Re: Linker problem with template specialisation

[Thomas Maeder]

tsotsi@gmail.com writes:

> 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; };

[NB: The above two lines only contain declarations of the static data
members called id. I don't see the corresponding definitions in the
code you posted. E.g.:

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

foo const map_foo_type<foo_1>::id;]


> Using the above code, I want to be able to do the following:
>
>     data<int> mid = map_foo_type<foo1>::id;
>
> When I compile with gcc 3.3.6 I get a linker error - undefined symbol
> mep_foo_type<foo1>::id

Funny, I get

test.cpp:50: error: type/value mismatch at argument 1 in template parameter 
list for 'template<class T> struct map_foo_type'
test.cpp:50: error:   expected a type, got 'foo1'

Did you meen foo_1 when you wrote foo1?

If I perform that change, I get

test.cpp:50: error: conversion from 'const foo' to non-scalar type 'data<int>' 
requested

because data<int> doesn't have a constructor that takes a foo const.


> If I have an intermediate step:
>
> foo id = map_foo_type<foo1>::id
> data<int> mid = id;
>
> then it links fine.

Hmm, this surely doesn't compile without a ';? at the end of the first
line and when the two lines are at global scope; is this code in a
function? If I move the fixed lines into a function, I still get the
error mentioned above (test.cpp:50).


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

I fail to see how the data assignment operator could play a role;
there is no assignment to a data<T> object in the entire code you posted.

To initialize objects, C++ uses constructors, not assignment operators.


> What is the problem please?

Primarily that you don't post the code that you have tested. :-)