Why would a copy constructor have more than one parameter?

Question

$12.8/2 - 'A non-template constructor for class X is a copy constructor if its first parameter is of type X&, const X&, volatile X& or const volatile X&, and either there are no other parameters or else all other parameters have default arguments (8.3.6).106)'

So far, I have not come across any example of a situation where there is a need to declare a copy constructor with additional default parameters.

Would like to know any real time use of such a copy constructor which take more than one parameter.

Answer 1

The old std::basic_string does have one too:

basic_string(const basic_string& s, 
         size_type pos = 0, size_type n = npos)

Answer 2

The BDE allocator [PDF Link] used this quirk. For example their array allocator looked like this:

template <typename T>
class bde::Array {
public:
    Array(bde::Allocator *allocator = 0);
    Array(const Array &rhs, bde::Allocator *allocator = 0);
};