default template arguments c++

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 ints: Stack<int> intStack; // stack of doubles 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 doubles that uses a std::deque<> to manage the elements internally.

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See Also:
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• Complete Tutorial of C++ Template's
• Standard Template Library Tutorial
• Inter Process Communication Tutorial
• Advance Programming in C & C++

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