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

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

Reading explanation of members :

explicit char_separator(const Char* dropped_delims,                         const Char* kept_delims = "",                         empty_token_policy empty_tokens = drop_empty_tokens) explicit char_separator() 

The explicit keyword for the 1st constructor requires explicit creation of objects of char_separator type. What does the explicit keyword mean in C++? covers the explicit keyword very well.

The explicit keyword for the 2nd constructor is a noise and is ignored.

EDIT

From the c++ standard :

7.1.2 p6 tells :

The explicit specifier shall be used only in declarations of constructors within a class declaration; see 12.3.1.

12.3.1 p2 tells :

An explicit constructor constructs objects just like non-explicit constructors, but does so only where direct-initialization syntax (8.5) or where casts (5.2.9, 5.4) are explicitly used. A default constructor may be an explicit constructor; such a constructor will be used to perform default-initialization or value-initialization (8.5). [Example:

class Z { public: explicit Z(); explicit Z(int); // ... }; Z a;               // OK: default-initialization performed Z a1 = 1;          // error: no implicit conversion Z a3 = Z(1);       // OK: direct initialization syntax used Z a2(1);           // OK: direct initialization syntax used Z* p = new Z(1);   // OK: direct initialization syntax used Z a4 = (Z)1;       // OK: explicit cast used Z a5 = static_cast<Z>(1); // OK: explicit cast used 

—end example]

So, the default constructor with the explicit keyword is the same as without this keyword.