Make copy assignment non-`virtual`, take the parameter by `const&`, and return by non-`const&`

Reason

It is simple and efficient. If you want to optimize for rvalues, provide an overload that takes an && (see F.18).

Example

class Foo {
public:
    Foo& operator=(const Foo& x)
    {
        // GOOD: no need to check for self-assignment (other than performance)
        auto tmp = x;
        swap(tmp); // see C.83
        return *this;
    }
    // ...
};

Foo a;
Foo b;
Foo f();

a = b;    // assign lvalue: copy
a = f();  // assign rvalue: potentially move

Note

The swap implementation technique offers the strong guarantee.

Example

But what if you can get significantly better performance by not making a temporary copy? Consider a simple Vector intended for a domain where assignment of large, equal-sized Vectors is common. In this case, the copy of elements implied by the swap implementation technique could cause an order of magnitude increase in cost:

template<typename T>
class Vector {
public:
    Vector& operator=(const Vector&);
    // ...
private:
    T* elem;
    int sz;
};

Vector& Vector::operator=(const Vector& a)
{
    if (a.sz > sz) {
        // ... use the swap technique, it can't be bettered ...
        return *this;
    }
    // ... copy sz elements from *a.elem to elem ...
    if (a.sz < sz) {
        // ... destroy the surplus elements in *this and adjust size ...
    }
    return *this;
}

By writing directly to the target elements, we will get only the basic guarantee rather than the strong guarantee offered by the swap technique. Beware of self-assignment.

Alternatives: If you think you need a virtual assignment operator, and understand why that's deeply problematic, don't call it operator=. Make it a named function like virtual void assign(const Foo&). See copy constructor vs. clone().

Enforcement

Look at the destructor to determine if the type has pointer semantics or value semantics.

• (Simple) An assignment operator should not be virtual. Here be dragons!
• (Simple) An assignment operator should return T& to enable chaining, not alternatives like const T& which interfere with composability and putting objects in containers.
• (Moderate) An assignment operator should (implicitly or explicitly) invoke all base and member assignment operators.