Does "explicit" keyword have any effect on a default constructor?

Question

Is there a reason to use the explicit keyword for a constructor that doesn't take any arguments? Does it have any effect? I'm wondering because I just came across the line

explicit char_separator()

near the end of the page documenting boost::char_separator, but it's not explained any further there.

Answer 1

Yes, it does have an effect.

Compare:

struct A
{
    A() {}
};

void foo(A) {}

int main()
{
    foo({}); // ok
}

and:

struct A
{
    explicit A() {}
};

void foo(A) {}

int main()
{
    foo({}); // error
}