C++ 如何初始化包含互斥锁的 class 的 static 变量

Question

I have a templated class with a mutex:

template <typename T> class A {
 public:
  std::mutex classMutex;
  T value;
  A(T initValue) : value(initValue){}
};   

and a second class with a static member of the first class:

class B{
  static A<double> test;
}; 
A<double> B::test = 0.0;

I keep getting an error: "copying variable of type 'A' invokes deleted constructor"

Answer 1

Thanks to @rafix07:

The solution is:

A<double> B::test{0.0};

"mutex is neither copyable nor movable, it means all copy and move operations are deleted by default (for class which contains mutex as data variable)

 A(const A&) = delete,
 A(A&&) = delete 

etc. When you call

A<double> B::test = 0.0 

double value 0.0 is converted to

A<double>(0.0) 

so you have

A<double> B::test = A<double>(0.0) 

because copy construction is deleted compiler refuse this line. With {} you are just calling A(0.0) constructor without any copy syntax." by @rafix07