On the book <Generic Programming and the STL>( Matthew . H . Austern ),this function is defined as
iterator set::begin() const.
However, why should a const object returns a non-const iterator?
Then, I found, in this book, the semantic of set::iterator is defined as same as set::const_iterator. Both of them must be const!
I tried to read the source of GNU STL(version 3.4.1).They were using a red-black tree to implant it (std::set has a _RBtree.) .Both set::iterator and set::const_iterator are simply defined as _RBtree::const_iterator.
But, in VC++, it is deferent.
There is a base class named xtree which also a red-black tree. Then the class map and set inherit from xtree (which implantation is better? is-a? has-a?).They don't defined iterator and const_iterator theirself,just inherit them from the base class xtree,and the member function begin() is defined as :
const_iterator set::begin() const;
iterator set::begin().
So ,we can initialize a mutable set, get the begin of it, then try to modify it!
The following code has been compiled successful under VC++2003
int a[4]={1,2,3,4};
std::set<int> s(a,a+4);
std::set<int>::iterator i=s.begin();
(*i) = 9;
std::cout<<"Now s = ";
std::copy(s.begin(),s.end(),std::ostream_iterator< int>(std::cout,","));
std::cout<<std::endl;
std::set<int>::const_iterator p=s.find(9);
if(p != s.end() )
std::cout<<(*p);
else std::cout<<"Cannot find the element special"<<std::endl;
A set must be a sorted container, but now it is not! So anything terrible things it would happen.
Is this a bug of VC++?
A bug of VC++.net STL
--
---------------snnn-------------
---http://snnn.blogone.net----- 11 3203
On Thu, 06 Jan 2005 01:18:22 +0800, snnn
<sn*****@gmail.com.haha.removeme> wrote: On the book <Generic Programming and the STL>( Matthew . H . Austern ),this function is defined as iterator set::begin() const. However, why should a const object returns a non-const iterator? Then, I found, in this book, the semantic of set::iterator is defined as same as set::const_iterator. Both of them must be const! I tried to read the source of GNU STL(version 3.4.1).They were using a red-black tree to implant it (std::set has a _RBtree.) .Both set::iterator and set::const_iterator are simply defined as _RBtree::const_iterator. But, in VC++, it is deferent. There is a base class named xtree which also a red-black tree. Then the class map and set inherit from xtree (which implantation is better? is-a? has-a?).They don't defined iterator and const_iterator theirself,just inherit them from the base class xtree,and the member function begin() is defined as : const_iterator set::begin() const; iterator set::begin(). So ,we can initialize a mutable set, get the begin of it, then try to modify it! The following code has been compiled successful under VC++2003 int a[4]={1,2,3,4}; std::set<int> s(a,a+4);
Shouldn't this be:
std::set<int> s(a,a+3);
??
Note that s is non-const...
std::set<int>::iterator i=s.begin(); (*i) = 9; std::cout<<"Now s = "; std::copy(s.begin(),s.end(),std::ostream_iterator< int>(std::cout,",")); std::cout<<std::endl; std::set<int>::const_iterator p=s.find(9); if(p != s.end() ) std::cout<<(*p); else std::cout<<"Cannot find the element special"<<std::endl;
A set must be a sorted container, but now it is not! So anything terrible things it would happen. Is this a bug of VC++? [or] A bug of VC++.net STL
You are mistaken... this is the declaration of begin() (actually two
declarations) for std::set as delivered with the Microsoft VCToolkit:
<quote>
iterator begin()
{ // return iterator for beginning of mutable
sequence
return (_TREE_ITERATOR(_Lmost()));
}
const_iterator begin() const
{ // return iterator for beginning of nonmutable
sequence
return (_TREE_CONST_ITERATOR(_Lmost()));
}
</quote>
As you can see, the "begin()" which returns a const_iterator is also
const, therefore you CANNOT modify the set if the set itself is const.
--
Bob Hairgrove No**********@Home.com
Bob Hairgrove wrote: On Thu, 06 Jan 2005 01:18:22 +0800, snnn <sn*****@gmail.com.haha.removeme> wrote:
int a[4]={1,2,3,4}; std::set<int> s(a,a+4);
Shouldn't this be: std::set<int> s(a,a+3); ??
That would only consume 3 elements of 's'.
Note that s is non-const...
What difference would that make? Values (r-values) used to
initialise the elements of the set<> cannot transfer their
const-ness, can they?
[...]
On Wed, 05 Jan 2005 15:36:30 -0500, Victor Bazarov
<v.********@comAcast.net> wrote: Note that s is non-const...
What difference would that make? Values (r-values) used to initialise the elements of the set<> cannot transfer their const-ness, can they?
The subject line says it all:
"which type should "std::set::begin() const" return?"
Answer:
std::set::const_iterator (which it does).
The OP seems to think it doesn't.
--
Bob Hairgrove No**********@Home.com
On Wed, 05 Jan 2005 15:36:30 -0500, Victor Bazarov
<v.********@comAcast.net> wrote: Bob Hairgrove wrote: On Thu, 06 Jan 2005 01:18:22 +0800, snnn <sn*****@gmail.com.haha.removeme> wrote:
int a[4]={1,2,3,4}; std::set<int> s(a,a+4);
Shouldn't this be: std::set<int> s(a,a+3); ??
That would only consume 3 elements of 's'.
Of course, you are right about this. I really need to RTFM some
more...
--
Bob Hairgrove No**********@Home.com
Bob Hairgrove wrote: On Wed, 05 Jan 2005 15:36:30 -0500, Victor Bazarov <v.********@comAcast.net> wrote:
Note that s is non-const...
What difference would that make? Values (r-values) used to initialise the elements of the set<> cannot transfer their const-ness, can they?
The subject line says it all: "which type should "std::set::begin() const" return?"
Answer: std::set::const_iterator (which it does).
The OP seems to think it doesn't.
Well, yes. I just was curious on your 'notice that s is non-const'
comment placed right after another comment about the initialisation
of 's'. It shouldn't matter whether 's' is const or not when 's'
is initialised, no? I mean, it will be initialised just as well if
it were const...
The second param is a past-the-end iterator
Whether s is const or non-const,a set should never return a non-const
iterator.We can never modify a set by a iterator pointing to it's
element.Because a set must be a sorted container.You can insert and
remove element into/off it,but never manual modify it.
"snnn" <sn*****@gmail.com> wrote... Whether s is const or non-const,a set should never return a non-const iterator.We can never modify a set by a iterator pointing to it's element.Because a set must be a sorted container.You can insert and remove element into/off it,but never manual modify it.
I think you need to submit this as a proposal to comp.std.c++.
snnn wrote: Whether s is const or non-const,a set should never return a non-const iterator.We can never modify a set by a iterator pointing to it's element.Because a set must be a sorted container.You can insert and remove element into/off it,but never manual modify it.
Untrue.
class X {
int a;
int b;
public:
X(int a, int b) : a(a),b(b) {}
bool operator<( X const& rhs ) { return a<rhs.a; }
};
std::set<X> sox = foo();
sox.begin()->b=0; // safe
Regards,
Michiel Salters
Have you got the rigth answer??
I've been thinking about it since we last meet. This thread has been closed and replies have been disabled. Please start a new discussion. Similar topics |
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