调用Singleton getInstance()时出现未定义的符号错误

Question

我不知道为什么会这样,我按照这里发布的例子.

class Song {

private:
    // singletong
    Song();
    Song(Song const&); // don't implement
    void operator = (Song const&); //don't implement

public:
    // Singleton method
    static Song &getInstance(){
        static Song song;
        return song;
    }

};

如果我不打电话给班级,那就没问题了.我一打电话给Song课程就像这样:

Song::getInstance();
// also tried: Song &song = Song::getInstance();

Xcode不再需要构建项目了.我收到这个错误:

任何想法为什么会这样？

Answer 1

您没有实现自从在getInstance()函数中实例化对象以来必须存在的构造函数:

static Song song;

要么实现它内联(不受欢迎):

private:
    // singletong
    Song() {
        // Your implementation goes here
    }

或者在编译单元(例如Sound.cpp)中实现它(首选):

Song::Song(){
    // Your implementation goes here
}