std::basic_ios::operator! – cppreference.com

Original:

Returns true if an error has occurred on the associated stream. Specifically, returns true if badbit or failbit is set in rdstate().

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Original:

(none)

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Original:

true if an error has occurred, false otherwise.

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#include <iostream>
#include <fstream>
#include <cstdlib>
int main()
{
    std::ifstream file("test.txt");
    if(!file)  // operator! is used here
    {  
        std::cout << "File opening failed\n";
        return EXIT_FAILURE;
    }

    // typical C++ I/O loop uses the return value of the I/O function
    // as the loop controlling condition, operator bool() is used here
    for(int n; file >> n; ) {
       std::cout << n << ' ';
    }
    std::cout << '\n';

    if (file.bad())
        std::cout << "I/O error while reading\n";
    else if (file.eof())
        std::cout << "End of file reached successfully\n";
    else if (file.fail())
        std::cout << "Non-integer data encountered\n";
}