Access static constexpr std::array without out-of-class definition

Question

I have a class that defines some arrays.

Points.hpp

class Points {

public:

    static constexpr std::array< double, 1 > a1 = { {
            +0.0 } };

    static constexpr std::array< double, 2 > a2 = { {
            -1.0 / std::sqrt( 3.0 ),
            +1.0 / std::sqrt( 3.0 ) } };

};

My main file then uses these arrays.

main.cpp

#include "Points.hpp"

int main()
{
    // Example on how to access a point.
    auto point = Points::a2[0];

    // Do something with point.
}

When I compile my code, using C++11 and g++ 4.8.2, I get the following linker error:

undefined reference to `Points::a2'

I attempted to create a Points.cpp file so that the compiler can create an object file from it.

Points.cpp

#include "Points.hpp"

But that did not fix the linker error.

I was under the impression that it was possible to initialize variables as static constexpr in C++11 in the class declaration, and then access them the way I'm doing it, as shown in this question: https://stackoverflow.com/a/24527701/1991500

Do I need to make a constructor for Points and then instantiate the class? What am I doing wrong?

Any feedback is appreciated! Thanks!

Did you add the path of Points.hpp to your header path for compiler? — SwiftMango
– SwiftMango, Commented Sep 21, 2014 at 0:42
You need to provide a definition for static data members if they're odr-used. — dyp
– dyp, Commented Sep 21, 2014 at 0:58
What do you mean @dyp? Should I not initialize them in the header file? — Hernan Villanueva
– Hernan Villanueva, Commented Sep 21, 2014 at 1:00
That is not the problem. A static data member is only declared inside the class body (declare => the name exists), it is defined outside the class body (define => associate an object with the name). The initialization of static data members within the class body is a strange feature that allows the compiler to use the value of the variable (at compile-time), without having to query the value from the actual object. — dyp
– dyp, Commented Sep 21, 2014 at 1:04

Community · Accepted Answer · 2017-05-23 10:28:34Z

2

As per @dyp suggestion, I looked into the definition of static data members.

My problem requires me to define the static member variables of my Points class.

Following the examples in these questions:

Is a constexpr array necessarily odr-used when subscripted?

and

Defining static members in C++

I need to add:

// in some .cpp
constexpr std::array< double, 1 > Points::a1;

edited May 23, 2017 at 10:28

CommunityBot

11 silver badge

answered Sep 21, 2014 at 3:28

Hernan Villanueva

5315 silver badges13 bronze badges

Sign up to request clarification or add additional context in comments.

1 Comment

M.M Over a year ago

The first thread you linked to is talking about built-in arrays , so it's not relevant to the std::array case.

Collectives™ on Stack Overflow

Access static constexpr std::array without out-of-class definition

Points.hpp

main.cpp

Points.cpp

1 Answer 1

1 Comment

Your Answer

Linked

Hot Network Questions

Collectives™ on Stack Overflow

Points.hpp

main.cpp

Points.cpp

1 Answer 1

1 Comment

Your Answer

Sign up or log in

Post as a guest

Linked

Related