Re: std::exception's constructors

"Doug Harrison [MVP]" <>
Fri, 01 Sep 2006 10:53:37 -0500
On Fri, 1 Sep 2006 12:52:03 +0100, "Paul" <vhr@newsgroups.nospam> wrote:

I have an exception class to which I added another constructor. These are
the existing and the added ones:

1) Existing:

Exception::Exception(const std::string& err_msg /* other parameters */);

2) Added:

Exception(const std::exception& except /* same other parameters */);

The following 'throw' caused ambiguity

throw Exception("SOME TEXT" /* other parameters */);

and this is a message from the compiler:

... : error C2668: 'Exception::Exception' : ambiguous call to overloaded
       ...: could be 'Exception::Exception(const std::exception & [OTHER
       ...: or 'Exception::Exception(const std::string & [OTHER
       while trying to match the argument list '(const char [...] [OTHER

I then browsed to the std::exception's definition in the STL library and
found the following among its constructors:

__CLR_OR_THIS_CALL exception(const char *const&);

So, the picture, as I see it, is as follows: the compiler tries to resolve
the call and finds two matches: one based on the std::string's non-explicit
constructor taking "const char*", and the other, on std::exception's
constructor accepting the same.

Whilst the std::string's behaviour is as expected, I am a little puzzled by
that of std::exception: I have double-checked the std::exception's standard
definition and have also checked on-line help, where I found only this:

Standard C++ Library Reference

exception Class

class exception {
  exception( ) throw( );
  exception(const exception& right) throw( );
  exception& operator=(const exception& right) throw( );
  virtual ~exception( ) throw( );
  virtual const char *what( ) const throw( );


How should it be?

I think you're right. You can report the problem here, though I have my
doubts they will fix this, as removing the ctor would probably break some
existing code:

Meanwhile, you can add a const char* ctor to your Exception class to
resolve the ambiguity.

Doug Harrison
Visual C++ MVP

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