访问静态constexpr std ::数组没有类的定义 [英] Access static constexpr std::array without out-of-class definition

问题描述

我有一个定义一些数组的类。

Points.hpp

  class Points {
 
 public：
 
 static constexpr std :: array<双，1> a1 = {{
 +0.0}}; 
 
 static constexpr std :: array<双，2> a2 = {{
 -1.0 / std :: sqrt（3.0），
 +1.0 / std :: sqrt（3.0）}}; 
 
}; 
  

我的主文件然后使用这些数组。

< h2> main.cpp

  #includePoints.hpp
 
 int main（）
 {
 //关于如何访问点的示例。 
 auto point = Points :: a2 [0]; 
 
 //用点做某事。 
} 
  

当我编译代码时，使用C ++ 11和g ++ 4.8.2 ，我得到以下链接器错误：

 未定义引用`Points :: a2'
  

我试图创建一个Points.cpp文件，以便编译器可以从中创建一个目标文件。

Points.cpp

  #includePoints.hpp
  
 
 
 
但是没有修复链接器错误。
 
 
 
可以在类声明中将变量初始化为C ++ 11中的static constexpr，然后按照我的方式访问它们，如下面的问题所示：http://stackoverflow.com/a/24527701/1991500  
 
 
 
我需要为Points制作一个构造函数，然后实例化班上？我做错了什么？
 
 
 
感谢任何反馈！感谢！
解决方案
根据@dyp建议，我研究了静态数据成员的定义。
 
 
 
我的问题需要我定义我的Points类的静态成员变量。
 
 
 
按照这些问题的例子：
 
 
 
 下标时constexpr数组是否必须使用odr？ 
 
 
 
和
 
 
 
 在C ++中定义静态成员 
 
 
 
我需要添加：
  // in some .cpp 
 constexpr std :: array<双，1>点数：a1; 
  
 
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: http://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!
解决方案
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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