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Section 13:
[13.9] What are some guidelines / "rules of thumb" for overloading operators?

Here are a few guidelines / rules of thumb (but be sure to read the previous FAQ before reading this list):

  1. Use common sense. If your overloaded operator makes life easier and safer for your users, do it; otherwise don't. This is the most important guideline. In fact it is, in a very real sense, the only guideline; the rest are just special cases.
  2. If you define arithmetic operators, maintain the usual arithmetic identities. For example, if your class defines x + y and x - y, then x + y - y ought to return an object that is behaviorally equivalent to x. The term behaviorally equivalent is defined in the bullet on x == y below, but simply put, it means the two objects should ideally act like they have the same state. This should be true even if you decide not to define an == operator for objects of your class.
  3. You should provide arithmetic operators only when they make logical sense to users. Subtracting two dates makes sense, logically returning the duration between those dates, so you might want to allow date1 - date2 for objects of your Date class (provided you have a reasonable class/type to represent the duration between two Date objects). However adding two dates makes no sense: what does it mean to add July 4, 1776 to June 5, 1959? Similarly it makes no sense to multiply or divide dates, so you should not define any of those operators.
  4. You should provide mixed-mode arithmetic operators only when they make logical sense to users. For example, it makes sense to add a duration (say 35 days) to a date (say July 4, 1776), so you might define date + duration to return a Date. Similarly date - duration could also return a Date. But duration - date does not make sense at the conceptual level (what does it mean to subtract July 4, 1776 from 35 days?) so you should not define that operator.
  5. If you provide constructive operators, they should return their result by value. For example, x + y should return its result by value. If it returns by reference, you will probably run into lots of problems figuring out who owns the referent and when the referent will get destructed. Doesn't matter if returning by reference is more efficient; it is probably wrong. See the next bullet for more on this point.
  6. If you provide constructive operators, they should not change their operands. For example, x + y should not change x. For some crazy reason, programmers often define x + y to be logically the same as x += y because the latter is faster. But remember, your users expect x + y to make a copy. In fact they selected the + operator (over, say, the += operator) precisely because they wanted a copy. If they wanted to modify x, they would have used whatever is equivalent to x += y instead. Don't make semantic decisions for your users; it's their decision, not yours, whether they want the semantics of x + y vs. x += y. Tell them that one is faster if you want, but then step back and let them make the final decision — they know what they're trying to achieve and you do not.
  7. If you provide constructive operators, they should allow promotion of the left-hand operand (at least in the case where the class has a single-parameter ctor that is not marked with the explicit keyword). For example, if your class Fraction supports promotion from int to Fraction (via the non-explicit ctor Fraction::Fraction(int)), and if you allow x - y for two Fraction objects, you should also allow 42 - y. In practice that simply means that your operator-() should not be a member function of Fraction. Typically you will make it a friend, if for no other reason than to force it into the public: part of the class, but even if it is not a friend, it should not be a member.
  8. In general, your operator should change its operand(s) if and only if the operands get changed when you apply the same operator to intrinsic types. x == y and x << y should not change either operand; x *= y and x <<= y should (but only the left-hand operand).
  9. If you define x++ and ++x, maintain the usual identities. For example, x++ and ++x should have the same observable effect on x, and should differ only in what they return. ++x should return x by reference; x++ should either return a copy (by value) of the original state of x or should have a void return-type. You're usually better off returning a copy of the original state of x by value, especially if your class will be used in generic algorithms. The easy way to do that is to implement x++ using three lines: make a local copy of *this, call ++x (i.e., this->operator++()), then return the local copy. Similar comments for x-- and --x.
  10. If you define ++x and x += 1, maintain the usual identities. For example, these expressions should have the same observable behavior, including the same result. Among other things, that means your += operator should return x by reference. Similar comments for --x and x -= 1.
  11. If you define *p and p[0] for pointer-like objects, maintain the usual identities. For example, these two expressions should have the same result and neither should change p.
  12. If you define p[i] and *(p+i) for pointer-like objects, maintain the usual identities. For example, these two expressions should have the same result and neither should change p. Similar comments for p[-i] and *(p-i).
  13. Subscript operators generally come in pairs; see on const-overloading.
  14. If you define x == y, then x == y should be true if and only if the two objects are behaviorally equivalent. In this bullet, the term "behaviorally equivalent" means the observable behavior of any operation or sequence of operations applied to x will be the same as when applied to y. The term "operation" means methods, friends, operators, or just about anything else you can do with these objects (except, of course, the address-of operator). You won't always be able to achieve that goal, but you ought to get close, and you ought to document any variances (other than the address-of operator).
  15. If you define x == y and x = y, maintain the usual identities. For example, after an assignment, the two objects should be equal. Even if you don't define x == y, the two objects should be behaviorally equivalent (see above for the meaning of that phrase) after an assignment.
  16. If you define x == y and x != y, you should maintain the usual identities. For example, these expressions should return something convertible to bool, neither should change its operands, and x == y should have the same result as !(x != y), and vice versa.
  17. If you define inequality operators like x <= y and x < y, you should maintain the usual identities. For example, if x < y and y < z are both true, then x < z should also be true, etc. Similar comments for x >= y and x > y.
  18. If you define inequality operators like x < y and x >= y, you should maintain the usual identities. For example, x < y should have the result as !(x >= y). You can't always do that, but you should get close and you should document any variances. Similar comments for x > y and !(x <= y), etc.
  19. Avoid overloading short-circuiting operators: x || y or x && y. The overloaded versions of these do not short-circuit — they evaluate both operands even if the left-hand operand "determines" the outcome, so that confuses users.
  20. Avoid overloading the comma operator: x, y. The overloaded comma operator does not have the same ordering properties that it has when it is not overloaded, and that confuses users.
  21. Don't overload an operator that is non-intuitive to your users. This is called the Doctrine of Least Surprise. For example, although C++ uses std::cout << x for printing, and although printing is technically called inserting, and although inserting sort of sounds like what happens when you push an element onto a stack, don't overload myStack << x to push an element onto a stack. It might make sense when you're really tired or otherwise mentally impaired, and a few of your friends might think it's "kewl," but just say No.
  22. Use common sense. If you don't see "your" operator listed here, you can figure it out. Just remember the ultimate goals of operator overloading: to make life easier for your users, in particular to make their code cheaper to write and more obvious.

Caveat: the list is not exhaustive. That means there are other entries that you might consider "missing." I know.

Caveat: the list contains guidelines, not hard and fast rules. That means almost all of the entries have exceptions, and most of those exceptions are not explicitly stated. I know.

Caveat: please don't email me about the additions or exceptions. I've already spent way too much time on this particular answer.