Implications of a const_cast in a copy constructor?

Question

So I've got an output stream class that owns a pointer to a class that actually does the writing, and I need a copy constructor so that I can return initialized instances from a function so that I can bind certain values transparently to the user. To have reasonable copy semantics I'd really like to clear the writer pointer in the copied object and close it rendering it unusable during copy.

I can do that just fine with a non-const copy constructor, a-la:

class Test {
  public:
    Test(Test& other);
};

But I want to be able to assign directly from the temporary returned by a function call:

Test test = make_test();

The copy constructor is required to be const. So I'm curious of what the implications of using a const_cast in the copy constructor would be. I'd cast the other reference to a non-const pointer and clear the writer pointer I mentioned. I know const_cast is generally considered evil but could it work in this case? I'm especially curious how it will interact with temporary objects returned from a function call.

Alternatively there's only like four creation functions that I really want to have access to the copy constructor, if there was a reasonable way to scope it so that it was only usable to those functions (including their return values) then I'd prefer that.

Answer 1

You would be violating the public contract of your object and the expectations of any client code by making your const copy constructor destructive.

Other than that, there's no problem with removing the constness of the parameter.

Answer 2

Sounds like a job for mutable:

struct A {

    A() : x(0) {}

    A( const A & a ) : x( a.x ) {
       a.x = 0;
    }

    mutable int x;
};

int main() {
    A a1;
    A a2 = a1;
}

Answer 3

You might wish to take a look at std::auto_ptr's implementation as its copy constructor also mutates the original object.

But be aware that this semantic is precisely why a lot of people dislike std::auto_ptr.