C++Talk.NET

Stefan Heinzmann · Guest

Hi all,

I have disagreement between two compilers when deducing template
parameters on instantiation of a function template and would like to
know which compiler is right. The problem is related to a const
qualification. Here's the code:

#include <algorithm>

// =============================================================
// Generic map element declarations.
// -------------------------------------------------------------

template<typename Elem> struct map_element_traits;

template<typename Elem> inline
const typename map_element_traits<Elem>::key_type&
key(const Elem& element)
{
return map_element_traits<Elem>::key(element);
}

template<typename Elem> inline
const typename map_element_traits<Elem>::mapped_type&
mapped_value(const Elem& element)
{
return map_element_traits<Elem>::mapped_value(element);
}

template<typename Elem>
struct key_value_compare
{
typedef typename map_element_traits<Elem>::value_type vtype;

bool operator()(const vtype& a, const vtype& b) const
{
return map_element_traits<Elem>::key(a)
< map_element_traits }
};

// =============================================================
// Generic map declarations.
// -------------------------------------------------------------

template<typename Map> struct map_traits;

template<typename Map> inline
typename map_traits<Map>::const_iterator
begin(Map const& map)
{
return map_traits<Map>::begin(map);
}

template<typename Map> inline
typename map_traits<Map>::const_iterator
end(Map const& map)
{
return map_traits<Map>::end(map);
}

template<typename Map> inline
typename map_traits<Map>::const_iterator
find(Map const& m, typename map_traits<Map>::key_type const& k)
{
return map_traits<Map>::find(m, k);
}

template<typename Map>
typename map_traits<Map>::mapped_type const&
lookup( Map const& map,
typename map_traits<Map>::key_type const& key,
typename map_traits<Map>::mapped_type const& def_value)
{
typename map_traits<Map>::const_iterator i = find(map, key);
return (i == end(map)) ? def_value : mapped_value(*i);
}

// =============================================================
// Types for lookup tables stored in ROM.
// -------------------------------------------------------------

template<typename Key, typename Val>
struct Pair
{
Key key;
Val val;
};

template<typename Key, typename Val>
struct map_element_traits< Pair
{
typedef Pair<Key,Val> value_type;
typedef Key key_type;
typedef Val mapped_type;

static const key_type& key(const value_type& e)
{ return e.key; }
static const mapped_type& mapped_value(const value_type& e)
{ return e.val; }
};

template<typename Key, typename Val, unsigned n>
struct map_traits< Pair //### const or not?
{
typedef Key key_type;
typedef Val mapped_type;
typedef Pair<Key,Val> value_type;
typedef const value_type* const_iterator;

static const_iterator begin(const value_type (&map)[n])
{ return &map[0]; }
static const_iterator end (const value_type (&map)[n])
{ return &map[n]; }

static const_iterator find(const value_type (&map)[n]
,const key_type& k)
{
value_type elem = { k };
const_iterator i = std::lower_bound(begin(map), end(map)
, elem, key_value_compare<value_type>());
return (i == end(map) || key(*i) != k) ? end(map) : i;
}
};

// =============================================================
// Usage test
// -------------------------------------------------------------

const Pair<int, const char*> table[] =
{
{ 0, "Ok" },
{ 6, "Minor glitch in self-destruction module" },
{ 13, "Error logging printer out of paper" },
{ 101, "Emergency cooling system inoperable" },
{ 2349, "Dangerous substances released" },
{ 32767, "Game over, you lost" }
};

int main()
{
const char *result = lookup(table, 5, "not found");
}

The problem is related to the struct map_traits. GCC 3.3 compiles the
code as shown, whereas VC++ 7.1 needs the marked line to be changed to this:

struct map_traits< const Pair //### const or not?

Obviously, the two compilers disagree when deducing the type of the
first argument to the lookup function template. Which one is right?

--
Cheers
Stefan

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tom_usenet · Guest

On Mon, 17 May 2004 17:18:12 +0000 (UTC), [email]stefan_heinzmann (AT) yahoo (DOT) com[/email]
(Stefan Heinzmann) wrote: