C++ C++；为什么；deque迭代器不可解引用“；

我正在做一个样本测试

    #include <stdexcept>
#include <iostream>
#include <deque>

class TrainComposition
{
    //std::vector<int> wagons;

public:

    std::deque<int> wagons;

    void attachWagonFromLeft(int wagonId)
    {
        wagons.push_front(wagonId);
    }

    void attachWagonFromRight(int wagonId)
    {
        wagons.push_back(wagonId);
    }

    int detachWagonFromLeft()
    {
        std::deque<int>::iterator it = wagons.begin();
        wagons.pop_front();
        return *it;
    }

    int detachWagonFromRight()
    {
        std::deque<int>::iterator it = wagons.end()-1;
        wagons.pop_back();
        return *it;
    }
};

#ifndef RunTests
int main()
{
    TrainComposition tree;
    tree.attachWagonFromLeft(7);
    tree.attachWagonFromLeft(13);
    std::cout << tree.detachWagonFromRight() << "\n"; // 7 
    std::cout << tree.detachWagonFromLeft() << "\n"; // 13
    return 0;
}
#endif

“列车组合是通过从左侧和右侧连接和分离货车来构建的。例如，如果我们先从左侧连接货车7，然后从左侧连接货车13，再从左侧连接货车13，我们得到两辆货车的组合（从左到右连接13和7）.现在，第一辆可以从右侧分离的货车是7辆，第一辆可以从左侧分离的货车是13辆。实施一个模拟此问题的列车组合。”

    #include <stdexcept>
#include <iostream>
#include <deque>

class TrainComposition
{
    //std::vector<int> wagons;

public:

    std::deque<int> wagons;

    void attachWagonFromLeft(int wagonId)
    {
        wagons.push_front(wagonId);
    }

    void attachWagonFromRight(int wagonId)
    {
        wagons.push_back(wagonId);
    }

    int detachWagonFromLeft()
    {
        std::deque<int>::iterator it = wagons.begin();
        wagons.pop_front();
        return *it;
    }

    int detachWagonFromRight()
    {
        std::deque<int>::iterator it = wagons.end()-1;
        wagons.pop_back();
        return *it;
    }
};

#ifndef RunTests
int main()
{
    TrainComposition tree;
    tree.attachWagonFromLeft(7);
    tree.attachWagonFromLeft(13);
    std::cout << tree.detachWagonFromRight() << "\n"; // 7 
    std::cout << tree.detachWagonFromLeft() << "\n"; // 13
    return 0;
}
#endif

我使用std:：deque修改了代码，但在构建代码时，出现了一个错误：“deque迭代器不可解引用”。为什么会发生此错误以及如何解决它

    #include <stdexcept>
#include <iostream>
#include <deque>

class TrainComposition
{
    //std::vector<int> wagons;

public:

    std::deque<int> wagons;

    void attachWagonFromLeft(int wagonId)
    {
        wagons.push_front(wagonId);
    }

    void attachWagonFromRight(int wagonId)
    {
        wagons.push_back(wagonId);
    }

    int detachWagonFromLeft()
    {
        std::deque<int>::iterator it = wagons.begin();
        wagons.pop_front();
        return *it;
    }

    int detachWagonFromRight()
    {
        std::deque<int>::iterator it = wagons.end()-1;
        wagons.pop_back();
        return *it;
    }
};

#ifndef RunTests
int main()
{
    TrainComposition tree;
    tree.attachWagonFromLeft(7);
    tree.attachWagonFromLeft(13);
    std::cout << tree.detachWagonFromRight() << "\n"; // 7 
    std::cout << tree.detachWagonFromLeft() << "\n"; // 13
    return 0;
}
#endif

#包括
#包括
#包括
班列组成
{
//std：：矢量货车；
公众：
标准：德克货车；
void attachWagonFromLeft（int wagonId）
{
货车。向前推（wagonId）；
}
void attachWagonFromRight（int-wagonId）
{
四轮马车。向后推（四轮马车）；
}
int detachWagonFromLeft（）
{
std:：deque:：iterator it=wagons.begin（）；
四轮马车；
归还它；
}
int detachWagonFromRight（）
{
std:：deque:：iterator it=wagons.end（）-1；
四轮马车；
归还它；
}
};
#ifndef运行测试
int main（）
{
列车组合树；
树。从左（7）起连接货车；
树。从左（13）到右（13）；
std:：cout在
detachWagonFromLeft（）
中的这一指令序列是一个问题：

    #include <stdexcept>
#include <iostream>
#include <deque>

class TrainComposition
{
    //std::vector<int> wagons;

public:

    std::deque<int> wagons;

    void attachWagonFromLeft(int wagonId)
    {
        wagons.push_front(wagonId);
    }

    void attachWagonFromRight(int wagonId)
    {
        wagons.push_back(wagonId);
    }

    int detachWagonFromLeft()
    {
        std::deque<int>::iterator it = wagons.begin();
        wagons.pop_front();
        return *it;
    }

    int detachWagonFromRight()
    {
        std::deque<int>::iterator it = wagons.end()-1;
        wagons.pop_back();
        return *it;
    }
};

#ifndef RunTests
int main()
{
    TrainComposition tree;
    tree.attachWagonFromLeft(7);
    tree.attachWagonFromLeft(13);
    std::cout << tree.detachWagonFromRight() << "\n"; // 7 
    std::cout << tree.detachWagonFromLeft() << "\n"; // 13
    return 0;
}
#endif

std::deque<int>::iterator it = wagons.begin();
wagons.pop_front();
return *it;

detachWagonFromRight（）
函数中存在类似的问题，可以修复为：

    #include <stdexcept>
#include <iostream>
#include <deque>

class TrainComposition
{
    //std::vector<int> wagons;

public:

    std::deque<int> wagons;

    void attachWagonFromLeft(int wagonId)
    {
        wagons.push_front(wagonId);
    }

    void attachWagonFromRight(int wagonId)
    {
        wagons.push_back(wagonId);
    }

    int detachWagonFromLeft()
    {
        std::deque<int>::iterator it = wagons.begin();
        wagons.pop_front();
        return *it;
    }

    int detachWagonFromRight()
    {
        std::deque<int>::iterator it = wagons.end()-1;
        wagons.pop_back();
        return *it;
    }
};

#ifndef RunTests
int main()
{
    TrainComposition tree;
    tree.attachWagonFromLeft(7);
    tree.attachWagonFromLeft(13);
    std::cout << tree.detachWagonFromRight() << "\n"; // 7 
    std::cout << tree.detachWagonFromLeft() << "\n"; // 13
    return 0;
}
#endif

int temp = wagons.back();
wagons.pop_back();
return temp;

wagons.end（）。