Question

This statement here doesn’t seem to be correct:

Bidirectional file streams, on the other hand, do not have the flag
set implicitly. This is because a bidirectional stream
does not have to be in both input and output mode in all cases. You
might want to open a bidirectional stream for reading only or writing
only. Bidirectional file streams therefore have no implicit input or
output mode. You must always set a bidirectional file stream’s open
mode explicitly.

otherwise, this code wouldn’t compile.

#include <iostream>
#include <fstream>

using namespace std;

int main()
{
    fstream file("novo.txt", ios::out);
    char i;
    file >> i;
} 

Answer 1

Although there has historically been some confusion regarding this matter, I believe the statement you quoted is indeed misleading, but not for the reason in your example. Bidirectional file streams do set the flag implicitly, as noted in the documentation:

void open(
const char *_Filename,
ios_base::openmode _Mode = ios_base::in | ios_base::out,
int _Prot = (int)ios_base::_Openprot
);

Based on the Apache statement, I would NOT expect your code sample to fail, as it explicitly sets the flag to ::out. I would, on the other hand, expect this code to fail:

int main()
{
    fstream file("novo.txt");
    char i;
    file >> i;
} 

But based on the MSDN documentation, this code will not fail either, and the file will default to ios_base::in & out (and, hence, the Apache statement is not entirely accurate).