Problem with iterators

[Geranimo]

Hi, I wrote a simple program to test equality between 2 containers, but it's not working and I can't figure out why.

  #include "stdafx.h"
#include <iostream>
#include <string>
#include <Windows.h>

using namespace std;

template<class In, class C>
bool mequal(In beg, In end, C c)
{
	while ((beg != end) && (*beg == *c.begin()))
	{
		beg++;
		c.begin()++;
	}
		
	if (beg == end)
		return 1;
	else
		return 0;

}


int _tmain(int argc, _TCHAR* argv[])
{
	string a = "lol";
	string b = "lol";

	cout << mequal(a.begin(), a.end(), b) << endl;

	system("PAUSE");
	return 0;
}

It only works for strings of 1 letter.

[Disch]

c.begin()++;

What are you trying to do here? This line is effectively a no-op.

If you want to keep track of an iterator for your 'c' string, you'll need to make a new iterator. You can't change where the string begins.

[Geranimo]

I did not specified the & so it's a copy of the string, therefore I don't change the string?

[AbstractionAnon]

c.begin() returns an iterator (a pointer to the front of the string).
You increment that pointer, but don't do anything with the result.

[Geranimo]

I do c.begin()++; and after (*beg == *c.begin()))
so I need to increment it.

[AbstractionAnon]

You're not understanding what Disch and I are saying.
You're NOT changing the value of begin().

Your line 14 is equivalent to the following:

{  // nested scope
    string::iterator  iter;
    iter = c.begin();
    iter++;
}  // iter goes out of scope  

[Geranimo]

Ok so I modified my program to create new iterators from those in the parameters:

#include "stdafx.h"
#include <iostream>
#include <string>
#include <Windows.h>
#include <iterator>

using namespace std;

template<class In, class C>
bool mequal(In beg, In end, C c)
{
	typedef typename In::iterator iter;
	typedef typename C::iterator iterc;
	
	iter b = beg;
	iter e = end;
	iterc cb = c.begin();
	iterc ce = c.end();

	
	while (*b == *cb)
	{
		++b;
		++cb;
		if (b == e && cb == ce)
			return 1;
		else if (b == e || cb == ce)
			return 0;
	}
		
	return 0;

}


int _tmain(int argc, _TCHAR* argv[])
{
	string a = "lol";
	string b = "lol";
	

	cout << mequal(a.begin(), a.end(), b) << endl;

	system("PAUSE");
	return 0;
}

and I get plenty of errors when I build.

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35

1

1> ConsoleApplication7.cpp 1>c:\users\ssmuel\desktop\programmation\c++\consoleapplication7\consoleapplication7\consoleapplication7.cpp(15): error C2039: 'iterator' : is not a member of 'std::_String_iterator<std::_String_val<std::_Simple_types<char>>>' 1> c:\users\ssmuel\desktop\programmation\c++\consoleapplication7\consoleapplication7\consoleapplication7.cpp(45) : see reference to function template instantiation 'bool mequal<std::_String_iterator<std::_String_val<std::_Simple_types<char>>>,std::string>(In,In,C)' being compiled 1> with 1> [ 1> In=std::_String_iterator<std::_String_val<std::_Simple_types<char>>> 1> , C=std::string 1> ] 1>c:\users\ssmuel\desktop\programmation\c++\consoleapplication7\consoleapplication7\consoleapplication7.cpp(18): error C2514: 'std::iterator' : class has no constructors 1> c:\program files (x86)\microsoft visual studio 12.0\vc\include\xutility(338) : see declaration of 'std::iterator' 1>c:\users\ssmuel\desktop\programmation\c++\consoleapplication7\consoleapplication7\consoleapplication7.cpp(19): error C2514: 'std::iterator' : class has no constructors 1> c:\program files (x86)\microsoft visual studio 12.0\vc\include\xutility(338) : see declaration of 'std::iterator' 1>c:\users\ssmuel\desktop\programmation\c++\consoleapplication7\consoleapplication7\consoleapplication7.cpp(24): error C2100: illegal indirection 1>c:\users\ssmuel\desktop\programmation\c++\consoleapplication7\consoleapplication7\consoleapplication7.cpp(24): error C2446: '==' : no conversion from 'int' to 'iter' 1> The target type has no constructors 1>c:\users\ssmuel\desktop\programmation\c++\consoleapplication7\consoleapplication7\consoleapplication7.cpp(24): error C2088: '==' : illegal for struct 1>c:\users\ssmuel\desktop\programmation\c++\consoleapplication7\consoleapplication7\consoleapplication7.cpp(26): error C2675: unary '++' : 'iter' does not define this operator or a conversion to a type acceptable to the predefined operator 1>c:\users\ssmuel\desktop\programmation\c++\consoleapplication7\consoleapplication7\consoleapplication7.cpp(28): error C2678: binary '==' : no operator found which takes a left-hand operand of type 'iter' (or there is no acceptable conversion) 1> c:\program files (x86)\windows kits\8.1\include\shared\guiddef.h(192): could be 'bool operator ==(const GUID &,const GUID &)' 1> c:\program files (x86)\microsoft visual studio 12.0\vc\include\system_error(410): or 'bool std::operator ==(const std::error_condition &,const std::error_code &) throw()' 1> c:\program files (x86)\microsoft visual studio 12.0\vc\include\system_error(402): or 'bool std::operator ==(const std::error_code &,const std::error_condition &) throw()' 1> c:\program files (x86)\microsoft visual studio 12.0\vc\include\exception(507): or 'bool std::operator ==(const std::exception_ptr &,std::nullptr_t)' 1> c:\program files (x86)\microsoft visual studio 12.0\vc\include\exception(502): or 'bool std::operator ==(std::nullptr_t,const std::exception_ptr &)' 1> c:\program files (x86)\microsoft visual studio 12.0\vc\include\exception(497): or 'bool std::operator ==(const std::exception_ptr &,const std::exception_ptr &)' 1> while trying to match the argument list '(iter, iter)' 1>c:\users\ssmuel\desktop\programmation\c++\consoleapplication7\consoleapplication7\consoleapplication7.cpp(30): error C2678: binary '==' : no operator found which takes a left-hand operand of type 'iter' (or there is no acceptable conversion) 1> c:\program files (x86)\windows kits\8.1\include\shared\guiddef.h(192): could be 'bool operator ==(const GUID &,const GUID &)' 1> c:\program files (x86)\microsoft visual studio 12.0\vc\include\system_error(410): or 'bool std::operator ==(const std::error_condition &,const std::error_code &) throw()' 1> c:\program files (x86)\microsoft visual studio 12.0\vc\include\system_error(402): or 'bool std::operator ==(const std::error_code &,const std::error_condition &) throw()' 1> c:\program files (x86)\microsoft visual studio 12.0\vc\include\exception(507): or 'bool std::operator ==(const std::exception_ptr &,std::nullptr_t)' 1> c:\program files (x86)\microsoft visual studio 12.0\vc\include\exception(502): or 'bool std::operator ==(std::nullptr_t,const std::exception_ptr &)' 1> c:\program files (x86)\microsoft visual studio 12.0\vc\include\exception(497): or 'bool std::operator ==(const std::exception_ptr &,const std::exception_ptr &)' 1> while trying to match the argument list '(iter, iter)' ========== Build: 0 succeeded, 1 failed, 0 up-to-date, 0 skipped ==========

[cire]

In is an iterator type which does not have a nested iterator type, so the compiler barfs on line 12 (which seems to correspond with line 15 in your actual code as opposed to that posted here.)

[keskiverto]

The 'beg' and 'end' are already iterators. Why did you touch them? Your problem was with 'c'.

[Geranimo]

I don't understand your advices. When I do c.begin()++; , it doesn't create a new iter because I tried to use it and it doesn't exists.

[cire]

I don't understand your advices. When I do c.begin()++; , it doesn't create a new iter because I tried to use it and it doesn't exists.

c.begin() is a member function begin invoked on an object c which returns an object of type C::iterator. It does create a new iterator. c.begin()++ increments that new iterator, however that iterator ceases to exist at the end of the statement and so it is as if you had done nothing.

[Geranimo]

Ok so if ++ doesn't work to increment an iterator permanently, should I use +=1 instead?

[keskiverto]

No. You should change a non-temporary iterator. Your problem is not on how you change, but what you change.

On your second attempt you already have non-temporary iterators cb and ce. Use them.

[Geranimo]

Ok I finally got it working, I think sleeping helps clarifying all that ^^

Here is my new code:

#include "stdafx.h"
#include <iostream>
#include <string>
#include <Windows.h>

using namespace std;

template<class In, class C>
bool mequal(In beg, In end, C c)
{
	if (beg == end && c.size() == 0)
		return 1;
	else if (beg == end || c.size() == 0)
		return 0;
	
	typedef typename C::iterator cit;
	
	cit cb = c.begin();
	cit ce = c.end();
	
	while (*beg++ == *cb++)
	{
		if (beg == end && cb == ce)
			return 1;
		
		else if (beg == end || cb == ce)
			return 0;
	}
		
	return 0;

}


int _tmain(int argc, _TCHAR* argv[])
{
	string a = "allo mon ami";
	string b = "allo mon ami";

	cout << mequal(a.begin(), a.end(), b) << endl;

	system("PAUSE");
	return 0;
}

Thanks everybody!