_gnu Iterator Error-g++

ashwinipahuja · 05-18-2004, 11:50 PM

Following piece of code is giving error under g++

#include<iostream>
#include<vector>
using namespace std;
class xyz
{
public:

};
int main()
{
vector<xyz *>::iterator *p;
vector<xyz *>::iterator *x=NULL;
p= x;
if( p && *p)
{
cout<<"P";
}
}

The error message is
gnu.cpp: In function `int main()':
gnu.cpp:14: no match for `__gnu_cxx::__normal_iterator<xyz**, std::vector<xyz*,
std::allocator<xyz*> > >*& && __gnu_cxx::__normal_iterator<xyz**,
std::vector<xyz*, std::allocator<xyz*> > >&' operator
gnu.cpp:14: candidates are: operator&&(bool, bool) <built-in>
gnu.cpp:18:2: warning: no newline at end of file

If anyone has some idea then please help
Thanx in advance
Ashwini

dakensta · 05-19-2004, 07:05 AM

As the compiler says, you are trying to perform this operation.

__gnu_cxx::__normal_iterator<xyz**, std::vector<xyz*,std::allocator<xyz*> > >*&
&&
__gnu_cxx::__normal_iterator<xyz**,std::vector<xyz*, std::allocator<xyz*> > >&

and the only available one is:
gnu.cpp:14: candidates are: operator&&(bool, bool) <built-in>

How do you propose to convert from an iterator to bool?

Code:

#include <iostream>
#include <vector>
using namespace std;
int main()
{
        vector<int>::iterator p;
        if (&p) cout << "Pointer OK\n";
        if (p)  cout << "Compiler will balk here - can't convert iter to bool\n";
        if (&p && p) cout << "Compiler will balk here, too, with the error above\n"
}

not to mention the fact that you are dereferencing a NULL pointer (through *p) which will result in undefined behaviour if at runtime you actually got to the bool(*p) bit.

You could get around this through operator overloading (which will be both error prone and most likely needlessly complex), however I have to ask why you are using pointers to iterators - what is wrong with just using an iterator?

ashwinipahuja · 05-26-2004, 12:57 AM

Hi dakensta,
But i dont want to mess up with the source code,
So i am looking for a solution in the existing code....
wut changes will i have to make.

dakensta · 05-26-2004, 08:14 AM

Every time you try and dereference p you must check it to be not null.

This is why you have the line (sorry, I missed this before, tut tut) here, but every check must be preceded by this:

if (p && *p)

Secondly you need to decide what makes your iterator valid. I can't answer this, because I don't know the context.

Maybe you could post a description of what you would like *p to give as true,

I would guess that you want to do is check that the iterator is in a certain container, in which case you are going to have to change either the iterator class (maybe derive from container::iterator) and add some means or add a contianer to the iterator validity check. Iterators don't know the container from which they reference objects.

Both will involve changes to the source code.

Sorry, I am a bit busy at the moment, I'll post back when I have had a chance to think about this a bit more.

ashwinipahuja · 05-26-2004, 11:17 PM

Ok dakensta, please let me know wut changes have to be made to the source code, whenever u get some time