如何修复尝试通过重载和模板化插入运算符显示字符串对象数组时的空白输出？

Question

我在 main() 中声明了 container<std::string> c1; 来存储 10 个元素的字符串对象，即 aa bb cc dd . . . jj；但是，当我编译代码时，输出是空白的。如果我将那些 angular 括号中的类型更改为 char 并使用一个字母，它就可以正常工作。我认为在重载插入运算符的参数中传递的内容有问题。我试过添加 const 并设置其中一个参数 container<T2> const &cobj——它并没有帮助解决问题。

#include <iostream>
#include <string>

template<typename T>
class container
{
    template <typename T2>
    friend std::ostream& operator<<(std::ostream& out, container<T2> &cobj);
    // Postcondition: display the contents of container object cobj in the format shown in the below sample outputs
public:
    container();
    // Postcondition: data member n is initialized to -1 and all elements in the empty arr array are initialized to zero
    bool isEmpty();
    // Postcondition: returns true if nothing is stored in the container; returns false otherwise
    bool isFull();
    // Postcondition: returns true if the container object (i.e., the arr array) is full; returns false otherwise
    int size() const;
    // Postcondition: returns the “size” which is the actual number of elements currently stored in the containe robject; size <= capacity
    int capacity();
    // Postcondition: returns the storage capacity of the container. 
    bool insertBack(const T& val);
    //  Precondition: the container object is not full
    // Postcondition: if arr array is not full, n is incremented by 1; returns true with val is inserted at the end of the arr array 
    //                 Otherwise, returns false; the value is not inserted and program execution continues.

private:
    static const int CAPACITY = 10;     // physical size of the arr array or the storage capacity of a container object
    T arr[CAPACITY];            // arr array can store up to CAPACITY  (10 in our case) elements of any type 
    int n;                      // n is used as the subscript for the arr array. n is initialized to -1 for an empty array
                                // Each time a new value is inserted into the arr array, n must first be incremented 
                                // by 1. Since n has been initialized to -1, the first inserted value is stored in arr[0],
                                // and the 2nd inserted value will be in arr[1], etc.  and the nth inserted value will be 
                                // stored in arr[n – 1]. Obviously, n + 1 represents the actual number of elements
                                // stored in the array after n rounds of insertion.         
};

template<typename T2>
std::ostream& operator<<(std::ostream& out, container<T2> &cobj)
{
    std::cout << "Container storage capacity = " << cobj.capacity() << std::endl
        << "Currently, container contains " << cobj.size() << " elements." << std::endl
        << "The contents of the container:" << std::endl;

    static int funcCalls = 0; // A variable that collects the calls made to this function.
                              // The first call is needed to later output an unique string.
    funcCalls++;

    if (cobj.isEmpty() && funcCalls == 1)
    {
        std::cout << "*** Container is currently empty!" << std::endl << std::endl
            << "The container object is empty!" << std::endl
            << "Currently, the container object contains " << cobj.size() << " element(s) or value(s)";
    }
    else if (cobj.isEmpty())
    {
        std::cout << "*** Container is currently empty!";
    }
    else
    {
        for (int i = 0; i < cobj.size(); ++i)
        {
            std::cout << cobj.arr[i] << ' ';
        }
    }

    std::cout << std::endl << std::endl;

    return out;
}

template<typename T>
container<T>::container()
{
    n = 0;
    arr[CAPACITY] = {0};
}

template<typename T>
bool container<T>::isEmpty()
{
    return n == 0;
}

template<typename T>
bool container<T>::isFull()
{
    return n == CAPACITY - 1;
}

template<typename T>
int container<T>::capacity()
{
    return CAPACITY;
}

template<typename T>
int container<T>::size() const
{
    return n;
}

template<typename T>
bool container<T>::insertBack(const T& val)
{
    if (!isFull())
    {
        n++;
        arr[n - 1] = val;
        return 1;
    }
    else
    {
        return 0;
    }
}

int main()
{
    container<std::string> c1;

    std::cout << "We now insert 10 values at the back of the array one at a time:" << std::endl;    

    c1.insertBack("aa");
    c1.insertBack("bb");
    c1.insertBack("cc");
    c1.insertBack("dd");
    c1.insertBack("ee");
    c1.insertBack("ff");
    c1.insertBack("gg");
    c1.insertBack("hh");
    c1.insertBack("ii");
    c1.insertBack("jj");
    std::cout << c1;

}

Answer 1

在默认构造函数中

template<typename T>
container<T>::container()
{
    n = 0;
    arr[CAPACITY] = {0};
}

您正在尝试用索引 CAPACITY 初始化数组中不存在的元素。因此你的程序有未定义的行为，

其实数组是不需要初始化的。它将被默认初始化。

template<typename T>
container<T>::container() : n( 0 )
{
}

所有这些函数，isEmpry、isFull、capacity 和 size 都应该用限定符 const 声明。

函数isFull有逻辑错误。当容器的大小等于其容量时容器将满

template<typename T>
bool container<T>::isFull() const 
{
    return n == CAPACITY;
}

友元函数可以是 class 定义中定义的非模板函数，例如

template<typename T>
class container
{
    friend std::ostream& operator<<(std::ostream& out, const container<T> &cobj)
    {
        //...
    }
    //...

如果您将进行所有这些更改，那么程序输出将是

We now insert 10 values at the back of the array one at a time:
Container storage capacity = 10
Currently, container contains 10 elements.
The contents of the container:
aa bb cc dd ee ff gg hh ii jj

如何修复尝试通过重载和模板化插入运算符显示字符串对象数组时的空白输出？

How do I fix blank output from trying to display an array of string objects through an overloaded and templated insertion operator?

c++

arrays

templates

class

definition