C++/Language/Static

A shared resource example.

<source lang="cpp">

1. include <iostream>
2. include <cstring>

using namespace std; class output {

 static char sharedResource[255]; // this is the shared resource
 static int inuse;        // buffer available if 0; in use otherwise
 static int oindex;       // index of sharedResource
 char str[80];
 int i;                   // index of next char in str
 int who;                 // identifies the object, must be > 0

public:

 output(int w, char *s) { 
    strcpy(str, s); 
    i = 0; 
    who = w; 
 }
 int putbuf() 
 {
   if(!str[ i ]) {         // done outputting
     inuse = 0;            // release buffer
     return 0;             // signal termination
   }
   if(!inuse)              // get buffer
       inuse = who; 
   if(inuse != who)        // in use by someone else
       return -1; 
   if(str[ i ]) {          // still chars to output
     sharedResource[oindex] = str[ i ];
     i++; oindex++;
     sharedResource[oindex] = "\0";// always keep null-terminated
     return 1;
   }
   return 0;
 }
 void show() { 
    cout << sharedResource << "\n"; 
 }

}; char output::sharedResource[255]; // this is the shared resource int output::inuse = 0; // buffer available if 0; in use otherwise int output::oindex = 0; // index of sharedResource int main() {

 output object1(1, "This is a test"), object2(2, " of statics");
 while(object1.putbuf() | object2.putbuf()) ; // output chars
 object1.show();
 return 0;

}

      </source>

A static member variable example.

<source lang="cpp">

1. include <iostream>

using namespace std; class myclass {

 static int i;

public:

 void setInt(int n) { 
    i = n; 
 }
 int getInt() { 
    return i; 
 }

};

int myclass::i; // Definition of myclass::i. i is still private to myclass. int main() {

 myclass object1, object2;
 object1.setInt(10); 
 cout << "object1.i: " << object1.getInt() << "\n"; // displays 10
 cout << "object2.i: " << object2.getInt() << "\n"; // also displays 10 
 return 0;

}

      </source>

Init static data before object creation

<source lang="cpp">

1. include <iostream>

using namespace std; class StaticMemberClass {

 static int i;

public:

 static void init(int x) {
    i = x;
 }
 void show() {
    cout << i;
 }

}; int StaticMemberClass::i; // define i int main() {

 StaticMemberClass::init(100);
 StaticMemberClass x;
 x.show(); 
 return 0;

}

      </source>

Static function and static variable

<source lang="cpp">

1. include <iostream>

using namespace std; class MyClass {

 static int i;

public:

 static void init(int x) {
    i = x;
 }
 void show() {
    cout << i;
 }

}; int MyClass::i; int main() {

 // init static data before object creation
 MyClass::init(30);
 MyClass x;
 x.show();
 return 0;

}

      </source>

Static function variable

<source lang="cpp">

1. include <iostream>

using namespace std; void printMessage(void); int main () {

  char choice;
  do {
     cout << "Enter Q to quit, any other character to continue: ";
     cin >> choice;
     if (choice == "Q")
        cout << "Input stopped";
     else
        printMessage(); 
     } while (choice != "Q");
  return 0;

} void printMessage (void) {

  static int times = 0;
  times++;
  cout << "This function called " << times << " times\n";

}

      </source>

Static Member Functions: its strictions

<source lang="cpp"> //A static member function does not have a this pointer. //cannot be a static and a non-static version of the same function. //A static member function may not be virtual. //static functions cannot be declared as const or volatile.

1. include <iostream>

using namespace std; class MyClass {

 static int resource;

public:

 static int getResource();
 void freeResource() { 
    resource = 0; 
 }

}; int MyClass::resource; // define resource int MyClass::getResource() {

 if(resource) return 0; // resource already in use
 else {
   resource = 1;
   return 1;            // resource allocated to this object
 }

} int main() {

 MyClass myObject1, myObject2;
 if(MyClass::getResource()) 
    cout << "myObject1 has resource\n";
 if(!MyClass::getResource()) 
    cout << "myObject2 denied resource\n";
 myObject1.freeResource();
 
 if(myObject2.getResource()) 
   cout << "myObject2 can now use resource\n";
 return 0;

}

      </source>

Static member functions: "preinitialize" private static data

<source lang="cpp">

1. include <iostream>

using namespace std; class static_type {

 static int i;

public:

 static void init(int x) {
    i = x;
 }
 void show() {
    cout << i;
 }

}; int static_type::i; int main() {

 static_type::init(100);   // init static data before object creation
 static_type x;
 x.show(); 
 return 0;

}

      </source>

Usage and effect of a static data member

<source lang="cpp">

1. include <iostream>

using namespace std; class shared {

 static int a;
 int b;

public:

 void set(int i, int j) {
   a=i; 
   
   b=j;
 }
 void show();

} ; int shared::a; void shared::show() {

 cout << "This is static a: " << a;
 cout << "\nThis is non-static b: " << b;
 cout << "\n";

} int main() {

 shared x, y;
 x.set(1, 1); 
 x.show();
 y.set(2, 2); 
 y.show();
 x.show(); 
 return 0;

}

      </source>

Use a static member variable independent of any object.

<source lang="cpp">

1. include <iostream>

using namespace std; class myclass { public:

 static int i;
 void setInt(int n) { 
    i = n; 
 }
 int getInt() { 
    return i; 
 }

}; int myclass::i; int main() {

 myclass object1, object2;
 
 myclass::i = 100;                      // set i directly,  no object is referenced.
 cout << "object1.i: " << object1.getInt() << "\n"; // displays 100
 cout << "object2.i: " << object2.getInt() << "\n"; // also displays 100
 return 0;

}

      </source>