2.7 Default Class Template Arguments
2.7 默认类模板实参
As for function templates, you can define default values for class 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:
就像函数模板一样,你也可以为类模板参数定义默认值。比如,在类Stack<>里,你可以把用于管理元素的容器定义为第2个模板参数,并且使用std::vector<>作为它的默认值:
#include <vector> #include <cassert> template<typename T, typename Cont = std::vector<T>> class Stack { private: Cont elems; // elements public: void push(T const& elem); // push element void pop(); // pop element T const& top() const; // return top element bool empty() const { // return whether the stack is emptyreturn 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 () { assert(!elems.empty()); elems.pop_back(); // remove last element } template<typename T, typename Cont> T const& Stack<T,Cont>::top () const { assert(!elems.empty()); 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:
可以看到:我们的类模板含有两个模板参数,因此每个成员函数的定义必须具有这两个参数:
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:
你仍然可以像前面例子一样使用这个栈(stack)。就是说,如果你只传递每一个类型实参给这个类模板,那么将会利用vector来管理stack的元素:
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:
另外,当在程序中声明Stack对象的时候,你还可以指定容器的类型:
#include "stack3.hpp" #include <iostream> #include <deque> int main() { // stack of ints: Stack< int> intStack; // stack of doubles:使用std::deque<>来管理元素 Stack< double,std::deque< double>> dblStack; // 操作 int stack intStack.push(7); std::cout << intStack.top() << ’ ’; intStack.pop(); // 操作 double stack dblStack.push(42.42); std::cout << dblStack.top() << ’ ’; dblStack.pop(); }
With Stack< double,std::deque< double>>
you declare a stack for doubles that uses a std::deque<> to manage the elements internally.
使用Stack< double,std::deque< double>>可以声明一个“元素类型为double类型,并且使用std::deque<>来管理内部元素”的栈。