CODEWITHSUNDEEP
c++
PangDae
PangDae

Reputation: 13

return struct value in C++

When I studied const reference type in C++, I learned returning reference type make use local variable in main function, so I tested how returned normal struct type work in lvalue. so I expected this source won't compile, but it compiled well... :( so I have two question

"Is returned struct variable?, why this code compiled well?"

"if returned struct is not variable, why this code compiled?, I assumed that returned value is rvalue.."

#include<iostream>
#include<cstring>
using namespace std;
struct travel_time
{
    int hour;
    int min;
};
travel_time sumTime(travel_time, travel_time);
int main(void)
{
    travel_time p1, p2,sum;
    cin >> p1.hour >> p1.min;
    cin >> p2.hour >> p2.min;
    sum=sumTime(p1, p2);
    cout << sum.hour << "hour " << sum.min<<"min";
    sumTime(p1, p2) = p1;   //********** why it works? **********
    return 0;
}
travel_time sumTime(travel_time t1, travel_time t2) 
{
    travel_time sum;
    sum.hour = t1.hour + t2.hour+(t1.min+t2.min)/60;
    sum.min = (t1.min + t2.min) % 60;

    return sum;
}

Upvotes: 0

Views: 335

Answers (1)

HolyBlackCat
HolyBlackCat

Reputation: 96959

Is returned struct variable?, why this code compiled well?

This doesn't make much sense.

travel_time sum; is a variable. When you call sumTime, you return a copy of this variable, and that copy itself isn't a variable. So the answer is no, the result of calling the function isn't a variable.

I have a feeling what you really wanted to ask is if the result of calling the function is an lvalue - it's not, the result is an rvalue (more specifically, prvalue).

if returned struct is not variable, why this code compiled?, I assumed that returned value is rvalue..

This again makes no sense, unless I replace "variable" with "lvalue".

In C++, you can assign to rvalues of class types (but not of privitive types such as int). Note that struct travel_time is a class, despite using the word "struct". Strictly speaking, there are no structures in C++, only classes.

There are ways to prevent this kind of assignment for your class, but you didn't do that.

Upvotes: 1

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