&)
{

}

I would personally prefer the Solution 1. It is more readable & Usually more often than not one would already have an overloaded << for the vector type being used and it could be leveraged.