6. 定义一个复数类Complex，重载运算符“-”，使之能用于复数的减法运算。将运算符函数重载为非成员、非友元的普通函数。编写程序，求两个复数之差。

好的，这个问题简单，以下是重载运算符“-”后的Complex类的定义和示例程序：

class Complex {
public:
    Complex(double real = 0.0, double imag = 0.0) {
        m_real = real;
        m_imag = imag;
    }

    double real() const { return m_real; }
    double imag() const { return m_imag; }

private:
    double m_real;
    double m_imag;
};

Complex operator- (const Complex& c1, const Complex& c2) {
    Complex result;
    result = Complex(c1.real() - c2.real(), c1.imag() - c2.imag());
    return result;
}

#include <iostream>
using namespace std;

int main() {
    Complex c1(3.0, 4.5);
    Complex c2(1.5, 2.0);

    Complex c3 = c1 - c2;

    cout << "c1 - c2 = " << c3.real() << " + " << c3.imag() << "i" << endl;

    return 0;
}

执行输出结果为：

c1 - c2 = 1.5 + 2.5i