单例为模板，未定义错误

Question

 #include <iostream>
 template <class T>
 class singleton
 {
  public :

  static T* instance() {sinstance = new T; return sinstance;}

 private :
 static T* sinstance;
 singleton(singleton const&); 
 singleton& operator=(singleton const&);

 protected:
 singleton();
 ~singleton() {delete sinstance;}
 };

 template <class T> T* singleton<T>::sinstance=NULL;

 class circle
 {public:

  circle( int radius) { pointer = new int; *pointer = radius ;}
  circle( const circle &r) { pointer = new int; *pointer=*r.pointer;}
  ~circle() { delete pointer;}
  circle& operator=(const circle&pr) 
 {       *pointer =*pr.pointer; }
 int getData(){ return *pointer;}
 private:
 int *pointer;
 };

 typedef singleton<circle> circlesingleton; 

 int main()
 {
  circle(7);
  circlesingleton::instance()->getData();
 }

你好。我正在使用单例 class 做作业。如何纠正这个问题？我是初学者。这些代码是我在网上搜索并添加了一些代码。谢谢。我用作模板的单例 class 中的第一个 class。 class 中的圆圈 class 我们的教授告诉我们使用规范形式

Errors :
project1singleton.cpp: In instantiation of ‘static T* 
singleton<T>::instance() [with T = circle]’:
project1singleton.cpp:40:19:   required from here
project1singleton.cpp:7:51: error: no matching function for call to 
‘circle::circle()’
static /*singleton*/ T* instance() {sinstance = new T /*singleton*/ ;return 
sinstance;}
                                               ^
project1singleton.cpp:7:51: note: candidates are:
project1singleton.cpp:26:7: note: circle::circle(const circle&)
   circle( const circle &r) { pointer = new int; *pointer=*r.pointer;}
   ^
project1singleton.cpp:26:7: note:   candidate expects 1 argument, 0 provided
project1singleton.cpp:25:7: note: circle::circle(int)
   circle( int radius) { pointer = new int; *pointer = radius ;}
   ^

Answer 1

您的 class circle 没有默认（无参数）构造函数，因此 new T 在 class singleton 中无效。

单例为模板，未定义错误

singleton as a template ，undefined error

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