Can a copy-constructor take a non-const parameter?
c++
Solution
To answer your titular question: Yes, any constructor for a class `T` that has one mandatory argument of type `T &` or `T const &` (it may also have further, defaulted arguments) is a copy constructor. In C++11, there's also a move constructor which requires one argument of type `T &&`.
Having a non-constant copy constructor that actually mutates the argument gives your class very unusual semantics (usually "transfer semantics") and should be extensively documented; it also prevents you from copying something constant (obviously). The old `std::auto_ptr<T>` does exactly that.
If at all possible, the new C++11-style mutable rvalue references and move constructors provide a far better solution for the problem of "moving" resources around when they're no longer needed in the original object. This is because an rvalue reference is a reference to a mutable object, but it can only bind to "safe" expressions such as temporaries or things that you have explicitly cast (via `std::move`) and thus marked as disposable.
Problem
I have this problem, there is a function `foo()` as follows, ``` vector<ClassA> vec; void foo() { ClassA a; //inside foo, a ClassA object will be created a._ptr = new char[10]; vec.push_back(a); //and this newly created ClassA object should be put into vec for later use } ``` And AFAIK, `vec` will invoke `ClassA`'s copy-ctor to make a copy of the newly created object `a`, and here is the problem. If I define `ClassA`'s copy-ctor the usual way, ``` ClassA::ClassA(const ClassA &ra) : _ptr(0) { _ptr = ra._ptr; } ``` then object `a` and its copy (created by vec) will have pointers `_ptr` pointing to the same area, when `foo` finishes, `a` will call the destructor to release `_ptr`, then `a`'s copy in `vec` will be a dangling pointer, right? Due to this problem, I want to implement `ClassA`'s copy-ctor this way, ``` ClassA::ClassA(ClassA &ra) : _ptr(0) //take non-const reference as parameter { std::swap(_ptr, a._ptr); } ``` Is my implementation ok? Or any other way can help accomplish the job?