Problem with specializing templates

[anon]

What I am trying to do is to detect automatically whether a container contains simple value_types or key-value pairs (whether the container is map or similar or not). For that I am hoping to instantiate a specialized template for containers whose value_type is a std::pair).

However, the pair specialization is not picked up, unless I specifically state the pair with its template arguments. (The int specialization is just to check that it in principle should work.)

What could I do to make maps automatically detected?

#include <vector>
#include <list>
#include <map>
#include <iostream>

template <class Cont, class ValueType = typename Cont::value_type>
struct S
{
    void print() { std::cout << "General\n"; }
};

template <class Cont>
struct S <Cont, int >
{
    void print() { std::cout << "Special: int\n"; }
};

template <class Cont>
struct S <Cont, std::pair<typename Cont::key_type, typename Cont::mapped_type> >
{
    void print() { std::cout << "Special: map\n"; }
};

int main()
{
    S<std::list<int> > s1;
    S<std::vector<double> > s2;
    S<std::map<int, double> > s3;
    S<std::map<int, double>, std::pair<int, double> > s4;
    S<std::map<int, double>, std::map<int, double>::value_type > s5;

    s1.print(); //Special: int   OK
    s2.print(); //General        OK
    s3.print(); //General        not OK, expected Special: map
    s4.print(); //Special: map   OK
    s5.print(); //General        not OK
}

[anon]

Nevermind, I figured it out by checking

std::cout << (typeid(std::map<int, double>::value_type) == typeid(std::pair<int, double>)) << '\n';
and
std::cout << (typeid(std::map<int, double>::value_type) == typeid(std::pair<const int, double>)) << '\n';

The specialization didn't match because the map's key_type is const.

---------------------
Just for the fun, here's a class to help print standard containers' contents.

#include <iostream>
#include <list>
#include <vector>
#include <set>
#include <map>
#include <string>
#include <limits>

template <class Container, class ValueType = typename Container::value_type >
struct Printer
{
    void print(std::ostream& os, typename Container::const_iterator it) const
    {
        os << *it;
    }
};

template <class Container>
struct Printer <Container, std::pair<const typename Container::key_type, typename Container::mapped_type> >
{
    void print(std::ostream& os, typename Container::const_iterator it) const
    {
        os << it->first << '=' << it->second;
    }
};


template <class Container>
class Repr
{
        const Container& c;
        Printer<Container> printer;
        const std::string separator;
        typename Container::size_type max;

    public:
        Repr(const Container& c, const std::string& sep, typename Container::size_type n): c(c), separator(sep), max(n) {}

    template <class C>
        friend std::ostream& operator << (std::ostream& os, const Repr<C>& cont);
};



template <class Container>
std::ostream& operator << (std::ostream& os, const Repr<Container>& cont)
{
    if (cont.max == 0 || cont.c.begin() == cont.c.end()) {
        return os;
    }
    typename Container::size_type counter = 1;
    typename Container::const_iterator it = cont.c.begin();
    cont.printer.print(os, it);
    for (++it; it != cont.c.end() && counter != cont.max; ++it, ++counter) {
        os << cont.separator;
        cont.printer.print(os, it);
    }
    if (it != cont.c.end()) {
        os << cont.separator << "...";
    }
    return os;
}

template <class Container>
Repr<Container> make_repr(
    const Container& c,
    const std::string& separator = std::string(" "),
    typename Container::size_type max = std::numeric_limits<typename Container::size_type>::max()
    )
{
    return Repr<Container>(c, separator, max);
}

int main()
{
    std::string data[] = {"One", "Two", "Three", "Four", "Five", "Six", "Seven" };
    typedef std::list<std::string> Container;
    Container array(data, data + sizeof(data) / sizeof(data[0]));
    std::cout << make_repr(array) << '\n'           //print using defaults
              << make_repr(array, ", ", 4) << '\n'  //print first 4 items, using user-defined delimiter
              << make_repr(array, "\n") << '\n';    //print each item on a new line


    std::map<std::string, int> Map;
    Map["One"] = 1;
    Map["Two"] = 2;
    Map["Three"] = 3;
    Map["Four"] = 4;
    std::cout << make_repr(Map, ", ") << '\n';    //works for std::map too
}

[anon]

Oh, the joy was short-lived. The std::pair<> specialization works with MingW but not with MSVC++ 2005. Am I doing something wrong or is one of the compilers wrong here?

Another question. If I wanted to move the Repr class into a separate header, how would I know what standard headers I'd need to include? Or, should I include nothing and expect the user to include required standard headers before including this file?

Sorry about reponding to myself...

[brewbuck]

You could use boost::enable_if, along with a helper template that determines if a type is a map type. The first part requires reading the docs, the second part might look like this:

template <typename T>
struct is_pair : public false_type {};

template <typename K, typename V>
struct is_pair<std::pair<K, V> > : public true_type {};

template <typename T>
struct is_map : public is_pair<typename T::value_type> {};

template <typename T>
struct is_map<T> : public false_type {};

That should produce a predicate is_map<> which is suitable for using with boost::enable_if.

EDIT: Of course, this is not perfect, because all it checks is whether value_type is a std:: pair or not. But that doesn't necessarily mean the container is a map -- it could be a vector of pairs, for instance.

[CornedBee]

You would include the headers you use explicitly. That would be <string> (std::string), <ostream> (std:stream) and <utility> (std:air).