由于使用静态变量导致的未定义的引用错误 [英] undefined reference error due to use of static variables

问题描述

我今天早些时候提出了一个关于单例的问题，我遇到了一些困难，我遇到了一些错误。我有以下代码：

Timing.h

  {
 
 public：
 static Timing * GetInstance（）; 
 private：
 Timing（）; 
 static Timing * _singleInstance; 
}; 
  

Timing.cpp

  #includeTiming.h
 
 static Timing * Timing :: GetInstance（）{//第一个错误
 if（！_singleInstance）{
 _singleInstance = new Timing（）; //第二个错误
} 
 return _singleInstance; 
} 
  

此代码中有两个错误，我无法弄清楚。 / p>

1. 方法 GetInstance（）在标题中声明为static。为什么在cpp文件中我必须省略字 static ？它给出错误：不能声明成员函数'static Timing * Timing :: GetInstance（）'有静态链接。正确的写法是：

     Timing * Timing :: GetInstance（）{...} 
     




2. 为什么我不能写 _singleInstance = new Timing（）; ？它给出错误：未定义引用Timing :: _ singleInstance。我通过定义 _singleInstance 作为cpp文件中的全局变量来解决此错误。

解决方案

1：static用于类声明之外的函数声明/定义时的本地链接。

本地链接意味着特定函数只能从该特定文件中的代码引用，并且对类中的方法没有太大意义。 p>

2：由于您的类声明可以多次包含，因此静态成员的实际存储应在cpp文件中定义：

  #includeTiming.h
 
 Timing * Timing :: _ singleInstance; 
 
 Timing * Timing :: GetInstance（）{//第一个错误
 if（！_singleInstance）{
 _singleInstance = new Timing //第二个错误
} 
 return _singleInstance; 
} 
  

I asked a question earlier today about singletons, and I'm having some difficulties understanding some errors I encountered. I have the following code:

Timing.h

class Timing {

public:
    static Timing *GetInstance();
private:
    Timing();
    static Timing *_singleInstance;
};

Timing.cpp

 #include "Timing.h"

 static Timing *Timing::GetInstance() {  //the first error
    if (!_singleInstance) {
        _singleInstance = new Timing();  //the second error
    }
    return _singleInstance;
}

There are two errors in this code which I can't figure out.

1. The method GetInstance() is declared in the header as static. Why in the cpp file do I have to omit the word static? It gives the error: "cannot declare member function ‘static Timing* Timing::GetInstance()’ to have static linkage". The correct way to write it is:

   Timing *Timing::GetInstance() { ... }  
   

2. Why can't I write _singleInstance = new Timing();? It gives the error: "undefined reference to Timing::_singleInstance". I solved this error by defining _singleInstance as a global var in the cpp file.

解决方案

1: static means "local linkage" when used for a function declaration/definition outside a class-declaration.

Local linkage means that the particular function can only be referenced from code inside this particular file, and that doesn't make much sense with a method in a class.

2: Since your class declaration can be included multiple times, the actual storage for the static member should be defined in the cpp-file:

#include "Timing.h"

Timing* Timing::_singleInstance;

Timing *Timing::GetInstance() {  //the first error
    if (!_singleInstance) {
        _singleInstance = new Timing();  //the second error
    }
    return _singleInstance;
}

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