如何在另一个类中设置静态变量？

Question

我想设置在Coeffs.cpp一个静态变量：

#include "Coeffs.h" 
class Coeffs 
{ 
    public: 
    double Coeffs::alpha5b = 0.0; 
}; 

与头文件

#ifndef COEFFS_H 
#define GOEFFS_H 
class Coeffs 
{ 
    public: 
    static double alpha5b; 
}; 
#endif 

用下面的代码：

#include <iostream> 
#include <fstream> 
#include <string> 
#include "json/json.h" 
#include "Coeffs.h" 

using namespace std; 

int main() 
{ 
    cout << "start" << endl; 

    string json; 
    ifstream inputStream; 
    inputStream.open("coeffTest.json"); 
    inputStream >> json; 

    Json::Value root; 
    Json::Reader reader; 
    bool parseSuccess = reader.parse(json, root); 
    if(!parseSuccess) 
    { 
     cout << "failed" << endl; 
    } 
    else 
    { 
     Coeffs::alpha5b = 1.1; 
     //Coeffs::alpha5b = root.get("alpha5b", "NULL").asDouble(); 
     //double item1[] = root.get("delta21b", "NULL").asDouble(); 
     //cout << "alpha5b is: " << Coeffs::alpha5b << endl; 
    } 
    cout << "done" << endl; 
} 

，但每次我编译得到这个：

pottsie@pottsie:~/Documents/CoeffsJSON$ g++ -o JsonToCoeffs JsonToCoeffs.cpp -ljson_linux-gcc-4.6_libmt 
/tmp/ccFxrr0k.o: In function `main': 
JsonToCoeffs.cpp:(.text+0x10b): undefined reference to `Coeffs::alpha5b' 
collect2: ld returned 1 exit status 

我看了一些其他类似的问题，并找不到任何有效的工作。我试着添加一个构造函数并创建一个对象，但是我仍然得到相同的错误。 任何人都知道该怎么办？

Answer 1

类声明应放置在头部（Coeffs.h）

#ifndef COEFFS_H 
#define COEFFS_H 
class Coeffs 
{ 
    public: 
    static double alpha5b; 
}; 
#endif 

但在源文件（静态部件的.cpp的初始化， .cxx）：

#include "Coeffs.h" 

double Coeffs::alpha5b = 0.0; 

Answer 2

在Coeffs.cpp：

#include "Coeffs.h" 

double Coeffs::alpha5b = 0.0;