具有参考返回类型的C ++类成员函数

Question

I am a newbie to programming and C++. I'm confused how reference works differently here.

The code below outputs 0 5 but removing the & from A &GotObj() would output 0 0 . Why didn't the second case output 0 5 ?

Thanks in advance.

#include <iostream>
using namespace std;

class A {
public:
    int val;
    A(int n=0) {
        val = n;

    }
    A &GetObj() {
        return *this;
    }
};

int main() {
    A a;
    cout << a.val << endl;
    a.GetObj() = 5;
    cout << a.val << endl;
    return 0;
}

Answer 1

When you return by value the expression a.GetObj() just creates a temporary. Assigning then only changes the value of that temporary.

Answer 2

This of a reference as just a pointer with a slightly different syntax. When GetObj is defined as

A& GetObj() { return *this: }

it returns a reference (ie a pointer) to a. Then, the assignment

a.GetObj() = 5

effectively invokes the assignment operator of a with the argument 5, which changes the value of a to 5.

But if you define GetObj() as

A GetObj { return *this; }

it returns a completely new temporary object that has the same value as a. So when you later assign 5 to it, it doesn't change the value of a.

Answer 3

Case 1: A GetObj()

• lets Create object of Class A :

A a;

• suppose this objects memory location is 0x87965432 . Now, when you call function of object a by a.GetObj() then it returns a temporary object whose memory location will be completely different then Class A's object a, lets say 0x98672345 . now if you assign value 5 by

a.GetObj() = 5

• then you are assigning value 5 to the object located at memory location 0x98672345 . now, you are going to print val variable of object which is located at memory location 0x87965432. thats why it will print 0.

Case 2: A &GetObj()

• as explained in Case 1, if you create object by

A a;

• lets consider its memory location 0x87965432 . now when you call function GetObj, by a.GetObj(), then return object will be of same memory location 0x87965432 , as its concept of return by Referance. now assigning value 5 to this object,

a.GetObj() = 5

will reflect effect in object a. and now printing value of variable val will be as expected 5.