C ++:对单例类中的实例的未定义​​引用 [英] C++: Undefined reference to instance in Singleton class

问题描述

我目前正在尝试将工厂实现为单例.我实际上使用了单例模式的教科书示例.这是 .h 文件:

I'm currently trying to implement a factory as a singleton. I practically used the textbook example of the Singleton pattern. Here's the .h file:

namespace oxygen{

class ImpFactory{

public:
    static boost::shared_ptr<ImpFactory> GetInstance();

private:
    static boost::shared_ptr<ImpFactory> mInstance;
};

这是 .cpp 文件:

and here's the .cpp file:

#include "impfactory.h"

using namespace oxygen;
using namespace boost;

shared_ptr<ImpFactory> ImpFactory::GetInstance(){
    if (mInstance.get() == 0)
        mInstance = shared_ptr<ImpFactory>(new ImpFactory());
    return mInstance;
}

代码编译，但出现链接器错误:

The code compiles, but I get a linker error:

../../lib/oxygen/liboxygen.so.3.2.4:未定义对 `oxygen::ImpFactory::mInstance' 的引用

../../lib/oxygen/liboxygen.so.3.2.4: undefined reference to `oxygen::ImpFactory::mInstance'

目前有三名学生难倒.有什么想法吗?

This currently has three students stumped. Any ideas?

推荐答案

您必须定义静态实例，而不仅仅是声明它.该定义创建了您引用的实际对象.

You must define the static instance, not just declare it. The definition creates the actual object you refer to.

在您的 cpp 文件中，添加以下行:

In your cpp file, add the line:

boost::shared_ptr<ImpFactory> ImpFactory::mInstance;

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