LNK2001静态属性和方法错误（Qt，C++）[重复]

Question

我有一个类EventType，它具有以下标题（仅相关行）：

#include<string>
#include<unordered_set>
#include<iostream>

class EventType
{
public:
    static EventType* getByName(std::string name);

    static EventType* getByID(std::string id);

    static void setAllEventTypes(std::unordered_set<EventType*> events);
    //...

private:
    static std::unordered_set<EventType*> allEvents; //stores all events
    std::string name;
    //...
    std::string akaID;
};

来源：

EventType* EventType::getByName(std::string name) {
    foreach(EventType * event, EventType::allEvents) {
        if(event->name == name) {
             return event;
        }
    }
    std::cout << "Error: Event with name " << name << "could not be found.\n";
}

EventType* EventType::getByID(std::string id) {
    foreach(EventType * event, EventType::allEvents) {
        if(event->akaID == id) {
            return event;
        }
    }
    std::cout << "Error: Event with aka.ID " << id << "could not be found.\n";
}

void EventType::setAllEventTypes(std::unordered_set<EventType*> events) {
    EventType::allEvents = events;
}

现在我收到了LNK2001-error：

eventtype.obj : error LNK2001: unresolved external symbol ""private: static class std::unordered_set<class EventType *,struct std::hash<class EventType *>,struct std::equal_to<class EventType *>,class std::allocator<class EventType *> > EventType::allEvents" (?allEvents@EventType@@0V?$unordered_set@PEAVEventType@@U?$hash@PEAVEventType@@@std@@U?$equal_to@PEAVEventType@@@3@V?$allocator@PEAVEventType@@@3@@std@@A)".

即使我没有使用 EventType 类外部的任何静态方法，我也会收到此错误。为什么会这样？ EventType不应该能够链接到自己吗？

Answer 1

您声明了allEvents，但没有定义它，您需要在源文件中这样做：

std::unordered_set<EventType*> EventType::allEvents;