NetNews Usenet Archive 1992 #20/NN_1992_20.iso/spool/comp/std/cplus/1140

Internet Message Format | 1992-09-09 | 2.7 KB

From: [e-mail] (Pablo Halpern )
Date: Thu, 3 Sep 1992 14:01:16 GMT
Subject: Re: Destruction of Temporaries, Yet Another Posting
Message-ID: <[e-mail]>
Organization: Hewlett-Packard Company
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Newsgroups: comp.std.c++
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In article <[e-mail]>, [e-mail] (Jerry Schwarz) writes:
|> In article <[e-mail]>, [e-mail] (John Hickin) writes:
|> |>
|> |> A compiler using the const temporary approach and a thoughtful programmer
|> |> can combine to make code like that of the String example safer:
|> |>
|> |>    class String { public:
|> |>        operator const char*(); // NON-const
|> |>        String( const char* );
|> |>        ...
|> |>    };
|> |>
|>
|> Making the function that gives the pointer to char a non-const is a mistake.
|> The function may modify the underlying representation, but it certainly
|> does not modify the represented character sequence.  There is no
|> reason that I shouldn't be able to apply it to a const String
|>
|> |>    String h( "hello " ), w( "world" );
|> |>    const char* p = h + w;
|>
|> A careful programmer might have written
|>
|>     const String h("hello "), w(" world") ;
|>
|> and be surprised that  (const char*)h isn't allowed.

Well, until the language support for abstract const is solidified, I
suggest the following:

    class String { public:
       operator char*();             // NON-const
       operator const char*() const; // const
       String( const char* );
       ...
    };

A non-const string can be converted to a non-const char*, a const string
can only be converted to a const char*.  If only conversion to a const
char* is desired, then I agree with Jerry that the non-const version should
be eliminated.

|> I believe the use of the implicitly invoked conversion operator
|> obscures the issue of lifetime of temporaries and that we should
|> be writing the above example out in full as.
|>
|>     const char* p = (h+w).pchar();

I tend to agree, although I am open to opposing views.

|>
|>   -- Jerry Schwarz

--

- Pablo

------------------------------------------------------------------------
Pablo Halpern             [phone]
HP Waltham                [e-mail]

I am self-employed, so my opinions *do* reflect those of my employer.
However, they may not reflect the opinions of my client.
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