Re: Linker problem with template specialisation

From:

Barry <dhb2000@gmail.com>

Newsgroups:

comp.lang.c++

Date:

Thu, 29 May 2008 04:49:14 -0700 (PDT)

Message-ID:

<f5393a2e-61b3-40ae-aa3b-4448922a4595@x41g2000hsb.googlegroups.com>

On May 29, 5:44 pm, tso...@gmail.com wrote:

I have the following code:

enum foo
{
    null_foo,
    foo1,
    foo2

};

template<typename T>
struct data
{
    T val;
    bool set;

    template<typename U>
    data<T>& operator=(const U &that)
    {
        val = that;
        set = true;
        return *this;
    }

};

struct foo_1 { };
struct foo_2 { };

template<typename T> struct map_foo_type { static foo const id =
null_foo; }; // worst match
template<> struct map_foo_type<foo_1> { static foo const id = foo1; };
template<> struct map_foo_type<foo_2> { static foo const id = foo2; };

initialize them outside of the class.

template<typename T> struct map_foo_type
{ static foo const id; }; // worst match

template <typename T>
foo const map_foo_type<T>::id = null_foo;

template<> struct map_foo_type<foo_1> { static foo const id; };
template<> struct map_foo_type<foo_2> { static foo const id; };

foo const map_foo_type<foo_1>::id = foo1;
foo const map_foo_type<foo_2>::id = foo2;

Using the above code, I want to be able to do the following:

data<int> mid = map_foo_type<foo1>::id;

foo_1

here calls copy constructor other than assignment operator as marked
else thread

When I compile with gcc 3.3.6 I get a linker error - undefined symbol
mep_foo_type<foo1>::id

If I have an intermediate step:

foo id = map_foo_type<foo1>::id
data<int> mid = id;

then it links fine.

Alternately, if I change operator= above to take const U data, rather
than a reference to const U, it also links fine!

What is the problem please?