类中的静态成员变量

Question

可能重复:
什么是未定义的引用/未解析的外部符号错误,如何解决？

为什么我Monitor::count对以下代码有"未定义的引用"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

因为你声明它但不定义它.将以下内容放在一个(也是唯一一个).cpp文件中:

int Monitor::count = 0;