Re: Can you solve this function chaining quiz?

From:

sp1ff@pobox.com (Michael Herstine <sp1ff@pobox.com>)

Newsgroups:

comp.lang.c++.moderated

Date:

Thu, 20 May 2010 21:34:06 CST

Message-ID:

<874oi2iet6.fsf@pobox.com>

I have a problem. I want to throw ExceptionA derived from Exception, i.e.

struct ExceptionA : Exception
{

};

ExceptionA, ExceptionB, ExceptionC, etc. shall be able to attach
text to the exception directly at construction via function
chaining, i.e.

throw ExceptionA().addText( "Hello" ).addText( "World" );

The quick way to implement that would be the following.

struct Exception
{
    Exception& addText( const std::string& str )
    {
       // Add str to some private string.
       return *this;
    }

};

Now, the problem comes with the throw.

throw ExceptionA().addText( "Hello" ).addText( "World" );

That will slice the exception and not throw an ExceptionA but
instead just a plain Exception since that is what addText()
returns.

I do not entirely understand why this is necessary. If this addText
function is proving difficult, why not do something more obvious?

     std::string exceptionText = "Hello ";
     exceptionText += "world";
     throw ExceptionA( exceptionText ); // Make ctor accept string.

Maybe this is overly simplistic. Maybe you want something to be done
to the string, other than concatenation. In that case, write a
function that concatenates the string with whatever extra you need!

I'm sorry if I was a bit unclear, but with the function chaining I want
to add more functions like addErrno(), addInnerException(),
addWhatever(). So the problem still remains.

[snip]

Is part of the specification for the problem "Must be done in one
line."? All this seems like a lot of work for just pretty syntax,
unless i'm missing something.

Yes, you are completely right, that is why I brought this question here.
Is it possible to make the syntax even better without drawbacks? I.e.

throw ExceptionA().addText("x").addText("y");

Maybe I'm not fully appreciating the problem, here, but it seems to me
you want to succinctly "attach" data to the exception in a way
expressable through interface (as oppossed to boost::exception). Why
not just use the constructor?

--
  Michael <sp1ff@pobox.com>

       web o http://www.unwoundstack.com
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