Re: How to detect const reference to temporary issues at compile or runtime?

From:

SG <s.gesemann@gmail.com>

Newsgroups:

comp.lang.c++

Date:

Tue, 30 Nov 2010 23:55:34 -0800 (PST)

Message-ID:

<9b1873a5-436e-41a7-9922-4a8d793e9304@j18g2000prn.googlegroups.com>

On 1 Dez., 02:56, Clinton Mead wrote:

I've found recently that most of the errors in my C++ programs are
of a form like the following example:

[...]

  class Y
  {
  public:
    Y(const Z& z) : z(z) {}
    const Z& z;
  };

[...reference to vanishing temporary...]

If you really need Y to store a reference to a Z object you can avoid
accidentally binding a reference to a temporary by makeing this
constructor *explicit* and taking a pointer parameter instead of a
reference. This forces you to use the address operator whenever you
try to invoke Y's constructor. (a) The address operator won't work on
temporary objects (unless you enabled some compiler extension perhaps)
and (b) The use of & should remind you to think about object life-
times.

This reminds me of one of the upcoming new standard library
facilities: reference_wrapper. If I remember correctly the
reference_wrapper has the following constructor overloads:

  template<class T>
  class reference_wrapper
  {
  public:
    ...
    reference_wrapper(T&);
    reference_wrapper(T&&) = delete; // #1
    ...
  };

where #1 explicitly forbids initializing a reference_wrapper with an
rvalue expression (includes expressions yielding temporary objects).

Cheers!
SG