ostream << operator not getting invoked

Question

I have created a class Animal with some basic properties and added a no data constructor. I have overloaded the ostream operator as well to print the properties.

Animal.cpp

#include<bits/stdc++.h> using namespace std; class Animal { string name; int action; public: Animal() { name = "dog"; action = 1; } ostream& write(ostream& os) { os << name << "\n" << action << "\n"; return os; } friend ostream& operator<<(ostream& os, Animal &animal) { return animal.write(os); } }; int main() { cout << "Animal: " << Animal() << "\n"; }

However I am getting error in the main that invalid operands to binary expression ostream and Animal. It works fine if I declare Animal and then call the cout. But how to make it work like this (initialize and cout at the same time) ?

On a side note: Why should I not #include <bits/stdc++.h>?

Remy Lebeau
– Remy Lebeau

2022-03-20 18:58:38 +00:00
Commented Mar 20, 2022 at 18:58 — Remy Lebeau
– Remy Lebeau, Commented Mar 20, 2022 at 18:58

songyuanyao · Accepted Answer · 2022-03-20 15:21:32Z

The 2nd parameter of operator<< is declared as Animal &; Animal() is a temporary and can't be bound to lvalue-reference to non-const.

You can change the type to const Animal &; temporary could be bound to lvalue-reference to const. (Then write needs to marked as const too.)

class Animal { string name; int action; public: Animal() { name = "dog"; action = 1; } ostream& write(ostream& os) const { os << name << "\n" << action << "\n"; return os; } friend ostream& operator<<(ostream& os, const Animal &animal) { return animal.write(os); } };

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