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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!

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Hernan Villanueva
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  • Did you add the path of `Points.hpp` to your header path for compiler? – SwiftMango Sep 21 '14 at 00:42
  • Points.hpp is included in main.cpp. – Hernan Villanueva Sep 21 '14 at 00:52
  • You need to provide a *definition* for static data members if they're odr-used. – dyp Sep 21 '14 at 00:58
  • What do you mean @dyp? Should I not initialize them in the header file? – Hernan Villanueva Sep 21 '14 at 01:00
  • 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 Sep 21 '14 at 01:04
  • Oh, and by the way: `sqrt` is not something that you can (portably) use in a context where a constant expression is required. IIRC, it only works due to intrinsics on g++. – dyp Sep 21 '14 at 01:06
  • Both [g++4.9 and clang++3.5 accept this code](http://coliru.stacked-crooked.com/a/332054844769510e) if putting it in one TU and using C++14 mode, though I do not know if that's portable. – dyp Sep 21 '14 at 01:09
  • Related: http://stackoverflow.com/a/23436665 – dyp Sep 21 '14 at 01:10

1 Answers1

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;
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Hernan Villanueva
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  • The first thread you linked to is talking about built-in arrays , so it's not relevant to the `std::array` case. – M.M Mar 11 '16 at 01:15