C++ compiler error on template specialization

Question

I would like to specialize a template method for a class C that is itself templated by an int parameter.

How do I do this?

template <int D=1>
class C {
    static std::string  foo () { stringstream ss; ss << D << endl; return ss.str();}    
};

template <class X>
void test() { cout << "This is a test" << endl;}

template <>
template <int D>
void test<C<D> > () {cout << C<D>::foo() << endl;}

The specialization for test() fails with "Too many template parameter lists in declaration of void test()".

log0 · Accepted Answer · 2010-06-18 15:33:37Z

2

Function template partial specialization is not allowed. Do

template <int D>  
void test () {cout << C<D>::foo() << endl;}

answered Jun 18, 2010 at 15:33

log0

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Comments

Mike Seymour · Accepted Answer · 2010-06-18 15:47:48Z

1

You don't want the first template<> on your partial specialisation of test<C<D>>. Moreover, you can only partially specialise class templates, not function templates. Something like this might work:

template <class X>
struct thing
{
    static void test() { cout << "This is a test" << endl;}
};

template <int D>
struct thing<C<D>>
{
    static void test() {cout << C<D>::foo() << endl;}
};

If your function template took an argument, and used that to infer the template argument, then you could get a similar effect using overloading, something like:

template <class X>
void test(const X&) { cout << "This is a test" << endl;}

template <int D>
void test(const C<D>&) {cout << C<D>::foo() << endl;}

test(3);  // calls first version
test(C<3>()); // calls second version

edited Jun 18, 2010 at 15:47

answered Jun 18, 2010 at 15:26

Mike Seymour

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3 Comments

user231536 Over a year ago

That does not work. Now I get a message: partial specialization 'test<C<D> >' is not allowed

Mike Seymour Over a year ago

Oh yes, I'd forgotten that. You can only partially specialise class templates.

k06a Over a year ago

What if C == std::tuple? And D == Args... variadic template?))

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