Re: References to references

From:

Thomas Maeder <maeder@glue.ch>

Newsgroups:

comp.lang.c++.moderated

Date:

Wed, 28 Dec 2011 12:29:30 -0800 (PST)

Message-ID:

<87zkec62nf.fsf@madbox3.site>

Kelvin Chung <kelvSYC@mac.com> writes:

I'm getting an unforseen problem when I'm trying to compile code for
older compilers. The following code compiles in clang 3.0 on a Mac:

template <class Key>
struct Foo {
typedef Key& key_type;

virtual Bar mapKey(const key_type& key) = 0;
};

However, when I compile with gcc 4.1.2 on SUSE Linux, I get "cannot
declare reference to 'Key&', specifically wrt to mapKey().

I'm trying to find the root cause of this: Is it a C++11-specifc
thing, and thus barfing on gcc 4.1.2, which IIRC has no C++11
support? Is it simply that I am expecting that "const key_type&"
resolves to "Key&" (ie. the references "collapse"), when it
shouldn't? What should be the workaround for getting this snippet
to compile under gcc 4.1.2?

This snippet? Simply remove one of the &s.

If this is a problem:

template <class Key>
struct Foo
{
  Bar mapKey(Key const &key);
};

int main()
{
  Foo<int &> foo; // oops - reference to reference
}

the remove_reference template from the Boost type traits library may
be helpful:

http://www.boost.org/doc/libs/1_48_0/libs/type_traits/doc/html/boost_typetraits/reference/remove_reference.html

template <class Key>
struct Foo
{
  typedef typename remove_reference<Key>::type key_type;

  Bar mapKey(key_type const &key);
};

int main()
{
  Foo<int &> foo; // ok
}

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