iterators as class members
Author
Hi, I've just begun to use STL and I have a question: I use an iterator for a list to iterate the list in a loop and the method that does this is called millions of times. Since now I create the two iterators each time the function is called (there are two lists) I thought that putting the iterator as class member and only initialize it when I need it could speed up a bit the algorithm (I don't expect to double the speed, just a small improvement).
Currently I get an error if I move the iterator in the class declaration. What should I do?
You will need to add typename as a prefix to it. I did not know that either until someone told me. Either that or typedef but I think the latter. Here's what I have:
typedef std::hash_map < const std::string, tBuffer >::iterator Iterator;
...
Iterator my_itr;
Although I swear I had to use typename somewhere...
typedef std::hash_map < const std::string, tBuffer >::iterator Iterator;
...
Iterator my_itr;
Although I swear I had to use typename somewhere...
yea, as Drew mention, thats a common problem with templates its called 'dependant typing'
template< class T >void myFunction() { T::N *a; // the compiler assumes a name qualified by a template parameter // refers to a static member (it has to assume something, since T // is unknown) so it basicaly thinks this is a multiplication op typename T::N *a; // typename tells the compiler, that T::N is a type}Quote:Original post by Drew_Benton
You will need to add typename as a prefix to it. I did not know that either until someone told me. Either that or typedef but I think the latter. Here's what I have:
typedef std::hash_map < const std::string, tBuffer >::iterator Iterator;
...
Iterator my_itr;
Although I swear I had to use typename somewhere...
You only have to use typename if the hash_map instantiation is dependent on a template parameter.
Author
That is what I have, following your idea:
typedef list<QLLight*>::iterator l_iter;
list<QLLight*> lights;
and then, in the class declaration:
l_iter l;
In the method I then use:
l = lights.begin();
But the compiler gives me the following error:
41 C:\Alessandro\progetti\quarklight\scene.cpp no match for 'operator=' in '((const QLScene*)this)->QLScene::l = (((const std::list<QLLight*, std::allocator<QLLight*> >*)((const QLScene*)this)) + 32u)->std::list<_Tp, _Alloc>::begin [with _Tp = QLLight*, _Alloc = std::allocator<QLLight*>]()'
[edit]I explained my problem in the wrong way: I'm not writing a template, I'm writing a class using a STL template (in this case list) but the compiler doesn't like a class member list.
typedef list<QLLight*>::iterator l_iter;
list<QLLight*> lights;
and then, in the class declaration:
l_iter l;
In the method I then use:
l = lights.begin();
But the compiler gives me the following error:
41 C:\Alessandro\progetti\quarklight\scene.cpp no match for 'operator=' in '((const QLScene*)this)->QLScene::l = (((const std::list<QLLight*, std::allocator<QLLight*> >*)((const QLScene*)this)) + 32u)->std::list<_Tp, _Alloc>::begin [with _Tp = QLLight*, _Alloc = std::allocator<QLLight*>]()'
[edit]I explained my problem in the wrong way: I'm not writing a template, I'm writing a class using a STL template (in this case list) but the compiler doesn't like a class member list.
Quote:Original post by cignox1
But the compiler gives me the following error:
41 C:\Alessandro\progetti\quarklight\scene.cpp no match for 'operator=' in '((const QLScene*)this)->QLScene::l = (((const std::list<QLLight*, std::allocator<QLLight*> >*)((const QLScene*)this)) + 32u)->std::list<_Tp, _Alloc>::begin [with _Tp = QLLight*, _Alloc = std::allocator<QLLight*>]()'
I've highlighted the problem, you must be using that piece of code inside a constant member function in which case you need to use a constant iterator instead i.e.:
struct foo {typedef list<QLLight*>::iterator l_iter;typedef list<QLLight*>::const_iterator const_l_iter;list<QLLight*> lights;..... void bar() const { const_l_iter l = lights.begin(); ...... }};Thats the great thing about constant correct-ness the compiler will stop you from doing something silly [smile]
Author
Thank you for pointing out this. I made the change, but now another error occours:
41 C:\Alessandro\progetti\quarklight\scene.cpp passing `const l_iter' as `this' argument of `std::_List_const_iterator<QLLight*>& std::_List_const_iterator<QLLight*>::operator=(const std::_List_const_iterator<QLLight*>&)' discards qualifiers
It compiles if I un-const the method. I will do this way for every method that will use the iterator.
Thank you all!
41 C:\Alessandro\progetti\quarklight\scene.cpp passing `const l_iter' as `this' argument of `std::_List_const_iterator<QLLight*>& std::_List_const_iterator<QLLight*>::operator=(const std::_List_const_iterator<QLLight*>&)' discards qualifiers
It compiles if I un-const the method. I will do this way for every method that will use the iterator.
Thank you all!
Quote:Original post by cignox1
Thank you for pointing out this. I made the change, but now another error occours:
41 C:\Alessandro\progetti\quarklight\scene.cpp passing `const l_iter' as `this' argument of `std::_List_const_iterator<QLLight*>& std::_List_const_iterator<QLLight*>::operator=(const std::_List_const_iterator<QLLight*>&)' discards qualifiers
Okay i think you need to should show us some code. You can't just put const on a non-constant iterator. a constant iterator is a real type well thats what it looks like your doing.
Quote:Original post by cignox1
It compiles if I un-const the method. I will do this way for every method that will use the iterator.
That is not a good idea, when you do operations on container that does not modify the state of the container at all you should use constant iterators.
Author
typedef list<QLLight*>::const_iterator l_iter;//Definition of the classesclass QLScene{ public: string name; vector<QLCamera*> cameras; list<QLLight*> lights; l_iter l;...};This is how I declared my class, lists and iterators, and the following method, if const, produces the already posted error:
QLLight* QLScene::GetLight(uint index) const{ if(index > lights.size()) return null; l = lights.begin(); //Here an error for(int i = 0; i < index; i++) ++l; //Here a similar one return *l;}If I remove const from the method, it compiles.
Okay i can see the problem [smile], your trying to modify the state of a data member inside a constant member function which you can't do unless it's a mutable data member. Either make the data member mutable (it's not generally a good idea but it depends on context) or just create a new local variable so either:
A:
or B:
notice how namespaces make prefixing type-names redundant and more cleaner [wink]
A:
#include <cstddef> // size_t#include <stdexcept> // out_of_range#include <iterator> // advance#include <list>#include <vector>#include <string>namespace QL { struct Camera; struct Light; struct Scene { typedef std::vector<Camera*> camera_list; typedef std::list<Light*> light_list; typedef light_list::iterator light_itr; typedef light_list::const_iterator const_light_itr; typedef std::size_t size_type; private: std::string name; std::vector<Camera*> cameras; std::list<Light*> lights; light_itr curr_itr; public: const Light& GetLight(size_type index) const { if(index > lights.size()) throw std::out_of_range("index out of range"); const_light_itr l = lights.begin(); std::advance(l, index); return **l; } };};or B:
#include <cstddef> // size_t#include <iterator> // advance#include <list>#include <vector>#include <string>namespace QL { struct Camera; struct Light; struct Scene { typedef std::vector<Camera*> camera_list; typedef std::list<Light*> light_list; typedef light_list::iterator light_itr; typedef light_list::const_iterator const_light_itr; typedef std::size_t size_type; private: std::string name; std::vector<Camera*> cameras; std::list<Light*> lights; mutable const_light_itr curr_itr; public: const Light& GetLight(size_type index) const { if(index > lights.size()) throw std::out_of_range("index out of range"); curr_itr = lights.begin(); std::advance(curr_itr, index); return **curr_itr; } };};notice how namespaces make prefixing type-names redundant and more cleaner [wink]
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