Using a class variable as a default argument to the class member function

Question

I am building a LinkedList in C++.
Signature for addNode function:

const bool LinkedList::addNode(int val, unsigned int pos = getSize());

getSize() is a public non-static member function:

int getSize() const { return size; }

size is a non-static private member variable.
However, the error that I am getting is a nonstatic member reference must be relative to a specific object

How do I achieve this functionality?

Just for reference, here's the whole code:

#pragma once

class LinkedList {
    int size = 1;
    struct Node {
        int ivar = 0;
        Node* next = nullptr;
    };
    Node* rootNode = new Node();
    Node* createNode(int ivar);
public:
    LinkedList() = delete;
    LinkedList(int val) {
        rootNode->ivar = val;
    }
    decltype(size) getSize() const { return size; }
    const bool addNode(int val, unsigned int pos = getSize());
    const bool delNode(unsigned int pos);
    ~LinkedList() = default;
};

Some other tries include:

const bool addNode(int val, unsigned int pos = [=] { return getSize(); } ());
const bool addNode(int val, unsigned int pos = [=] { return this->getSize(); } ());
const bool addNode(int val, unsigned int pos = this-> getSize());

The current workaround I am currently using:

const bool LinkedList::addNode(int val, unsigned int pos = -1) {
    pos = pos == -1 ? getSize() : pos;
    //whatever
}

songyuanyao · Accepted Answer · 2020-08-07 08:21:35Z

4

The default argument is provided from the caller side context, which just doesn't know which object should be bound to be called on. You can add another wrapper function as

// when specifying pos
const bool LinkedList::addNode(int val, unsigned int pos) {
    pos = pos == -1 ? getSize() : pos;
    //whatever
}

// when not specifying pos, using getSize() instead
const bool LinkedList::addNode(int val) {
    return addNode(val, getSize());
}

edited Aug 7, 2020 at 8:21

answered Aug 7, 2020 at 8:12

songyuanyao

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4 Comments

Some programmer dude Over a year ago

@d4rk4ng31 One function with the pos argument (but without a default value), and one without a pos argument. The one without calls the function with (as shown in this answer).

RoQuOTriX Over a year ago

@d4rk4ng31 that's not unnecessary reproducal of code if only the top-most function is doing the main part. All other functions are calling up to them

songyuanyao Over a year ago

@d4rk4ng31 Because as the default argument this pointer needs to be provided at caller side context, but there isn't.

NotAProgrammer Over a year ago

@d4rk4ng31 It may be helpful to look at how C++ classes are laid out in memory, especially functions. There is only one set of class functions in memory for all objects of the same class type.

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