What is a static constructor?

This question was asked to me in an interview:

What is a static constructor开发者_StackOverflow社区?

Does it exist in C++? If yes, please explain it with an example.

C++ doesn’t have static constructors but you can emulate them using a static instance of a nested class.

class has_static_constructor {
    friend class constructor;

    struct constructor {
        constructor() { /* do some constructing here … */ }
    };

    static constructor cons;
};

// C++ needs to define static members externally.
has_static_constructor::constructor has_static_constructor::cons;

In C++, there is no static constructor. In C# (and probably in Java too), you can define static constructor which is called automatically by the runtime so as to initialize static members.

For further question and interest you can read this topic:

What is the rationale for not having static constructor in C++?

Since we do not technically have static constructors in C++, you have to decide whether it is worth it to do something tricky to force the issue (e.g. using a static instance of a nested class), or to just slightly restructure your code to call a static initializer early in your program's life.

#include <iostream>           // cout, endl

class Foo {
   public:
      static int s_count;

      // Constructor definition
      Foo (int l, int w, int h)
      {
         cout <<"Foo ctor called." << endl;
         length = l;
         width  = w;
         height = h;

         // Increase every time object is created
         s_count++;
      }

      int vol ()
      {
         return length * width * height;
      }

      static void initCount()
      {
         s_count = 0;
      }

      static int getCount()
      {
         return s_count;
      }

   private:
      double length;     // Length of a box
      double width;      // Width  of a box
      double height;     // Height of a box
};

// Initialize static member of class Foo
int Foo::s_count;  // Initializing here is non-deterministic

int main(void) {

   Foo::initCount();  // Initializing here is deterministic

   // Print total number of objects before creating object.
   cout << "Inital Count: " << Foo::getCount() << endl;

   Foo Foo1(3, 1, 1);    // Declare box1
   Foo Foo2(8, 6, 2);    // Declare box2

   // Print total number of objects after creating object.
   cout << "Final Count: " << Foo::getCount() << endl;

   return 0;
}

Output:

$ static_init_test
Inital Count: 0
Foo ctor called.
Foo ctor called.
Final Count: 2

I like this approach better; as a silver lining, it takes the non- out of non-deterministic initialization.

There is one gotcha though -- this technique is insufficient if you are trying to initialize static const variables. For static const variables, you will have to make them private to the class and provide getters for outsiders to read them.

Note: I updated this code -- it compiles and runs successfully with no warnings via:

g++ static_init_test.cpp -std=c++11 -o static_init_test

Static constructors exist in C# and Java.
They are used to initialize static members of a class.
The runtime executes them before the class is first used.

There is no such thing in C++. Constructors and destrcutors typically used to create or destruct instance of object. It's meaningless to call them without corresponding object instance. You can emulate them using a singleton pattern.

A static constructor is used to initialize static data of a class. C++ doesn't have static constructor. But a static constructor can be emulated by using a friend class or nested class as below.

class ClassStatic{
private:
    static char *str;
public:
    char* get_str() { return str; }
    void set_str(char *s) { str = s; }
    // A nested class, which used as static constructor
    static class ClassInit{
    public:
        ClassInit(int size){ 
            // Static constructor definition
            str = new char[size];
            str = "How are you?";
        }
    } initializer;
};

// Static variable creation
char* ClassStatic::str; 
// Static constructor call
ClassStatic::ClassInit ClassStatic::initializer(20);

int main() {
    ClassStatic a;
    ClassStatic b;
    std::cout << "String in a: " << a.get_str() << std::endl;
    std::cout << "String in b: " << b.get_str() << std::endl;
    a.set_str("I am fine");
    std::cout << "String in a: " << a.get_str() << std::endl;
    std::cout << "String in b: " << b.get_str() << std::endl;
    std::cin.ignore();
}

Output:

String in a: How are you?
String in b: How are you?
String in a: I am fine
String in b: I am fine

May be they mean this:

class Cat
{
private:
Cat();
public:
static Cat getCat() {return Cat(); }
}

In C++, there is no such thing as a static constructor.

I think static constructor is used to initialize any static data, or to perform a particular action that needs to be performed once only. It is called automatically before the first instance is created or any static members are referenced. In++, we dont have anything called static constructor but you can mimic the functionality of the static constructor. Take a look at this C# static constructor:

public class Bus  {
     // Static variable used by all Bus instances.
     // Represents the time the first bus of the day starts its route.
     protected static readonly DateTime globalStartTime;

     // Property for the number of each bus.
     protected int RouteNumber { get; set; }

     // Static constructor to initialize the static variable.
     // It is invoked before the first instance constructor is run.
     static Bus()
     {
         globalStartTime = DateTime.Now;

         // The following statement produces the first line of output, 
         // and the line occurs only once.
         Console.WriteLine("Static constructor sets global start time to {0}",
             globalStartTime.ToLongTimeString());
     }

     // Instance constructor.
     public Bus(int routeNum)
     {
         RouteNumber = routeNum;
         Console.WriteLine("Bus #{0} is created.", RouteNumber);
     }

     // Instance method.
     public void Drive()
     {
         TimeSpan elapsedTime = DateTime.Now - globalStartTime;

         // For demonstration purposes we treat milliseconds as minutes to simulate
         // actual bus times. Do not do this in your actual bus schedule program!
         Console.WriteLine("{0} is starting its route {1:N2} minutes after global start time {2}.",
                                 this.RouteNumber,
                                 elapsedTime.Milliseconds,
                                 globalStartTime.ToShortTimeString());
     }  }

 class TestBus  {
     static void Main()
     {
         // The creation of this instance activates the static constructor.
         Bus bus1 = new Bus(71);

         // Create a second bus.
         Bus bus2 = new Bus(72);

         // Send bus1 on its way.
         bus1.Drive();

         // Wait for bus2 to warm up.
         System.Threading.Thread.Sleep(25);

         // Send bus2 on its way.
         bus2.Drive();

         // Keep the console window open in debug mode.
         System.Console.WriteLine("Press any key to exit.");
         System.Console.ReadKey();
     }  }  /* Sample output:
     Static constructor sets global start time to 3:57:08 PM.
     Bus #71 is created.
     Bus #72 is created.
     71 is starting its route 6.00 minutes after global start time 3:57 PM.
     72 is starting its route 31.00 minutes after global start time 3:57 PM.      
*/

Here is the simplest static constructor in c++17:

#include <iostream>
class h {
    public:
    static inline int i;
    private:
    struct constructor {
        constructor() {
         i=3;
        }
    };
    static inline constructor cons;
};

int main() {
  std::cout << h::i;
}

See my answer to a similar question. C#'s static-constructor metaphor can be done in C++.

In C++, if someone says "static constructor", they are generally referring to "static initialization" (and destruction). It's not uncommon to use this terminology.

Other way to emulate static constructor behaviour is to use instance variable with private constructor and static factory method.

Cat* Cat::give_birth() {
  static Cat *myone = NULL;
  if (myone == NULL) {
    myone = new Cat();
  }
  return myone;
}