C++ STL: list with Pointers - Iterator cannot access?

Question

I am struggeling with an STL list that holds Pointers of my "Object" object.

I declared:

list<Object*> objectlist;

and inserted via:

this->objectlist.push_back(new Object(address,value,profit));

and tried to iterate like in maps and others:

list<Object*>::iterator iter;

iter = this->objectlist.begin();

while(iter != this->objectlist.end())
{
    iter->print();
}

Where print() is a public Method of class Object;

Whats wrong here?

I cannot access via iterator to the objects in the list ?

Stack Overflow is garbage · Accepted Answer · 2011-06-23 12:58:06Z

40

You need (*iter)->print();

Since you have an iterator to a pointer, you have to first de-reference the iterator (which gets you the Object*) then the arrow de-references the Object * and allows the call to print.

edited Jun 23, 2011 at 12:58

Stack Overflow is garbage

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answered Jun 23, 2011 at 12:54

RC.

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1 Comment

crashmstr Over a year ago

Yes, your iterator is not a pointer to your object type, but an iterator. Also don't forget your increment or you will have an infinite loop.

Kerrek SB · Accepted Answer · 2011-06-23 12:55:31Z

7

You are not incrementing your iterator! Change your while loop to a for loop like so:

for (list<Object*>::const_iterator iter = this->objectlist.begin(),
     end = this->objectlist.end();
     iter != end;
     ++iter)
{
    (*iter)->print();
}

(Also iter dereferences to a pointer, like the other answers have pointed out.)

answered Jun 23, 2011 at 12:55

Kerrek SB

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Comments

Juho · Accepted Answer · 2011-06-23 12:55:51Z

3

You can access the value pointed by iterator with *iter

Also, remember to increment the iterator in each iteration. Otherwise you get stuck in an endless loop.

Like this:

iter = this->objectlist.begin();

while(iter != this->objectlist.end())
{
    (*iter)->print();
    iter++;
}

answered Jun 23, 2011 at 12:55

Juho

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Comments

alanw · Accepted Answer · 2017-08-28 13:28:24Z

2

You could try the C++11 way (if possible). The dereference of the iterator comes free :)

for (auto& iter: this->objectlist)
{
     iter->print();
}

Also if print() is a const function you should use

for (const auto& iter: this->objectlist)

edited Aug 28, 2017 at 13:28

answered Feb 7, 2013 at 22:38

alanw

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1 Comment

GeertVc Over a year ago

2 questions: - "i" should be "iter", right? - is the "&" needed? Seems to work fine without it too

ollo · Accepted Answer · 2014-05-26 17:17:39Z

Instead of the for-loop you should use STL's for_each() - see the advantages of for_each over a simple for-loop.

Syntax

#include <algorithm> // 'for_each()' is in there

std::for_each(InputIterator first, // Iterator start position
              InputIterator last,  // Iterator end position
              Function fn);        // Unary function - executed on all elements in range

`for_each` example

Here's how your example looks std::for_each() instead of for( ... ):

list<Object*> objectlist;

// insert some elements …

// Deletes all elements of 'objectlist'
std::for_each(objectlist.begin(), objectlist.end(), DeleteObj<Object*>());

Unary function

The unary function is implemented using a template, so you can use it with any type. Just make sure T is a pointer type!

template<class T> class DeleteObj
{
public:
    bool operator()(T obj) const
    {
        delete obj;
        return true;
    }
};

Btw. you don't have to implement this as template. And: you can bind a member function instead of such a implementation too!

Documentation

小武哥 · Accepted Answer · 2011-06-23 12:55:05Z

0

you should use the following way:

(*iter)->print()

answered Jun 23, 2011 at 12:55

小武哥

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1 Comment

小武哥 Over a year ago

Something more to say: *iter means the element, so you should first get the element, and then to use it.

Frerich Raabe · Accepted Answer · 2011-06-23 12:56:27Z

0

An iterator references an element in the list, so for a list of type T, dereferencing an iterator yields a value of type T.

In your case, T is actually a pointer - Object*. Hence, the iter-> call (which dereferences the iterator) yields a pointer. You have to dereference that pointer to get to the actuala object.

Try using

(*iter)->print()

instead.

answered Jun 23, 2011 at 12:56

Frerich Raabe

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