字符串不能作为临时对象初始化对象嘛

大佬们，帮我看看这个代码那里错误啊！
#include
#include
#include
using namespace std;
class Complex {
private:
double r, i;
public:
void Print() {
cout << r << "+" << i << "i" << endl;
}
// 在此处补充你的代码
Complex() {};
Complex(const std::string& str)
{
int pos = str.find("+", 0);
string stmp = str.substr(0, pos);//分离出代表实部的字符串
r = atoi(stmp.c_str()); //将字符串转换为整数
stmp = str.substr(pos + 1, str.length() - pos - 2);//分离出代表虚部的字符串
i = atoi(stmp.c_str());
}
};
int main() {
Complex a;
//string str = "3+4i";
//a = str;a.Print();
a = "3+4i"; a.Print();
system("pause");
return 0;
}

#include <iostream>
using namespace std;
class Complex {
private:
    double r, i;
public:
    void Print() {
        cout << r << "+" << i << "i" << endl;
    }
    // 在此处补充你的代码
    Complex& operator =(const std::string& str)
    {
        int pos = str.find("+", 0);
        string stmp = str.substr(0, pos);//分离出代表实部的字符串
        this->r = atoi(stmp.c_str()); //将字符串转换为整数
        stmp = str.substr(pos + 1, str.length() - pos - 2);//分离出代表虚部的字符串
        this->i = atoi(stmp.c_str());
        return *this;
    }
};
int main() {
    Complex a;
    //string str = "3+4i";
    //a = str;a.Print();
    a = "3+4i"; a.Print();
    system("pause");
    return 0;
}