c++ template笔记(2)模板类

 

1.自定义Stack模板类

#include <vector>
#include <stdexcept>

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

template<typename T>
void Stack<T>::pop ()
{
    if (elems.empty()) {
        throw std::out_of_range("Stack<>::pop(): empty stack");
    }
    elems.pop_back();         // remove last element
}

template <typename T>
T Stack<T>::top () const
{
    if (elems.empty()) {
        throw std::out_of_range("Stack<>::top(): empty stack");
    }
    return elems.back();      // return copy of last element
}

测试

#include <iostream>
#include <string>
#include <cstdlib>
#include "stack1.hpp"

int main()
{
    try {
        Stack<int>         intStack;       // stack of ints
        Stack<std::string> stringStack;    // stack of strings

        // manipulate int stack
        intStack.push(7);
        std::cout << intStack.top() << std::endl;

        // manipulate string stack
        stringStack.push("hello");
        std::cout << stringStack.top() << std::endl; 
        stringStack.pop();
        stringStack.pop();
    }
    catch (std::exception const& ex) {
        std::cerr << "Exception: " << ex.what() << std::endl;
        return EXIT_FAILURE;  // exit program with ERROR status
    }
}

由于多掉了一次pop所以报错

2.用typdef产生一个特定模板类别名

比如Stack<int>

typedef Stack<int> IntStack;
int main()
{
    IntStack intStack;
    intStack.push(7);
    std::cout << intStack.top() << std::endl;
}

3.模板类的特化

#include <deque>
#include <string>
#include <stdexcept>
#include "stack1.hpp"

template<>
class Stack<std::string> {
  private:
    std::deque<std::string> elems;  // elements

  public:
    void push(std::string const&);  // push element
    void pop();                     // pop element
    std::string top() const;        // return top element
    bool empty() const {            // return whether the stack is empty
        return elems.empty();
    }
};

void Stack<std::string>::push (std::string const& elem)
{
    elems.push_back(elem);    // append copy of passed elem
}

void Stack<std::string>::pop ()
{
    if (elems.empty()) {
        throw std::out_of_range
                ("Stack<std::string>::pop(): empty stack");
    }
    elems.pop_back();         // remove last element
}

std::string Stack<std::string>::top () const
{
    if (elems.empty()) {
        throw std::out_of_range
                ("Stack<std::string>::top(): empty stack");
    }
    return elems.back();      // return copy of last element
}

基于模板类之上，对于特定类型进行重新实现，比如上面就是一个对std:: string的特例实现,内部用deque实现

测试

#include <iostream>
#include <string>
#include <cstdlib>
#include "stack2.hpp"

int main()
{
    try {
        Stack<int>         intStack;       // stack of ints
        Stack<std::string> stringStack;    // stack of strings

        // manipulate int stack
        intStack.push(7);
        std::cout << intStack.top() << std::endl;
        intStack.pop();

        // manipulate string stack
        stringStack.push("hello");
        std::cout << stringStack.top() << std::endl; 
        stringStack.pop();
        stringStack.pop();
    }
    catch (std::exception const& ex) {
        std::cerr << "Exception: " << ex.what() << std::endl;
        return EXIT_FAILURE;  // exit program with ERROR status
    }
}

上面Stack<std::string>使用了特化的类

4.局部特化

若模板类有两个参数，那么确定其中一个参数,另一个未确定的话，则可以说是局部特化

这些概念与c#的泛型是相同的.

5.预设模板

与局部特化概念差不多,但保留了参数，给参数设置了一个默认的类型

#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
}

示例:这样如果用默认的类型，就可以少指定一个模板类的类型

#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 manage 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
    }
}