Создание класса квадратичных функций

#include <iostream>
 
class QuadraticFunction {
private:
    double a, b, c;
 
public:
    QuadraticFunction() : a(0), b(0), c(0) {}
    QuadraticFunction(double _a, double _b, double _c) : a(_a), b(_b), c(_c) {}
    double getA() const { return a; }
    double getB() const { return b; }
    double getC() const { return c; }
 
    double calculate(double x) const {
        return a * x * x + b * x + c;
    }
 
    QuadraticFunction& operator+=(const QuadraticFunction& other) {
        a += other.a;
        b += other.b;
        c += other.c;
        return *this;
    }
 
    QuadraticFunction& operator*=(double scalar) {
        a *= scalar;
        b *= scalar;
        c *= scalar;
        return *this;
    }
 
    friend std::ostream& operator<<(std::ostream& out, const QuadraticFunction& func) {
        out << "f(x) = " << func.a << "x^2 + " << func.b << "x + " << func.c;
        return out;
    }
};
 
int main() {
    QuadraticFunction f1(1, 2, 1);
    QuadraticFunction f2(2, -1, 3);
    std::cout << "f1(x) = " << f1 << std::endl;
    std::cout << "f2(x) = " << f2 << std::endl;
    std::cout << "f1(2) = " << f1.calculate(2) << std::endl;
    std::cout << "f2(2) = " << f2.calculate(2) << std::endl;
    f1 += f2;
    std::cout << "f1(x) + f2(x) = " << f1 << std::endl;
    f2 *= 2.5;
    std::cout << "f2(x) * 2.5 = " << f2 << std::endl;
}

...
double calculate(double x) const
{
    return a * x * x + b * x + c;
}
...

...
double operator()(double x) const
{
    return a * x * x + b * x + c;
}
...

QuadraticFunction operator+(QuadraticFunction a, const QuadraticFunction &b) {
    return a += b;
}
 
QuadraticFunction operator*(QuadraticFunction a, double b) {
    return a *= b;
}
 
QuadraticFunction operator*(double a, QuadraticFunction b) {
    return b *= a;
}