Initialisation of static members in traits

Question

I have always read that you can define a trait like

enum MyEnum { val_1, val_2, val_3 };

template< typename T > 
struct my_trait { 
  static const MyEnum value = MyEnum::val_1;
};

and then specialise it

template<> 
struct my_trait < void >{ 
  static const MyEnum value = val_3; 
};

When I have tried it I always get a linker error since the static member is not defined, so I have to explicitly specialise it in the source file as

MyEnum my_trait < void >::value = val_3;

and change the definition in the header to

template<> 
struct is_void< void >{ 
  static const MyEnum value; 
};

Is there any way to define the trait directly in the header without having to redefine it in the header?

It doesn't have to be defined if it isn't odr-used. AFAICR, you're not supposed to odr-use the standard `::value`s. — chris, Apr 14 '17 at 13:16
@Lezkus "odr-used" is a term in the standard. Nuances aside, it means "the object itself rather than its value is used." Such as bound to a reference or having its address taken. — Angew is no longer proud of SO, Apr 14 '17 at 13:27
If you want a full definition of odr-used, the standard or cppreference is your best bet honestly. But most sensible uses of traits don't do it. It's also worth pointing out that C++17 has inline variables and `static constexpr bool value = ...;` is implicitly inline. — chris, Apr 14 '17 at 13:28
@Angew I edited the question with the actual use case. I thought that they were equivalent, but I guess they are not. — Lezkus, Apr 14 '17 at 13:32
Can you provide a reproducer too? Some code which uses the trait and reproduces the error? — Angew is no longer proud of SO, Apr 14 '17 at 13:34

max66 · Accepted Answer · 2017-04-14T13:39:54.990

0

Not exactly an answer to your question but... in your example (if you can use a C++11 compliler) you can use std::integral_constant.

A full example

#include <iostream>
#include <type_traits>

enum MyEnum { val_1, val_2, val_3 };

template< typename T > 
struct my_trait : public std::integral_constant<MyEnum, MyEnum::val_1>
 { };

template<> 
struct my_trait<void> : public std::integral_constant<MyEnum, MyEnum::val_3>
 { };

int main ()
 {
   std::cout << my_trait<void>::value << std::endl; // print 2 (aka val_3)
   std::cout << my_trait<int>::value << std::endl;  // print 0 (aka val_1)
 }

edited Apr 14 '17 at 13:39

answered Apr 14 '17 at 13:24

max66

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I just presented an example. The real code I am working with is a little bit more complicated. – Lezkus Apr 14 '17 at 13:25
1

@Lezkus - I see... Well, I suppose you should propose a minimal example that show how to get the error – max66 Apr 14 '17 at 13:26
I edited the question with the actual use case. I though that the formulation with any other type would be equivalent, but apparently it is not. – Lezkus Apr 14 '17 at 13:33
@Lezkus - answer modified to use `My_enum` – max66 Apr 14 '17 at 13:40
I see, that's a very good point. Does this generalise to a situation like this one: http://stackoverflow.com/questions/43190790/linking-against-a-shared-library-with-explicitly-instantiated-templates ? – Lezkus Apr 14 '17 at 13:44
@Lezkus - I suppose you can define `static constexpr const char charsStr[] = "chars";` as global constant and inherit from `std::integral_constant` – max66 Apr 14 '17 at 13:54

score 0 · Answer 2 · answered Apr 14 '17 at 13:29

This will cause linker error:

std::cout << &is_void<void>::value << std::endl;

because you odr-use value - it means you want its address. I am not sure but this might also happen with references - as they might be implemented internally as pointers. So until c++14 you would have to define your value inside implementation file.

Since C++17 you can use inline variables to keep it all in one place even if you need to odr-use it:

template< typename T > 
struct is_void{ 
  inline static const bool value = false;
  ^^^^^^ 
};

Initialisation of static members in traits

2 Answers2