2018.2.9

1、临时变量不可以作为非常量传引用的参数
例：

#include <iostream>
using namespace std;
class MyInt  {
    int nVal;
    public:
        MyInt(int n) { nVal = n; }
        int ReturnVal() { return nVal; }
        MyInt & operator -(const MyInt & n){
            return MyInt(nVal-n.nVal);//本身的成员变量值应该也要变        }
};
int main ()  {
    MyInt objInt(10);
    objInt-2-1-3;
    cout << objInt.ReturnVal();
    cout <<",";
    objInt-2-1;
    cout << objInt.ReturnVal();
    return 0;
}

报错：
4w4.cpp: In member function ‘MyInt& MyInt::operator-(const MyInt&)’:
4w4.cpp:10:29: error: invalid initialization of non-const reference of type ‘MyInt&’ from an rvalue of type ‘MyInt’
return MyInt(result);

修改：

#include <iostream>
using namespace std;
class MyInt  {
    int nVal;
    public:
        MyInt(int n) { nVal = n; }
        int ReturnVal() { return nVal; }
        MyInt & operator -(const MyInt & n){
            nVal-=n.nVal;
            return *this;
        }
};
int main ()  {
    MyInt objInt(10);
    objInt-2-1-3;
    cout << objInt.ReturnVal();
    cout <<",";
    objInt-2-1;
    cout << objInt.ReturnVal();
    return 0;
}

输出：
4，1