# std::rel_ops::operator!=,>,<=,>=

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< cpp‎ | utility

Defined in header `<utility>` |  |
template< class T > bool operator!=( const T& lhs, const T& rhs ); | (1) | (deprecated in C++20)
template< class T > bool operator>( const T& lhs, const T& rhs ); | (2) | (deprecated in C++20)
template< class T > bool operator<=( const T& lhs, const T& rhs ); | (3) | (deprecated in C++20)
template< class T > bool operator>=( const T& lhs, const T& rhs ); | (4) | (deprecated in C++20)

Given a user-defined operator== and operator< for objects of type `T`, implements the usual semantics of other comparison operators.

1) Implements operator!= in terms of operator==.

2) Implements operator> in terms of operator<.

3) Implements operator<= in terms of operator<.

4) Implements operator>= in terms of operator<.

### Parameters

lhs | - | left-hand argument
rhs | - | right-hand argument

### Return value

1) Returns true if `lhs` is not equal to `rhs`.

2) Returns true if `lhs` is greater than `rhs`.

3) Returns true if `lhs` is less or equal to `rhs`.

4) Returns true if `lhs` is greater or equal to `rhs`.

### Possible implementation

First version
    namespace rel_ops {
        template< class T >
        bool operator!=( const T& lhs, const T& rhs )
        {
            return !(lhs == rhs);
        }
    }
Second version
    namespace rel_ops {
        template< class T >
        bool operator>( const T& lhs, const T& rhs )
        {
            return rhs < lhs;
        }
    }
Third version
    namespace rel_ops {
        template< class T >
        bool operator<=( const T& lhs, const T& rhs )
        {
            return !(rhs < lhs);
        }
    }
Fourth version
    namespace rel_ops {
        template< class T >
        bool operator>=( const T& lhs, const T& rhs )
        {
            return !(lhs < rhs);
        }
    }

### Notes

Boost.operators provides a more versatile alternative to `std::rel_ops`

As of C++20, `std::rel_ops` are deprecated in favor of `operator<=>`.

### Example

Run this code

    #include <iostream>
    #include <utility>

    struct Foo {
        int n;
    };

    bool operator==(const Foo& lhs, const Foo& rhs)
    {
        return lhs.n == rhs.n;
    }

    bool operator<(const Foo& lhs, const Foo& rhs)
    {
        return lhs.n < rhs.n;
    }

    int main()
    {
        Foo f1 = {1};
        Foo f2 = {2};
        using namespace std::rel_ops;

        std::cout << std::boolalpha;
        std::cout << "not equal?     : " << (f1 != f2) << '\n';
        std::cout << "greater?       : " << (f1 > f2) << '\n';
        std::cout << "less equal?    : " << (f1 <= f2) << '\n';
        std::cout << "greater equal? : " << (f1 >= f2) << '\n';
    }

Output:

    not equal?     : true
    greater?       : false
    less equal?    : true
    greater equal? : false

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