C++ namespaced function template specialization alternative using overloading with class pointer?

Question

So I've come across a problem where using function template specialization forces me to write the function in the same namespace as the function template. I cannot do this because some other code requires to define some function template within another specific namespace.

To work-around the namespace issue, I'm trying to use function overloading instead. Furthermore, these functions return value need to be constexpr, so I could not overload using a class reference, so I used a class pointer instead.

So I ended up with this:

template<class T> constexpr auto getName(T* const) { return ""; }

and then I overload them like this:

inline constexpr auto getName(int* const) { return "int"; }

This works fine and allows me to call the correct overload using a nullptr trick like this:

auto name = getName((int* const)nullptr);

My problem is it seems I am unable to overload this when the type is an array. I would like to be able to overload when the type is int[], and return the string "int[]" in that case.

Perhaps there is a better solution to avoid the namespace hell for function template specialization? I know this all stinks but I could not find a better alternative yet.

Thank you!

@Rakete1111 I forgot to mention that I cannot use rtti in my case. — Erunehtar
– Erunehtar, Commented Aug 8, 2018 at 19:57
@acade unfortunately I can't use class reference in my case. Your solution works fine if I could use references. — Erunehtar
– Erunehtar, Commented Aug 8, 2018 at 20:29

Jarod42 · Accepted Answer · 2018-08-08 20:28:00Z

1

You might use "tag" instead:

template <typename T> struct Tag{};

// template <typename T>
//constexpr auto getName(Tag<T>) { return ""; }

inline constexpr auto getName(Tag<int>) { return "int"; }

inline constexpr auto getName(Tag<int[]>) { return "int[]"; }

template <std::size_t N>
constexpr auto registerName(Tag<int[N]>) { return "int[N]"; }

Demo

edited Aug 8, 2018 at 20:28

answered Aug 8, 2018 at 19:32

Jarod42

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4 Comments

SergeyA Over a year ago

I do not see how this tag can be used in the real life, without calling code actually knowing it deals with the array - at which point it can simply call a different function instead.

Jarod42 Over a year ago

@SergeyA: constexpr auto name = getName((int* const)nullptr); would be constexpr auto name = getName(Tag<int>{}); (where I suppose int would be T in generic code). but indeed getName<T>() would also work but handling getName<std::vector<T>>() would be more problematic.

Erunehtar Over a year ago

@Jarod42 I tried your solution but it didn't work for the array type, online test: cpp.sh/9mxh7

Erunehtar Over a year ago

@Jarod42 oh nice I didn't think to combine it with the template size constant. It works now, cheers!

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