二进制表达式的无效操作数

Question

我收到以下错误：

二进制表达式的操作数无效（“basic_ostream<char,std::_1::char_traits<char>>”和“value_type”（又名“qElem”）） 发生在：

cout << "Your first task is to: " << tasks.front() << endl;

代码建议我在&tasks.front() 处放置一个&，但我不想接收0xfdlkajd 的值，我希望将第一个值存储在我的向量中。任何帮助将不胜感激。

我的代码：

#ifndef Queue_queue_h
#define Queue_queue_h

#include <iostream>
#include <string>
#include <vector>

using namespace std;


struct qElem { //qElem Struct

    string s;
    string p;
    qElem(string task, string priority) : s(task), p(priority) {}

};


//Establishing my Template and PriQueue Class
template <class T> //Template
class PriQueue
{
public:

    vector<qElem> tasks;

    //PriQueue();
    void enqueue(T str, int pri); //Adds to queue
    void dequeue(); //Deletes from queue
    void peek(); //Prints the first value in queue
    void size(); //Prints how many in queue
    void sort(vector<qElem*> &tasks); //Sort according to priority

private:

    int count = 0;

};

template <class T1>
void PriQueue<T1>::enqueue(T1 str, int pri) //Adding an element to the queue
{

    tasks.push_back(qElem(str, pri));

    sort(tasks); //NEW ERROR IS HERE

    count++;

}

template <class T1>
void PriQueue<T1>::dequeue() //Removing an element from the front of the queue
{
    //tasks.erase(tasks.begin());
    tasks.erase(tasks.begin());

    if (tasks.empty()) {
        cout << "You have no tasks!" << endl;
}

    else {


    }

    count--;

}

template <class T1>
void PriQueue<T1>::peek() //Returning a value at front of the queue (NOT removing it)
{
    if (tasks.empty()) {
        cout << "You have no tasks!" << endl;
}

else {
        cout << "Your first task is to: " << tasks.front().s << endl;

}

//Testing Purposes only
/*
 cout << "Your tasks are:";
 for (typename vector<T1>::iterator i = tasks.begin() ; i != tasks.end(); ++i)
 cout << " " << *i << ",";
 cout << endl;
 */

}

template <class T1>
void PriQueue<T1>::size() //Returning the number of items in the queue.
{
    cout << "You have " << count << " tasks in queue." << endl;

}

template <class T>
void PriQueue<T>::sort(vector<qElem*> &tasks) {
bool sortUp = true;
for(int i = 0; i < tasks.size();i++)
    for(int j = i+1; j < tasks.size(); j++)
    {
        if(sortUp)
        {
            if(tasks[i] > tasks[j])
                swap(tasks[i],tasks[j]);
        }
        else if(tasks[i] < tasks[j]) //else sortDown
            swap(tasks[i],tasks[j]);
    }
}


#endif

Answer 1

编译器不知道如何打印出qElem。如果您只想打印任务，请使用cout << "..." << tasks.front().s << endl;。编译器知道如何打印std::string。 （您也可以为qElem 实现自己的operator << 重载，但在这种情况下这可能有点过头了。）

请注意，您的代码有很多问题。当您的qElem 仅存储std::strings 时，您为什么要使用模板PriQueue？为什么要使用类成员（ss 和 pp）在构造函数中存储临时值？你的优先级是int（构造函数）还是std::string（qElem）还是T？

Answer 2

qElem 必须实现操作符

struct qElem { //qElem Struct

    string s;
    string p;
    qElem(string task, string priority) : s(task), p(priority) {}

    std::ostream& operator<<(std::ostream& os, const qElem & obj)
    {
      os << s << "/" << p;
      return os;
    }
};