c++ - Why can't I specialize with a nullptr template parameter on a dependent type? -

this question has answer here:

• (partially) specializing non-type template parameter of dependent type 4 answers

i'd make specialization of struct 1 uses function pointer , doesn't. when this:

template <typename t, t*> struct test;  template <typename t> struct test<t, nullptr> { };  template <typename r, typename...args, r(*fn)(args...)> struct test<r(args...), fn> { }; 

i error:

error: type 't*' of template argument 'nullptr' depends on template parameter                                                                                                                                                     struct test<t, nullptr> 

what mean , how make work?

the linked question describes main issue , solutions, since involved i've applied them particular problem.

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version 1

we can use std::integral_constant hold pointers, can leverage fact around restriction on non-type parameters:

template <typename t, typename u> struct test;  template <typename t> struct test<t, std::integral_constant<t*,nullptr>> {     void operator()() { std::cout << "nullptr" << std::endl; } };  template <typename r, typename...args, r(*fn)(args...)> struct test<r(args...), std::integral_constant<r(*)(args...),fn>> {     void operator()() { std::cout << "fnptr" << std::endl; } }; 

unfortunately usage looks pretty ugly, works:

test<int,std::integral_constant<int*,nullptr>> a; a(); test<decltype(foo), std::integral_constant<decltype(&foo), foo>> b; b(); 

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version 2

this version uses std::enable_if checking rather partial specialization:

template <typename t, t*, typename = void> struct test;  template <typename t, t* p> struct test<t, p, typename std::enable_if<(p==nullptr)>::type> {     void operator()() { std::cout << "nullptr" << std::endl; } };  template <typename t, t* p> struct test<t, p, typename std::enable_if<std::is_function<t>::value>::type> {     void operator()() { std::cout << "fnptr" << std::endl; } }; 

the usage of intended:

test2<int, nullptr> c; c(); test2<decltype(foo), foo> d; d(); 

with version don't have instant access parameter types of function pointers used template arguments, write other trait them if necessary.

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here's live demo shows both versions working.