Purpose of Explicit Default Constructors

Question

I recently noticed a class in C++0x that calls for an explicit default constructor. However, I\'m failing to come up with a scenario in which a default constructor can be c

Answer 1

This declares an explicit default constructor:

struct A {
  explicit A(int a1 = 0);
};

A a = 0; /* not allowed */
A b; /* allowed */
A c(0); /* allowed */

In case there is no parameter, like in the following example, the explicit is redundant.

struct A {
  /* explicit is redundant. */
  explicit A();
};

In some C++0x draft (I believe it was n3035), it made a difference in the following way:

A a = {}; /* error! */
A b{}; /* alright */

void function(A a);
void f() { function({}); /* error! */ }

But in the FCD, they changed this (though, I suspect that they didn't have this particular reason in mind) in that all three cases value-initialize the respective object. Value-initialization doesn't do the overload-resolution dance and thus won't fail on explicit constructors.