3.5 Default Template Arguments

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For class templates you can also define default values for template parameters. These values are called default template arguments . They may even refer to previous template parameters. For example, in class Stack<> you can define the container that is used to manage the elements as a second template parameter, using std::vector<> as the default value:

  // basics/stack3.hpp  #include <vector>  #include <stdexcept>  template <typename T, typename CONT = std::vector<T> >  class Stack {    private:      CONT elems;  // elements  public:      void push(T const&);  // push element  void pop();  // pop element  T top() const;  // return top element  bool empty() const {  // return whether the stack is empty  return elems.empty();      }  };  template <typename T, typename CONT>  void Stack<T,CONT>::push (T const& elem)  {      elems.push_back(elem);  // append copy of passed  elem  }  template <typename T, typename CONT>  void Stack<T,CONT>::pop ()  {      if (elems.empty()) {          throw std::out_of_range("Stack<>::pop(): empty stack");      }      elems.pop_back();  // remove last element  }  template <typename T, typename CONT>  T Stack<T,CONT>::top () const  {      if (elems.empty()) {          throw std::out_of_range("Stack<>::top(): empty stack");      }      return elems.back();  // return copy of last element  } 

Note that we now have two template parameters, so each definition of a member function must be defined with these two parameters:

 template <typename T, typename CONT>  void Stack<T,CONT>::push (T const& elem)  {      elems.push_back(elem);  // append copy of passed  elem  } 

You can use this stack the same way it was used before. Thus, if you pass a first and only argument as an element type, a vector is used to manage the elements of this type:

 template <typename T, typename CONT = std::vector<T> >  class Stack {    private:      CONT elems;  // elements    }; 

In addition, you could specify the container for the elements when you declare a Stack object in your program:

  // basics/stack3test.cpp  #include <iostream>  #include <deque>  #include <cstdlib>  #include "stack3.hpp"  int main()  {      try {  // stack of  int  s:  Stack<int> intStack;  // stack of  double  s which uses a  std::deque<>  to mange the elements  Stack<double,std::deque<double> > dblStack;  // manipulate  int  stack  intStack.push(7);          std::cout << intStack.top() << std::endl;          intStack.pop();  // manipulate  double  stack  dblStack.push(42.42);          std::cout << dblStack.top() << std::endl;          dblStack.pop();          dblStack.pop();      }      catch (std::exception const& ex) {          std::cerr << "Exception: " << ex.what() << std::endl;          return EXIT_FAILURE;  // exit program with ERROR status  }  } 

With

 Stack<double,std::deque<double> > 

you declare a stack for double s that uses a std::deque<> to manage the elements internally.

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