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Question

class test {
public:
    test &operator=(const test & other){} // 1
    const test & operator+(const test& other) const {} // 2
    const test & operator+(int m) {} //3
private:
    int n;
};

int main()
{
test t1 , t2, t3;

// this works fine  t1 = t2.operator+(t3) , calls  2 and 1
t1 = t2 + t3; 

// this works fine    t1 = t2.operator+ (100). calls 3 and 1
t1 = t2 + 100;

//this works fine t1 = (t2.operator+ (100)).operator+ (t3)
t1 = t2 + 100 +t3;

//why is the error in this one  ????
// it should be t1 = (t2.operator+ (t3)).operator+ (100)
t1  =  t2 + t3 + 100; 

return 0;
}

Answer 1

因为t2 + t3返回的对象是const,所以你不能调用它的非const函数(3).

它在"t1 = t2 + 100 + t3;"的情况下工作正常,因为t2 + 100返回的对象也是const,但是你正在调用它的const函数(2),这没关系.