Check for same instance of object using this pointer?

Question

I have a std::vector which contains some objects. How can I use memory addresses (do I want to use the this pointer for this purpose?) to check whether I am doing something with the same object?

For example:

void particle::calcFrc(std::vector<particle>& particles)
{
    vector3d tRel;
    mFrc.reset();
    for(unsigned int j = 0; j < particles.size(); j ++){

        if(... what goes here? ...){

            tRel = particles.at(j).mPos - mPos;

            if(tRel != zero()){
                // do stuff
            }
        }
    }
}

I want to do a check in the if statement to see whether particles.at(j) is referring to the same object from which this method was called.

`j` doesn't exist at that point. Is your `if` block in the wrong place? And have you thought about the `&` address-of operator? — Lightness Races in Orbit, Feb 13 '13 at 23:41
Much better :) You also removed unnecessary stuff, which is great. Now investigate `&`; your intuition about `this` is correct. — Lightness Races in Orbit, Feb 13 '13 at 23:43
I tried `if(&this == &particles.at(j))` however this didn't seem to work... :s What does a `this` pointer do? Does it point to the object which is currently doing the calling of some function? — FreelanceConsultant, Feb 13 '13 at 23:44
I was correct: http://publib.boulder.ibm.com/infocenter/lnxpcomp/v8v101/index.jsp?topic=%2Fcom.ibm.xlcpp8l.doc%2Flanguage%2Fref%2Fcplr035.htm — FreelanceConsultant, Feb 13 '13 at 23:45
`this` is already a pointer - by applying `&` to it you are obtaining a pointer to a pointer. More information about what `this` is and does may be found in your C++ book. — Lightness Races in Orbit, Feb 13 '13 at 23:47
The one that you have been learning from so far, selected from [this list](http://stackoverflow.com/questions/388242/the-definitive-c-book-guide-and-list). — Lightness Races in Orbit, Feb 14 '13 at 00:00
On a totally unrelated tangent, you're probably better off doing `particles[j]` instead of `particles.at(j)`. The `.at()` accessor does runtime bounds checking, which you don't need with how you are traversing your vector. (If your real code is doing anything fancy for computing the index rather than just iterating through as shown here, keeping the runtime check may nevertheless be prudent.) — Nicu Stiurca, Feb 14 '13 at 00:02
I haven't been learning from a book... I have been learning from StackOverflow, of course. @SchighSchagn thanks for your comment, that is a good idea. — FreelanceConsultant, Feb 14 '13 at 01:06
@EdwardBird: Then that's the problem determined. BTW please use `@` notification syntax - I stumbled upon your reply only by chance. — Lightness Races in Orbit, Feb 15 '13 at 15:43

score 0 · Accepted Answer · answered Feb 14 '13 at 04:12

if(... what goes here? ...){

I think what you are looking for is:

if (this != &particles[j]) {

... and the test will work fine, assuming that this object actually is one of the objects in the vector that was passed in to the method (and not just a temporary copy of one of them).

Check for same instance of object using this pointer?

1 Answers1