初始化构建器的调用

Question

我开始用C ++学习OOP编程。 我写了一个代码，显示读取的复数和数字的总和。

#include "stdafx.h"
#include<iostream>
using namespace std;

class complex
{
    double real;
    double imag;
public:
    complex(double x = 0, double y = 0);
    complex(const complex&);
    double modul();
    void display();
};
complex::complex(double x, double y)
{
    real = x;
    imag = y;
}
complex::complex(const complex& z)
{
    real = z.real;
    imag = z.imag;
}
double complex::modul()
{
    return(sqrt(real*real + imag*imag));
}
void complex::display()
{
    cout << real << " + i" << imag << endl;
}
int main()
{
    double x, y, sumax = 0, sumay = 0, i = 0, n;
    cout << "n = "; cin >> n;
    while (i < n)
    {
        cout << "Real = ";  cin >> x;
        cout << "Imaginary = "; cin >> y;
        sumax += x;
        sumay += y;
        complex z(x, y);
        cout << endl;
        z.display();
        i++;
    }
    complex sz = complex(sumax, sumay);
    cout << "Sum = ";
    sz.display();

}

complex sz = complex(sumax, sumay);和complex sz(sumax,sumay);之间有什么区别吗？ 如果是我应该使用哪一个？ 该计划适用于两者。