Re: stream operator << overload resolution: temporaries vs non-tem

From:
"Igor Tandetnik" <itandetnik@mvps.org>
Newsgroups:
microsoft.public.vc.language
Date:
Sun, 20 Aug 2006 10:44:02 -0400
Message-ID:
<uG3RDcGxGHA.1224@TK2MSFTNGP03.phx.gbl>
"Paul" <vhr@newsgroups.nospam> wrote in message
news:F8312E9E-01F7-4278-BE98-E0A1A2B86EDF@microsoft.com

Thank you, Igor, I have indeed missed on that. But this still does not
explain a couple of things, I think.

1) Why would it work for strings (std::string): string operator
functions too take a reference to a stream:


As an extension, MSVC allows binding temporaries to non-const
references. This conversion is the lowest ranking one, only used when no
other alternative is available. Try building with /Za (strict ANSI
conformance), you should get an error.

std::ostream& operator <<(std::ostream&, const std::string&);

std::ofstream("test.txt") << std::string("Hello!") << std::endl;

(I debugged and checked: it does call the above operator << for
strings.)


Don't you get a warning about a non-standard feature being used?

2) Why would it work on subsequent insertions:

std::ofstream("test.txt") << "Hello!" /*returns address*/ << ' ' <<
"Hello!" << std::endl;


The subsequent insertion is called on a reference returned by the
previous insertion. The compiler has no way to know this reference
refers to a temporary. Consider:

struct C {
    C& detemporize() {return *this;}
};
void f(C&);

// does not compile: can't bind temporary to non-const reference
f(C());

f(C().detemporize()); // works

If the stream is treated as a constant object:

const std::ofstream& tmp = std::ostream("test.txt");
tmp << "Hello!";

then the reference to the stream returned from this operation will
also be a constant?


This should not compile - you can't call non-const function through a
const reference.
--
With best wishes,
    Igor Tandetnik

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