指针返回值地址

Question

I have the following .h file

class Node
{
private :
    int m_data;
    Node* m_left;
    Node* m_right;
public:
    Node(int data) : m_data(data), m_left(nullptr), m_right(nullptr) {}
    Node* getLeft() { return m_left; }
    Node* getRight() { return m_right;  }
    int getData() { return m_data;  }
};

class Tree
{
private :
    Node * m_root;
    unsigned int m_size;
    void freeNode(Node*);
    bool insert(Node**, int );
public:
    Tree() : m_size(0) {}
    ~Tree();
    int findMaximumVericalSum();
    bool insert(int);
};

The implementation, I get an error - what is wrong and how should I fix it

'&' requires l-value

The address-of operator ( & ) must have an l-value as operand.

bool Tree::insert(Node** root, int data)
{
    Node* newNode = new Node(data);
    if (*root == nullptr) {
        *root = new Node(data);
    }
    else if (data < (*root)->getLeft()->getData())
    {
        // error '&' requires l-value
        insert(&((*root)->getLeft()), data);
    }
    else if (data > (*root)->getLeft()->getData())
    {

    }
}

Answer 1

To my surprise, I wasn't able to find a good duplicate for the question as asked (other issues notwithstanding).

The error you see comes from the following piece of code:

&((*root)->getLeft())

And can be vastly simplified into following very little piece:

int bar();
void foo(int*);

void baz() {
    foo(&bar());
}

(also can be seen on https://gcc.godbolt.org/z/PuwtuT )

This produces exactly the same compiler error. And the reason for this is that C++ prohibits taking an address of prvalues, and while whole rvalue/lvalue discussion is super complicated, it is easy to remember that when function returns a value, it is always an prvalue. (If function returns a reference, this is a different story). You can, however, pass an rvalue by either const or rvalue reference.

I can not immediately offer a solution to your problem, because I am not super clear what is your end goal. But I do hope that this answer could at least steer your thoughts and allow you to come up with another question.

Answer 2

 Node* getLeft() 

This function returns a prvalue.

 return m_left; 

The prvalue is initialised from this expression. It is not possible to get address of m_left from the prvalue. More generally, it is not possible to get the address of any prvalue. The operand of the addressof operator must be an lvalue.

 &((*root)->getLeft() 

what is wrong

Here, the operand of the addressof operator is a prvalue. The program is ill-formed.

how should I fix it

Don't try to apply the addressof operator to a prvalue expression.

If your intention is to pass a pointer to (*root)->m_left into the function, then you need some way to access that member.

• A solution is to give access to the member by it public, or through friendship.
• Another is to provide a public getter that returns a reference, rather than a copy of the member.
• A third solution is to add a setter function. In that case, you can use a local variable as argument to the recursion, and then call the setter with the variable after it was modified by the function.

---

Furthermore, Tree::insert is declared to return bool , but lacks any return statements. Behaviour of the program is undefined (or would be if it was well-formed in the first place).

PS Your function doesn't handle the case of data == (*root)->getLeft()->getData() .