如何将带有默认参数的模板函数传递给std :: call_once [英] How to pass template function with default arguments to std::call_once

问题描述

我需要在我的模板化单例类中使用std :: call_once，但是当前下面的示例代码无法编译:

I need to use std::call_once in my templatized singleton class but currently below sample code is not compiling :

std::once_flag flag;
class LifeTrackerHelper
{
public:
template<class T>
inline static int SetLongevity(std::unique_ptr<T>& pobj,unsigned int longevity = 0)
{
    return 0;
}
};
template<class T>
class Singleton
{
   public:    
   inline static T* getInstance()
   {
     static std::unique_ptr<T> ptr(new T());  
     std::call_once(flag,&LifeTrackerHelper::SetLongevity<T>,ptr);  
     //static int i = LifeTrackerHelper::SetLongevity<T>(ptr);
     // if call_once is commented and above line uncommented this will work
     return ptr.get();
   }
};
class Test
{
    public:
    void fun()
    {
        std::cout<<"Having fun...."<<std::endl;
    }
};
int main()
{

  Singleton<Test>::getInstance()->fun(); 
}

因此需要帮助以了解如何正确使用std :: call_once.

So need help in understanding how to properly use std::call_once here.

推荐答案

您的问题是&LifeTrackerHelper::SetLongevity<T>是一个期望unique_ptr和unsigned int的函数指针，但它仅获得一个参数.尽管实际函数的第二个参数具有默认值，但在由函数指针调用时，它需要两个参数.

Your problem is &LifeTrackerHelper::SetLongevity<T> is a function pointer expecting a unique_ptr and an unsigned int, but it only gets the one argument. While the actual function has a default value for the second argument, it needs both arguments when called by a function pointer.

您可以通过传递另一个参数来解决它:

You can fix it by passing another argument:

std::call_once(flag, &LifeTrackerHelper::SetLongevity<T>, ptr, 0);

或者您也可以将其包装在lambda中:

Or you can wrap it in a lambda:

std::call_once(flag, [](std::unique_ptr<T>& p){ return LifeTrackerHelper::SetLongevity<T>(p); }, ptr);

---

根据 cppreference ，在C ++ 17之前，call_once将被复制或移动.到目前为止，通过unique_ptr并没有出现任何错误，但是在其上使用std::ref可能是明智的选择.

---

According to cppreference, before C++17 the arguments to call_once will be copied or moved. So far, I haven't gotten any errors passing a unique_ptr, but it might be wise to use std::ref on it.

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