CODEWITHSUNDEEP
c++fstream
Shamil Mukhetdinov
Shamil Mukhetdinov

Reputation: 579

How is fstream / ifstream / ofstream objects convert to bool

I want to know, how these objects convert to bool. I mean what the compiler relies on when casting. On bit flags? As I think it works like that: the compiler checks the flags and if !goodbit then returns false else true. For example:

#include <iostream>
#include <fstream>

int main() {
    std::ifstream file("someDir.txt");

    if(!file) { \\ Checks the flags? If !goodbit (eofbit, badbit or failbit) return false else true?
        \\some code
    }

    return 0;

P.S. I have bad English, sorry

Upvotes: 0

Views: 828

Answers (1)

Philip Nelson
Philip Nelson

Reputation: 1231

The compiler uses the operator bool to convert the stream to a boolean. In the documentation you can read that std::basic_ios<CharT,Traits>::operator bool

Checks whether the stream has no errors.

1) Returns a null pointer if fail() returns true, otherwise returns a non-null pointer. This pointer is implicitly convertible to bool and may be used in boolean contexts.

2) Returns true if the stream has no errors and is ready for I/O operations. Specifically, returns !fail().

There is a really helpful table on the same page that details which bits (eofbit, failbit, badbit) result in fail() returning true or false.

Upvotes: 1

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