what's the difference between Class(const Class & c) and Class(Class & c)?

Question

I want to know the difference between the Class(const Class & c) constructor and the Class(Class & c) constructor in C++.

class Class
{
public:
    Class()
    {
       cout << "running Class()..." << endl;
    }
    Class(Class& c)
    {
        cout << "running Class(Class& c)..." << endl;
    }
    Class(const Class& c)
    {
        cout << "runing Class(const Class& c)..." << endl;
    }
};

Class is defined as above.

I know that Class(const Class& c) is a copy constructor. It will be called when new Class object is created. For example:

Class c;  // output->running Class()...
Class cc(c); // output->runing Class(const Class& c)...

Now, I want to know when the Class(Class& c) constructor will be called.

_I know that `Class(const Class& c)` is a copy constructor._ `Class(Class& c)` is a copy constructor [as well](http://eel.is/c++draft/class.copy.ctor#1.sentence-1). Const-reference variant is usually preferred (e.g., since copied object is typically not needed to be modified and non-const reference cannot bind const objects nor rvalues). — Daniel Langr, Dec 19 '19 at 09:46
For the record, I did *not* vote to close this question as a duplicate, contrary to what the note at the top of the post is claiming, and I don’t think it’s a duplicate (though it may be sufficient to help clear up OP’s misunderstanding). — Konrad Rudolph, Dec 19 '19 at 09:59
If you wish the change the thing being copied as you copy it (e.g., `have_been_copied = true;`) then you want the one without the `const` — Neil Gatenby, Dec 19 '19 at 10:00

score 1 · Answer 1 · answered Dec 19 '19 at 09:48

Both are copy constructor.

class.copy.ctor#1

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 ([dcl.fct.default]). [ Example: X::X(const X&) and X::X(X&,int=1) are copy constructors.

score 1 · Answer 2 · answered Dec 19 '19 at 09:48

I know that Class(const Class& c) is a copy constructor. It will be called when new Class object is created. For example:
Class c;  // output->running Class()...
Class cc(c); // output->runing Class(const Class& c)...

No. Your code actually outputs

running Class()...
running Class(Class& c)...

And there you have it: the non-const copy constructor is called for non-const arguments. That’s all there is to it.

what's the difference between Class(const Class & c) and Class(Class & c)?

2 Answers2