类中的静态成员变量[重复]

Question

可能重复：
What is an undefined reference/unresolved external symbol error and how do I fix it?

为什么以下代码出现“未定义对Monitor::count 的引用”错误？谢谢！

#include <iostream>

using namespace std;

class Monitor
{
    static int count;
public:
    void print() { cout << "incident function has been called " << count << " times" << endl; }
    void incident() { cout << "function incident called" << endl; count++; }
};

void callMonitor()
{
    static Monitor fooMonitor;
    fooMonitor.incident();
    fooMonitor.print();
}

int main()
{
    for (int i = 0; i < 5; i++)
        callMonitor();
    return 1;
}

Answer 1

因为您声明它但不定义它。将以下内容放入您的一个（并且仅一个）.cpp 文件中：

int Monitor::count = 0;

Answer 2

您尚未定义静态变量count。

class Monitor
{
     // ...
};

int Monitor::count = 0 ;

// ...