template and Visual : unable to match function definition to an existing declaration

[Locus06]

Hi,

Here's a piece of code that compiled with GCC, but not with Visual 2010:

template < class TPRIORITY, class T >
class cf_list :
    public std::list< std::pair<TPRIORITY, T> >
{
    public:
        ...

        /// \brief Returns a reference to the element referenced by iterator _it.
        T get_value( const typename std::list< std::pair< TPRIORITY, T > >::iterator& _it );

        ...
};


template <class TPRIORITY, class T>
T cf_list<TPRIORITY, T>::get_value(
        const typename std::list< std::pair< TPRIORITY, T > >::iterator& _it
        )
{
    // Second element is the stored value
    return ( *_it ).second;
}

The error is the following:

cf_list.hpp(230): error C2244: 'cf_core::cf_list<TPRIORITY,T>::get_value' : unable to match function definition to an existing declaration
...
see declaration of 'cf_core::cf_list<TPRIORITY,T>::get_value'
1>          definition
1>          'T cf_core::cf_list<TPRIORITY,T>::get_value(const std::list<std::pair<_Other1,_Other2>>::iterator &)'
1>          existing declarations
1>          'T cf_core::cf_list<TPRIORITY,T>::get_value(const std::_List_iterator<std::_List_val<_Ty,_Alloc>> &)'

Thanks in advance for any ideas...

[superbonzo]

a possible workaround should be

template < class TPRIORITY, class T >
class cf_list :
    public std::list< std::pair<TPRIORITY, T> >
{
    public:
        ...

        using std::list< std::pair< TPRIORITY, T > >::iterator; // pull in the base member type

        /// \brief Returns a reference to the element referenced by iterator _it.
        T get_value( const iterator& _it );

        ...
};


template <class TPRIORITY, class T>
T cf_list<TPRIORITY, T>::get_value(
        const iterator& _it
        )
{
    // Second element is the stored value
    return ( *_it ).second;
}

this should be also preferred as it would eventually resolve (or make less error prone) other name lookup issues ( eg. bad things will happen if later on you forget to fully qualify "iterator" while another type named "iterator" is visible in the same scope ... ).

[Locus06]

It works, and it is cleaner... Thanks a lot !

[D_Drmmr]

template < class TPRIORITY, class T >
class cf_list :
    public std::list< std::pair<TPRIORITY, T> >
{
    public:
        ...

        /// \brief Returns a reference to the element referenced by iterator _it.
        T get_value( const typename std::list< std::pair< TPRIORITY, T > >::iterator& _it );

        ...
};

It's a bad idea to inherit publicly from std::list (or any other container from the STL). I also don't see the need for it from the code you posted. 'get_value' does not access the list; only the element through the iterator you passed, so there is no need to make this a member function. Moreover, since the list is not changed, the function should be const.
What I really don't get, is why you would want to return a copy of the element value when an iterator (not const_iterator) is passed. You'll be spending some late-night debugging sessions when you start writing code like this:

cf_list<int, int> myList;
//...
myList.get_value(myList.begin()) = 42;

The last line does nothing, because it only modifies the copy.