Using == Operator with Classes

This is a discussion on Using == Operator with Classes within the C++ Programming forums, part of the General Programming Boards category; Hi, Like the tile says, would it be possible to use the == operator with a class? For example: Code: ...

  1. #1
    Registered User mikeman118's Avatar
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    Using == Operator with Classes

    Hi,

    Like the tile says, would it be possible to use the == operator with a class? For example:
    Code:
    class Class
    {
        ...
    }
    Class cls1, cls2;
    if (cls1 == cls2)
    {
        ...
    }
    If it's difficult or something complicated then never mind, but if it's not too difficult can anyone tell me how to do this? Thanks.

  2. #2
    and the hat of wrongness Salem's Avatar
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    http://www.parashift.com/c++-faq-lit...erloading.html
    Any decent reference material should tell you how to do this.
    If you dance barefoot on the broken glass of undefined behaviour, you've got to expect the occasional cut.
    If at first you don't succeed, try writing your phone number on the exam paper.
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  3. #3
    Cat without Hat CornedBee's Avatar
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    Yes. You just have to overload it. Either as a free function:
    Code:
    bool operator ==(const Class &lhs, const Class &rhs)
    {
      return codeThatDeterminesIfLhsAndRhsAreEqual();
    }
    Or as a member function.
    Code:
    bool Class::operator ==(const Class &rhs)
    {
      return codeThatDeterminesIfThisAndRhsAreEqual();
    }
    All the buzzt!
    CornedBee

    "There is not now, nor has there ever been, nor will there ever be, any programming language in which it is the least bit difficult to write bad code."
    - Flon's Law

  4. #4
    Kernel hacker
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    You need to declare an "operator==(const Class &rhs)" in your class - or you can declare an external "operator==(const Class &lhs, const Class &rhs)". [1]

    You then need to "compare" your two elements (*this, rhs or lhs, rhs depending on which you choose). For example:
    Code:
    class C 
    {
    private:
       int x;
       int y;
    ...
    public:
       bool operator==(const C &rhs)
       {
          if (rhs.x == x && rhs.y == y) return true;
          return false;
       };
    };
    Of course, depending on the contents of your class, the comparison may be more or less complex, and you don't necessarily need to compare ALL the fields. If a record is a "person", then "social security number" or similar may be the component that says two persons are the same [obviously, that would have to be checked elsewhere that two different persons aren't entered with the same number]. Or a car-registration number for a car, serial number for a computer, etc.


    [1] lhs stands for Left Hand Side and rhs stands for Right Hand Side, and corresponds to the left and right hand side operands to the == operator.

    --
    Mats
    Compilers can produce warnings - make the compiler programmers happy: Use them!
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  5. #5
    Registered User mikeman118's Avatar
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    Thank you everyone, it worked! (That was easy)

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