C++: is “this” pointer useless?

Question

in the code below, it does not matter whether i put "this->" or remove it. it gives same output and result in both cases. So, what is the point of having the "this" pointer in C++? Are there other usages where it is essential? Thanks.

#include<iostream>
using namespace std;

class square{
    int l;
    int w;
    public:
        square(int x, int y){
            w = x;
            l = y;
        }
        int getArea(){
            return w * l;
        };
        bool AreaSmallerThan(square c){
            if(this->getArea() < c.getArea())
                return true;
            else
                return false;
        }

};

int main(){
    square A(2,3);
    square B(1,3);
    if(A.AreaSmallerThan(B))
        cout<<"A is smaller than B."<<endl;
    else
        cout<<"A is NOT smaller than B."<<endl;
    return 0;
}

Answer 1

Yes there are times it is essential. A classic case is in operator= , to avoid destroying resources during self-assignment.

for example https://stackoverflow.com/a/3975092/103167

Set& Set::operator=(const Set& setEqual)
{
  //first check for self assignment
  if (&setEqual == this)
    cout << "this is self assignment";
  return *this;
}

(Note that this isn't needed when using the copy-and-swap idiom)

Accessing members via this is also frequently seen in template code which inherits from a template parameter. Those inherited names can only be found during second phase lookup, which means they need to be qualified with this->

Answer 2

TL;DR: It has it's uses. If you choose good naming practices, you generally won't need to use it often.

There are a number of cases where you would want a "pointer to the current object", for example:

struct Foo
{
    void MakeCallback(eventid_t eventId)
    {
        scheduleCallback(eventId, callbackProxyFn, this);
    }

    static void callbackProxyFn(eventid_t eventId, Foo* foo)
    {
        // call 'callback' on the relevant object instance.
        foo->callback(eventId);
    }

    void callback(eventid_t eventId);
};

It can also be used to resolve conflicts between names in the current object and other scopes, if you choose to use terrible naming conventions.

void Foo::bar(int n)
{
    this->n = n;
}

You could avoid this (pun intended) scenario, as is common practice, by prefixing statics, globals and members:

class Player {
    int m_score;
public:
    Player(int score) : m_score(score) {}
};

Player g_player1;
static Player s_login; // yeah, I know, terrible, just an example tho.

A common use is in eliminating self in copy/comparison operators:

bool Foo::operator==(const Foo& rhs) const
{
    if (this == &rhs)
         return true;
    ...
}

You can also use it to produce a reference to the current object:

foo(const Foo&);

void foo(*this);

Answer 3

Random example...what if you had passed an int w, int l into your getArea() function...then you need to use the this keyword to differentiate between the local parameter

int getArea(int w, int l){
        return this->w * this->l;
    };

Another common example might be move assignment . I have pasted an example from a Tree datastructure project I coded.

     /* move assignment */
    TreeSet& operator= (TreeSet&& rhs)
    {
        clearAll(rootPtr);
        this->rootPtr = rhs.rootPtr;
        rhs.rootPtr = nullptr;
        return *this;
    }

And finally another example for an iterator I wrote...when overloading the ++ operator on an iterator, you want to return the resulting iterator..

 /* Update the current pointer to advance to the node
         * with the next larger value
         */
        const_iterator& operator++ () {
            //I have deleted all the logic for the sake of not taking up a ton of space..
            return *this;
        }

Answer 4

一般来说，当一个类的实例调用某个方法并且实例本身需要从该方法内部传递给该类之外的某个函数时，'this'指针会很有用。