靜態成員變量

Question

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

爲什麼我有一個「未定義參考Monitor::count」錯誤下面的代碼？謝謝！

#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

因爲你聲明，但不定義它。把下面的在你的.cpp文件之一（和僅一個）：

int Monitor::count = 0; 

Answer 2

您尚未定義靜態變量count。

class Monitor 
{ 
    // ... 
}; 

int Monitor::count = 0 ; 

// ...