Re: Why extracting string from stringstream(a) fails?

From:

Maxim Yegorushkin <maxim.yegorushkin@gmail.com>

Newsgroups:

comp.lang.c++

Date:

Mon, 26 Oct 2009 15:36:09 +0000

Message-ID:

<4ae5c205$0$9753$6e1ede2f@read.cnntp.org>

Victor Bazarov wrote:

Maxim Yegorushkin wrote:

Alf P. Steinbach wrote:

* Maxim Yegorushkin:

On 25/10/09 18:13, Johannes Schaub (litb) wrote:

Maxim Yegorushkin wrote:

Here is another trick to turn a temporary into an l-value, so that
any
operator>> can work on a temporary stream object:

      template<class T>
      inline T& lvalue(T const& t) {
          return const_cast<T&>(t);
      }

Although this will be ill-formed because it requires the stream to
have a
copy constructor.

Interesting.

My understanding is that a copy constructor is only required when
copy initialization is involved or when a conversion is made to
initialize a function argument. In this case there is no copy
initialization or conversion happening. Therefore, the code must be
well formed.

Could you elaborate you point please?

It's different in C++98 and C++0x (C++03 is just C++98 with
corrections).

The argument passing is defined as copy initialization. And in C++98
the implementation is allowed to make any number of copies of an
rvalue actual argument passed to 'T const&' formal argument, or for
any copy initialization. Which means that the type must provide a
suitable copy constructor. For example, that means that you can't do
this thing with a std::auto_ptr. Or a stream.

Even more interesting.

Given the following declaration:

void foo(int& ref);

Could you explain how ref argument can possibly be copy-initialized
please?

I am guessing the same way 'r2' is a "copy" of 'r1' here:

    int a = 42;
    int &r1 = a;
    int &r2 = r1;

:-)

Very well, this is what I wanted to hear. ;)

In this case no copy constructor is required to copy-initialize a reference to
non-const. By induction, the same should hold true for references to const
(although they can be initialized with r-values, but only if necessary). Hence,
the above lvalue function template is well-formed.

Comeau online compiles lvalue(std::istringstream(...)) just fine.

--
Max