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Quick q. on string::const_reverse_iterator

Posted on 2001-07-27
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Last Modified: 2012-05-04
Hi,
why the following code does not work for me?

std::string test("test");
  std::string::const_reverse_iterator backward = test.rbegin();


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Question by:proskig
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LVL 86

Expert Comment

by:jkr
ID: 6326799
Because if 'test' isn't a 'const string', 'reverse_iterator rbegin();' is used instead of 'const_reverse_iterator rbegin() const;', thus leading to "error C2440: 'initializing' : cannot convert from 'class std::rever
se_iterator<char *,char,char &,char *,int>' to 'class std::reverse_iterator<char
 const *,char,char const &,char const *,int>'"

Make it either

const std::string test("test");

or

std::string::reverse_iterator backward = test.rbegin();

and it'll work...

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Accepted Solution

by:
nietod earned 50 total points
ID: 6326837
But you can still generate a constant interator.  Use

std::string test("test");

std::string::const_reverse_iterator backward (&(*test.rbegin()));
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Expert Comment

by:nietod
ID: 6326851
Note that oen way in which pointers differs from interators is that there is not always a convertion from iterator to const_iterator  (and same for reverse.).

For this reason and because virtually all of the STL uses interators, not const_iterators, it is often recommended that you do not use const_iterator (or reverse) except for very specific cases.
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Author Comment

by:proskig
ID: 6326858
OK, I knew about jkr's solution, so I would prefer nietod's.

But can anyone explain why it happens or why the following works:

std::string::const_iterator forward = test.begin();
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Author Comment

by:proskig
ID: 6326878
Sorry, because string::iterator is typedef for char*, so that's why it works.

Sorry, again.
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LVL 86

Expert Comment

by:jkr
ID: 6326897
>>OK, I knew about jkr's solution

Then, why do you ask

>>why the following code does not work for me?

Because that's what I answered...
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Expert Comment

by:nietod
ID: 6327059
Note that the conversion I suggested is not necessarily safe when the string has 0 length.  i..e it doesn't necessarily work on an empty container.

As I think you understand now, a string classes' iterator is just typedefed as a char * and a constant iterator is typedefed as a "const char *"   So there is a conversion between iterator and const_iterator for this class, but the existance of this conversion is not guaranteed.  i.e. its not true for all containers and its not true for all implimentations.  (Someome could  write a string class where this wasn't true.)
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Author Comment

by:proskig
ID: 6327187
jkr:
because I had function, which accepted non-const string and I did not want to create another temporary object.

So my real question was coming from the point: why for begin() C++ compiler is able to construct const_iterator from iterator and it cannot do that for rbegin.

However, I posted points for you.
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Expert Comment

by:jkr
ID: 6327379
Thanx, proskig!
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Expert Comment

by:Axter
ID: 6327838
proskig,
Are you trying to reverse a string.
If so, you can do so using the following method:
#include <algorithm>

std::string test("A test");
std::reverse(test.begin(),test.end());
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Author Comment

by:proskig
ID: 6333487
Axter: Almost, but mainly I was playing around with some samples
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