Return vector from function

Question

I have two classes, the first that contains a vector and a get-method like this:

std::vector<mytype> const& MyClassA::getvector(std::vector<mytype>::iterator &it)
{
    it = this->myiterator;

    return this->myvector;
}

std::vector<mytype> myvector;
std::vector<mytype>::iterator myiterator;

and I have another class where I should use the vector:

MyClassB::myfunction()
{
    std::vector<mytype>::iterator it;
    std::vector<mytype> vector = MyClassA->getvector(it);

    if (it > vector.begin())
    {
        ....
        (--it)->dosomestuff();
        ....
    }
}

in the if I get a runtime exception: iterators incompatible. Why?

EDIT 1

void MyClassA::setvector(mytype myelement)
{
    this->myvector.push_back(myelement);
    this->myiterator = this->myvector.end() - 1;
}

Where are you initializing myiterator?

Rocky Pulley
– Rocky Pulley

2011-06-23 14:44:31 +00:00
Commented Jun 23, 2011 at 14:44 — Rocky Pulley
– Rocky Pulley, Commented Jun 23, 2011 at 14:44
@Rocky added one more function to the answer

Stefano
– Stefano

2011-06-23 14:48:43 +00:00
Commented Jun 23, 2011 at 14:48 — Stefano
– Stefano, Commented Jun 23, 2011 at 14:48

Mark Ransom · Accepted Answer · 2011-06-23 14:45:59Z

10

You have made a copy of the vector in the calling code, and the iterator isn't valid because it's still pointing to the original vector.

You probably intended something like this:

const std::vector<mytype> & vector = MyClassA->getvector(it);

answered Jun 23, 2011 at 14:45

Mark Ransom

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11 Comments

Stefano Over a year ago

@Mark it works but I have still a problem, I should move the iterator in the MyClassB function and save the current position in the original variable stored in class MyClassA

Šimon Tóth Over a year ago

@Stefano Switch the parameter and the result. Store a copy in the parameter and return the new iterator. newvec.begin()+(olditer-oldvec.begin()) should work.

Stefano Over a year ago

@Let I need to get a vector as return from the function. It all works now but it doesn't save in the original iterator variable the new value, MyFunctionB is working on an iterator copy

Šimon Tóth Over a year ago

@Stefano Well, then take int as a parametr and store the distance there.

Stefano Over a year ago

@Let, good solution, anyway just for curiosity why isn't the function passing the original iterator rather than a copy?

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vines · Accepted Answer · 2011-06-23 14:57:40Z

1

Since you return std::vector<mytype> _const_&, the type of its iterator is std::vector::const_iterator.

EDIT:

Ah yes, I'm wrong and Mark Ransom is right. I leave this answer here because of the discussion in the comments, which itself may be useful to someone else.

edited Jun 23, 2011 at 14:57

answered Jun 23, 2011 at 14:43

vines

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7 Comments

Šimon Tóth Over a year ago

@sehe You can't get an iterator out of constant vector, but this is totally irrelevant in this case.

Ben Voigt Over a year ago

@sehe: @vines is correct, from a const vector&, only const member functions will be called, which return const_iterator (not const iterator).

Šimon Tóth Over a year ago

@Ben But he doesn't have a const vector.

Ben Voigt Over a year ago

@Let_Me_Be: He has a const vector&, the return type of getvector().

Mike Seymour Over a year ago

@Ben: that's irrelevant; the code compares an iterator to a const_iterator, which is valid (or would be if they came from the same vector). The problem has nothing to do with what is const and what isn't.

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