CODEWITHSUNDEEP
c++liststlconstants
Miles
Miles

Reputation: 1437

Accessing non-const members in list of objects

I would like to access members of objects contained in a list of such objects.

I have a class CApp with a member std::list<Particle> PList serving as a list of particles.

A class Particle has a member void Update() which from my understanding of const can't be const because it affects the instance (euler integration and stuff).

I would like to iterate through PList to update all of the particles.

A Particle constructor includes:

Particle::Particle(std::list<Particle>* PList_In) {
    PList = PList_In;
    PList->push_back(*this);
}

The following is called several times:

Particle(&PList);

So the list appears to be set up. As a side note, if anyone could explain what's actually there in terms of memory (pointers, references) at this point that would be great.

But basically this errs:

// Update all particles
std::list<Particle>::const_iterator iter;
for (iter = PList.begin(); iter != PList.end(); iter++) {
    iter->Update();
}

with

error: passing ‘const Particle’ as ‘this’ argument of ‘void Particle::Update()’ discards qualifiers

Not sure what to do with this, if more information/explanation is required, let me know.

Thanks in advance!

Upvotes: 0

Views: 407

Answers (2)

Naveen
Naveen

Reputation: 73443

By using const_iterator you are telling the compiler that you are not going to use this iterator on methods which modify the pointed to objects. Whether a method modifies the object or not is determined by its const qualifier. For example if you have a method declaration like int get() const; inside class A then this method is guaranteeing that it will not modify objects of class A. In your case it seems Update is not having the const qualifier hence the compiler is complaining that you can not call a non-const function using a const_iterator. You need to change the iterator type std::list<Particle>::iterator.

As a side note consider passing the list object by reference instead of using the pointer in your function.

Upvotes: 1

maxim1000
maxim1000

Reputation: 6365

By using const_iterator you say that you don't want to change list elements.

If you want to, use std::list::iterator.

Upvotes: 1

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