“未定义参考"；声明C ++静态成员变量 [英] "Undefined reference" to declared C++ static member variable

问题描述

我已经开始使用Java进行编程，我刚刚达到了我认为在语言知识方面的良好"水平.

I have started programming with Java, I just achieved which I consider as a "good" level in matter of language knowledge.

为了娱乐起见，我决定开始使用C ++进行编程，虽然我对这种语言还很陌生，但是我是一个学习速度很快的人，而且我认为它与Java并不太远.

For fun I decided to start programming using C++, I'm fairly new to this language but I'm a fast learner and I think it's not that far from Java.

我创建了一个测试类，该类具有一个值和一个名称作为属性，一个对象计数器作为全局变量.

I've created a test class which has a value and a name as attributes, and an objects counter as a global variable.

 #include<iostream>


/* local variable is same as a member's name */
class Test
{
    private:
       double x;
       std::string name;
       static int nb;
    public:
        Test(double x, std::string n)
        {
            this->x=x;
            this->name=n;
            nb=nb+1;
        }
       void setX (double x)
       {
           // The 'this' pointer is used to retrieve the object's x
           // hidden by the local variable 'x'
           this->x = x;
       }
       double getX()
       {
           return this->x;
       }
       std::string getName()
       {
           return this->name;
       }

       static int getNb()
       {
           return nb;
       }

};


int main()
{
   Test obj(3.141618, "Pi");
   std::cout<<obj.getX()<<" "<<obj.getName()<<" "<<Test::getNb()<<std::endl;
   return 0;
}

执行程序时输出此错误:

When executed the programm outputs this error :

In function `Test::Test(double, std::string)':
 (.text._ZN4TestC2EdSs[_ZN4TestC5EdSs]+0x4a): undefined reference to `Test::nb'
 (.text._ZN4TestC2EdSs[_ZN4TestC5EdSs]+0x53): undefined reference to `Test::nb'
In function `Test::getNb()':
 (.text._ZN4Test5getNbEv[_ZN4Test5getNbEv]+0x6): undefined reference to `Test::nb'
error: ld returned 1 exit status

一些中文给我.

我不明白.

推荐答案

在C ++中， static 变量本质上是围绕全局变量的语法糖.就像全局变量一样，必须在一个完整的源文件中定义，并带有:

In C++, static variables are essentially syntactic sugar around global variables. Just like global variables, they must be defined in exactly one source file, with:

int Test::nb;

，如果要使用特定值对其进行初始化，

and if you want to initialize it with a particular value,

int Test::nb = 5; // or some other expression

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