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Templates and Static variables in C++
  • Difficulty Level : Medium
  • Last Updated : 29 May, 2017

Function templates and static variables:
Each instantiation of function template has its own copy of local static variables. For example, in the following program there are two instances: void fun(int ) and void fun(double ). So two copies of static variable i exist.




#include <iostream>
  
using namespace std;
  
template <typename T>
void fun(const T& x)
{
  static int i = 10;
  cout << ++i;
  return;
}
  
int main()
{    
  fun<int>(1);  // prints 11
  cout << endl;
  fun<int>(2);  // prints 12
  cout << endl;
  fun<double>(1.1); // prints 11
  cout << endl;
  getchar();
  return 0;
}

Output of the above program is:

  11
  12
  11

Class templates and static variables:
The rule for class templates is same as function templates
Each instantiation of class template has its own copy of member static variables. For example, in the following program there are two instances Test and Test. So two copies of static variable count exist.




#include <iostream>
  
using namespace std;
  
template <class T> class Test
{  
private:
  T val; 
public:
  static int count;
  Test()
  {
    count++;
  }
  // some other stuff in class
};
  
template<class T>
int Test<T>::count = 0;
  
int main()
{
  Test<int> a;  // value of count for Test<int> is 1 now
  Test<int> b;  // value of count for Test<int> is 2 now
  Test<double> c;  // value of count for Test<double> is 1 now
  cout << Test<int>::count   << endl;  // prints 2  
  cout << Test<double>::count << endl; //prints 1
     
  getchar();
  return 0;
}

Output of the above program is:

  2
  1

Please write comments if you find anything incorrect, or you want to share more information about the topic discussed above.

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