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Showing content from https://www.tutorialspoint.com/cpp_standard_library/cpp_ios_operator.htm below:

C++ IOS::operator!() function

The C++ std:ios::operator!() function is a user defined operator that allows objects of a class to be used with the logical NOT operator(!). By implementing this operator, we can customize how the NOT operator works for the instances of that class.

For example, if an object represents a success status, operator!() might return true when the status is a failure.

Following is the syntax for std::ios::operator!() function.

bool operator!() const;

It does not accept any parameter.

It retuns true if either failbit or badbit is set or else false.

If an exception is thrown, there are no changes in the stream.

Accesses the stream object.

Concurrent access to the same stream object may cause data races.

Let's look at the following example, where we are going to overload the operator!() function to return the negation of the boolean value.

#include <iostream>
class a
{
public:
    a(bool value) : b(value) {}
    bool operator!() const
    {
        return !b;
    }
private:
    bool b;
};
int main()
{
    a x1(true);
    a x2(false);
    std::cout << "Result Of Object1 : " << !x1 << std::endl;
    std::cout << "Result Of Object2 : " << !x2 << std::endl;
    return 0;
}

Output of the above code is as follows −

Result Of Object1 : 0
Result Of Object2 : 1

Consider the following example, where we are going to check whether the object is empty or not.

#include <iostream>
#include <string>
class a
{
public:
    a(const std::string& c) : b(c) {}
    bool operator!() const
    {
        return b.empty();
    }
private:
    std::string b;
};
int main()
{
    a x1("");
    a x2("TutorialsPoint");
    std::cout << "Result : " << !x1 << std::endl;
    std::cout << "Result : " << !x2 << std::endl;
    return 0;
}

Following is the output of the above code −

Result : 1
Result : 0

ios.htm

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