error C2106: '=': left operand must be l-value when trying to change value from function-CodePudding

I have a linked list that contains a pointer to the first and last node and size which indicates how many nodes are there in the list. I have a function that returns the data in the first node.

I want to be able to change it using queue1.front() = 3;. However, I am getting

lvalue required as left operand of assignment

error while compiling

Node class

template<class T> class Node 
{
public:
    Node(const T& t);
    Node(const Node&) = default; // Copy Constructor set to default
    Node& operator=(const Node&) = default; // Assignment operator set to default

    T getData();
private:
    T m_data;
    Node* m_nextNode;
};

template<class T> Node<T>::Node(const T& t) 
{
    this->m_data = t;
    this->m_nextNode = nullptr;
}

template<class T> T Node<T>::getData()
{
    return this->m_data;
}

Queue class

template<class T> class Queue 
{
public:
    static const int DEFAULT_FIRST_INDEX = 0;
    static const int SIZE_EMPTY = 0;
    Queue();
    Queue(const Queue&) = default; // Copy Constructor set to default
    Queue& operator=(const Queue&) = default; // Assignment operator set to default
    T front();
private:
    Node<T>* m_head;
    Node<T>* m_tail;
    int m_size;
};

template<class T>
T Queue<T>::front() 
{
    if (this->m_size == Queue<T>::SIZE_EMPTY)
    {
        throw Queue<T>::EmptyQueue();
    }
    return this->m_head->getData();
}

CodePudding user response：

The member functions

T Node::getData() ...
T Queue<T>::front() ...

returning a copy of the member m_data, which is an r-value temporary. In order to work with assignment, you need non-const l-value reference qualified T. Hence, the compiler error.

Therefore, you need the following fix:

template<class T> class Node 
{
public:
    T& getData()
//  ^^
    {
        return this->m_data;
    }
    const T& getData() const;  // you might also need
//  ^^^^^^^^^          ^^^^^
};

template<class T> class Queue
{
public:   
    T& front()
//  ^^
    {
        // ....
        return this->m_head->getData();
    }
    const T& front() const; // you might also need
//  ^^^^^^^^         ^^^^^
};