Re: References to references

Thomas Maeder <>
Wed, 28 Dec 2011 12:29:30 -0800 (PST)
Kelvin Chung <> 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:

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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