Assigning integer values from variadic template list to static const std::array member

Question

Consider the following code snippet:

template<unsigned... IDs>
class MyClass{
public:
    static const std::array<unsigned, sizeof...(IDs)> ids { IDs... };
    PinIDs() = default;
};

Then use the class as:

MyClass<1,5,7,9> myClass;

The Objective would be to have ids with a size of 4, and contain the values: (1,5,7,9) respectively.

Is this type object possible or would I have to remove the static qualifier? If not how would one write this with the static qualifier. The object needs to be default constructible.

EDIT:

~~I tried Apple Apple's first solution and with MS Visual Studio 2017 CE on Win7 I got this compiler error:~~

1>------ Build started: Project: PracticeMath, Configuration: Debug Win32 ------ 1>stdafx.cpp 1>PracticeMath.cpp 1>c:\users\skilz80\documents\visual studio 2017\projects\practicemath\practicemath\practicemath.cpp(33): error C2988: unrecognizable template declaration/definition 1>c:\users\skilz80\documents\visual studio 2017\projects\practicemath\practicemath\practicemath.cpp(33): error C2143: syntax error: missing ';' before '<' 1>c:\users\skilz80\documents\visual studio 2017\projects\practicemath\practicemath\practicemath.cpp(33): error C2059: syntax error: '<' 1>c:\users\skilz80\documents\visual studio 2017\projects\practicemath\practicemath\practicemath.cpp(33): error C2039: 'ids': is not a member of '`global namespace'' 1>c:\users\skilz80\documents\visual studio 2017\projects\practicemath\practicemath\practicemath.cpp(33): error C2143: syntax error: missing ';' before '{' 1>c:\users\skilz80\documents\visual studio 2017\projects\practicemath\practicemath\practicemath.cpp(33): error C2447: '{': missing function header (old-style formal list?) 1>c:\users\skilz80\documents\visual studio 2017\projects\practicemath\practicemath\practicemath.cpp(38): error C2065: 'myId': undeclared identifier 1>c:\users\skilz80\documents\visual studio 2017\projects\practicemath\practicemath\practicemath.cpp(39): error C2065: 'myId': undeclared identifier 1>c:\users\skilz80\documents\visual studio 2017\projects\practicemath\practicemath\practicemath.cpp(40): error C2065: 'myId': undeclared identifier 1>c:\users\skilz80\documents\visual studio 2017\projects\practicemath\practicemath\practicemath.cpp(44): error C2065: 'c': undeclared identifier 1>Done building project "PracticeMath.vcxproj" -- FAILED. ========== Build: 0 succeeded, 1 failed, 0 up-to-date, 0 skipped ==========

With full original source like this:

#include <iostream> #include <array> template<unsigned... IDs> class PinIDs{ public: static const std::array<unsigned, sizeof...(IDs)> ids; PinIDs() = default; const unsigned& operator[]( unsigned idx ) const { return ids[idx]; } }; template<unsigned... IDs> const std::array<unsigned, sizeof...(IDs)> PinIDs<IDs...>::ids { IDs... }; int main() { PinIDs<4, 17, 19> myId; std::cout << myId[0] << " "; std::cout << myId[1] << " "; std::cout << myId[2] << " "; std::cout << "\nPress any key and enter to quit." << std::endl; char c; std::cin >> c; return 0; }

Thanks to StoryTeller bringing up the fact that when I tried to apply Apple Apple's 1st method I accidently mixed up MyClass as opposed to the actual name of the class in my solution - project. Once I corrected that it does compile, build and run as expected.

Yes; but was having trouble... don't know if the template syntax business, or just because of using static... — Francis Cugler
– Francis Cugler, Commented Dec 20, 2017 at 6:36
@PasserBy I understand the errors; it's just the template syntax that gets me at times... — Francis Cugler
– Francis Cugler, Commented Dec 20, 2017 at 6:43

apple apple · Accepted Answer · 2017-12-20 06:50:59Z

3

you can try this

#include <array>

template<unsigned... IDs>
class MyClass{
public:
   static const std::array<unsigned, sizeof...(IDs)> ids;
   MyClass() = default;
};

template<unsigned... IDs>
const std::array<unsigned, sizeof...(IDs)> MyClass<IDs...>::ids {IDs...};

int main(){
   MyClass<1,5,7,9> myClass;
   return myClass.ids[0];
}

or use constexpr/inline (both need c++17)

#include <array>
template<unsigned... IDs>
class MyClass{
public:
    //static constexpr std::array<unsigned, sizeof...(IDs)> ids{IDs...};//or this
    static inline const std::array<unsigned, sizeof...(IDs)> ids{IDs...};
    MyClass() = default;
};

int main(){
MyClass<1,5,7,9> myClass;
    return myClass.ids[0];
}

edited Dec 20, 2017 at 6:50

answered Dec 20, 2017 at 6:37

apple apple

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7 Comments

Francis Cugler Over a year ago

Hmm pretty close; I was able to get my class to work as expected without the static qualifier. Just that when I tried to make the member variable of static type; is where I was getting into a bit of trouble.

StoryTeller - Unslander Monica Over a year ago

@FrancisCugler - This is how you'd do it with a static member. It need to be defined outside the class.

Francis Cugler Over a year ago

Also I would like to add a operator[] to the class that will index directly into the array so I don't have to use object.ids[] or MyClass::ids[]

Francis Cugler Over a year ago

I know that much; same as having const static member variables; I've used them before. Just not with templates... and that is where I was struggling because of the syntax.

apple apple Over a year ago

@FrancisCugler maybe your compiler not support it yet, here is a working example

|

StoryTeller - Unslander Monica · Accepted Answer · 2017-12-20 06:51:32Z

1

Refer to @apple apple's answer for the basics. I'll just add the C++17 way to do it. Which is quite close to your original attempt. Just add an inline specifier to the variable:

template<unsigned... IDs>
class MyClass{
public:
    static inline const std::array<unsigned, sizeof...(IDs)> ids{ { IDs... } };
    MyClass() = default;
};

Now the declaration can double as a definition. Oh, and mind the braces. std::array needs to be initialized as an aggregate. So one pair of {} for the std::array, and one for the internal raw array it holds.

edited Dec 20, 2017 at 6:51

answered Dec 20, 2017 at 6:42

StoryTeller - Unslander Monica

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5 Comments

Francis Cugler Over a year ago

hmm I knew you could use inline on member functions; just didn't know you could use it on member variables...

StoryTeller - Unslander Monica Over a year ago

@FrancisCugler - Only from C++17 onward.

Francis Cugler Over a year ago

Oh okay, I see; a new language feature that MS Visual Studio is probably lacking on...

StoryTeller - Unslander Monica Over a year ago

@FrancisCugler - Depends on your MSVC version, as ever :)

Francis Cugler Over a year ago

yeah my version states C2433: ... : 'inline' not permitted on data declarations...

Andrei R. · Accepted Answer · 2017-12-20 07:02:27Z

0

why don't you just try it with online compiler, supporting c++17?

template<unsigned... IDs>
class MyClass{
public:
    static constexpr std::array<unsigned, sizeof...(IDs)> ids { IDs... };
    MyClass() = default;
};

works fine. You don't need static const inline for variables, which are computed at compile time, just use static constexpr

answered Dec 20, 2017 at 7:02

Andrei R.

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5 Comments

Francis Cugler Over a year ago

are constexpr default constructible?

Andrei R. Over a year ago

@FrancisCugler, constexpr values are computed at compile-time. static fields in no way interfere with type traits of a class (including default constructibility).

StoryTeller - Unslander Monica Over a year ago

"constexpr values are computed" - No, they may be computed at compile time. And even then the array is still likely gonna occupy storage. Because operator[] is an ODR-use.

Andrei R. Over a year ago

@StoryTeller, occupies storage doesn't imply not computed at compile time. Values of this array (along with its size) are defined at compile-time and may be statically checked.

StoryTeller - Unslander Monica Over a year ago

@AndreiR. - I never said the storage precludes it. The storage argument is to clarify that it isn't something that exists "just at compile time". And yes, the values may be used when compiling. Emphasis on may. constexpr is great, but sometimes it fails us. Only way to know is to examine the generated object files.

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