storing iterators

[Thomas Grund]

Hi,

If I store an iterator to a std::vector and then push_back some elements
then the iterator is not valid anymore since the internal data of the
vector is reallocated.

Can I store an iterator to a std::map?
So can I assume that the internal data of a map (or set) is not
reallocated?


#include <vector>
#include <map>
#include <iostream>

using namespace std;

int main()
{
// ----------- first case, this does not work
vector<int> v;
v.push_back(1);
vector<int>::iterator i = v.begin();

cout << *i << '\n';

v.push_back(2);
v.push_back(2);
v.push_back(2);

cout << *i << '\n'; // undefined, reallocation of internal data

// -----------second case works ?
map<int, int> m;
m[1] = 1;
map<int, int>::iterator j = m.find(1);

cout << j->second << '\n';

m[2] = 3;
m[3] = 2;
m[1] = 5;

cout << j->second << '\n';
// question: Is it sure that I get 5 here ?
}

If this works, can I also change values such as j->second = 10; ?


Thanks a lot!

Thomas

 

[loufoque]

If I store an iterator to a std::vector and then push_back some elements
then the iterator is not valid anymore since the internal data of the
vector is reallocated.

Why not store the position instead since vectors give random access for
O(1) ?

 

[Thomas Grund]

Why not store the position instead since vectors give random access for O(1) ?

My question is concerning the map case. I want to build up some sparse
matrix structure. For this case I would like to have O(1) access to some
elements.
The example with the vector was just to tell that I am aware of possible
problems with using iterators.

Thomas

 

[Kai-Uwe Bux]

Hi,

If I store an iterator to a std::vector and then push_back some elements
then the iterator is not valid anymore since the internal data of the
vector is reallocated.

Can I store an iterator to a std::map?
Yes.

So can I assume that the internal data of a map (or set) is not
reallocated?

Iterators and references are guaranteed to remain valid: erase-ops only
invalidate the iterators to the erased items; insert-ops do not invalidate
any iterators.

#include <vector>
#include <map>
#include <iostream>

using namespace std;

int main()
{
// ----------- first case, this does not work
vector<int> v;
v.push_back(1);
vector<int>::iterator i = v.begin();

cout << *i << '\n';

v.push_back(2);
v.push_back(2);
v.push_back(2);

cout << *i << '\n'; // undefined, reallocation of internal data

// -----------second case works ?
map<int, int> m;
m[1] = 1;
map<int, int>::iterator j = m.find(1);

cout << j->second << '\n';

m[2] = 3;
m[3] = 2;
m[1] = 5;

cout << j->second << '\n';
// question: Is it sure that I get 5 here ?
}
Yes.

If this works, can I also change values such as j->second = 10; ?

Yes.


Best

Kai-Uwe Bux

 

[Thomas Grund]

(...)

Yes.


Best

Kai-Uwe Bux

Thanks!

Thomas