Error C2280 'Class1 &Class1::operator =(const Class1 &)': attempting to reference a deleted function

I have a

class Class1 { private:

const int classNumber;

public:

Class1(int passedNum)

:classNumber(passedNum)

{}

int GetClassNumber() const { return classNumber; }

};

class Class2

{

private:

vector class1Vector;

public:

void CreateNewClass(int classNumber)

{

Class1 newClass1(classNumber); class1Vector.push_back(newClass1); sort(class1Vector.begin(), class1Vector.end());

}

};

The sort function causing the error to happen, I guess even if I manually sort the vector, I will assign a temp =, so it is the = sign with a constant variable in class1 that makes things complicated, how do I fix this? while keeping the variable constant, and still using a sort function ( so a = operator )