VC++ 10 impact on existing code?

[Russco]

There is one change that fell out of the implementaion of rvalue references that has broken some code bases internally. It is code like the following:

struct S
{
    S();
    explicit S(const S&);
};
 
void f(S);
 
int main()
{
    S s;
 
    f(s);
}

This code compiles with Visual C++ 2008 SP1 (and with previous compilers) but it is, correctly, rejected by Dev10. If you make the default copy-constructor of a class 'explicit' you are saying that you can't pass an instance of the class to a function. The "fix" in every case we have seen so far is to remove the 'explicit' keyword from the default copy-constructor - it was usually added as a result of over enthusiasm.

Learnt something new. I never understood why people make copy constructors explicit because as far as I knew it only prevented assignment-looking calls to the copy constructor.

ie.

classname a;
classname b(a); // is ok if copy constructor is explicit
classname c = a; // fails if classname has explicit copy constructor.

I suppose thinking about it passing by value would be an implicit call and not an explicit call.

Is there ever a good reason to make copy constructors explicit?
Personally i have only used explicit constructors for single argument or multiargument constructors where all but one argument has a default value.

While talking about explicit, is it so hard to implement explicit conversion operators which is something I felt c++ needed for years?