Why C++ allows returning ifstream object?

Question

In C++98, the following code does not compile because the ifstream has no copy constructor:

#include <iostream>
#include <fstream>
using namespace std;

ifstream f() {
    return ifstream("main.cpp");
}

int main() {
    ifstream st= f();
}

However using multiple GCC versions with C++11, this compiles without warnings. What is the reason of this?

Verified: [https://ideone.com/NpWsRU](https://ideone.com/NpWsRU) — drescherjm, Apr 13 '21 at 14:15
RVO / copy elision : https://stackoverflow.com/questions/12953127/what-are-copy-elision-and-return-value-optimization — Hatted Rooster, Apr 13 '21 at 14:15
@HattedRooster: Guaranteed RVO is a C++17 thing, not C++11 as in this question. — MSalters, Apr 13 '21 at 14:17
@HattedRooster Copy/move constructors are still needed in C++11 when RVO is applied. This is caused by additional move constructor, which did not exist in C++98. — Daniel Langr, Apr 13 '21 at 14:18

score 7 · Accepted Answer · answered Apr 13 '21 at 14:16

7

C++11 added move constructors. The stream is now moved. The source object here is a temporary in the return expression, which can be moved to the st object in main.

answered Apr 13 '21 at 14:16

MSalters

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Why C++ allows returning ifstream object?

1 Answers1