第2章类模板：2.7 默认类模板实参

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mob60475707d572 2020-04-15 22:36:00

文章标签 #include sed 类模板默认值 ios 文章分类 代码人生

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() << ’\n’;
    intStack.pop();

    // 操作 double stack
    dblStack.push(42.42);
    std::cout << dblStack.top() << ’\n’;
    dblStack.pop();
}