C++：赋值运算符：按值传递（复制和交换）与按引用传递

Question

Considering the advantages of copy-and-swap idiom ...

Why do we still need copy-assignment operator accepting references as the mainstream?

class T {
public:

    // 1: Why still commonly this?
    T& operator=(const T& rhs);

    // 2: Why not mostly that?
    T& operator=(T rhs);
}

There are answers suggesting to use the latter ( here and here ).

However, most of the SO examples are still around pass-by-reference operator= .

Even consolidated C++ FAQ points out (yes, it's about const , but...):

A class Fred 's copy constructor and assignment operator should have const in the parameter: respectively Fred::Fred(const Fred&) and Fred& Fred::operator=(const Fred&)

Obviously, copy-and-swap is implementable via pass-by-reference - it is just unnecessary if copy is to be made anyway. One may also want to avoid copying immediately on call (perform it conditionally in the body) - isn't that the less frequent case (possibly premature optimization) then?

Shouldn't copy-and-swap with pass-by-value assignment be the default approach?

Answer 1

Pass by reference is for avoiding unnecessary copy when you need faster executions, and pass by a const reference is when you want to pass it fast and read-only. And pass by copy is when you want to copy the object to be able to manipulate it in the course of the execution/implementation of your function.

Answer 2

More than it creates spurious non useful copies, second option may not work on every class as copying may be deleted.

First option have many, if not all, advantages: no copy, read-only semantic, always available.

Answer 3

I was pointed to the valuable hint :

For example, ever wonder why high performance / often used std::types like std::vector and std::string don't use copy/swap? | Howard Hinnant

Everything I found against copy-on-swap was about optimization (avoid copy, re-purpose lhs ):

• Obviously, self-assignment is noop.

• Another common possibility happens in (STL) container when lhs has space for rhs :

   class container<T> { public: // If passed by value, created copy would unconditionally increase capacity: T& operator=(const container<T>& rhs) { //... if (this->capacity() >= rhs.size()) { // reuse capacity... } else { // increase capacity... } } }

  Notice that the hint also mentions (STL) containers .

The trade-off for operator= is:

• by value + copy-and-swap: simplifies (saves human time) with strong exception guarantees
• by reference: allows optimization (saves machine time) likely weakening guarantees

---

Related answers:

• Why SGI STL don't use the copy-and-swap idiom?
• Is copy-and-swap always the best solution?