Static counter in C ++

Question

I am trying to create a Data class whose objects contain a unique identifier.

I want the 1st identifier of the object to be 1, the second to 2, etc. I should use static int , but all objects have the same identifier, not 1, 2, 3 ...

This is the Data class:

 class Data { private: static int ID; public: Data(){ ID++; } }; 

How can I do this so that the first ID is 1, the second is 2, etc.?

Answer 1

It:

 class Data { private: static int ID; const int currentID; public: Data() : currentID(ID++){ } }; 

Besides a static counter, you also need an element associated with the instance.

Answer 2

If the identifier is static, then it will have the same value for all instances of the class.

If you want each instance to have consistent id values, you could combine a static attribute with a class variable, for example:

 class Data { private: static int ID; int thisId; public: Data(){ thisId = ++ID; } }; int Data::ID = 0; 

If the application is multithreaded, you will have to synchronize it with something like a pthread mutex.

Answer 3

Each instance of Data needs its own non-static member variable, which retains its identifier. A static variable can be used to store the last identifier used, which will be incremented in the Data constructor.

Instead of a static counter that is not thread safe, consider using boost uuid :

 #include <boost/lexical_cast.hpp> #include <boost/uuid/uuid.hpp> #include <boost/uuid/uuid_generators.hpp> #include <boost/uuid/uuid_io.hpp> using boost::lexical_cast; using boost::uuids::uuid; using boost::uuids::random_generator; std::string id_ = lexical_cast<std::string>((random_generator())()); 

Answer 4

Initialization of the static variable inside the function is allowed, so the solution may be something like this

  class Data { private: static int ID () { static int ID = 0; return ID ++; } int myId; public: Data(): myId (ID ()) { } }; 

Answer 5

where is the instance (not static) here? you need to declare a new instance id field like this

 int m_ID; 

then in your constructor do

 Data(){m_ID = ::InterlockedIncrement(&ID);} 

in a blocked or other thread safe way