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Bart Decuypere
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 Posted: Sat Nov 18, 2006 6:30 am Post subject: Problem with templates and iterators |
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Hi,
can you help me out with the question why the construct in main.cpp does
compile and the one in CTest.h does not (sources below).
error from g++ on windows/MinGW:
In file included from ../main.cpp:1:
..../CTest.h: In function `std::istream& operator>>(std::istream&, const
CTest<T>&)':
..../CTest.h:26: error: expected `;' before "it"
main.cpp
----------
#include "CTest.h"
#include <vector>
using std::vector;
int main(int argc, char **argv) {
vector<vector<int> > metType;
//compiles OK
vector<vector<int> >::iterator it=metType.begin();
}
CTest.h
---------
#ifndef CTEST_H_
#define CTEST_H_
#include <iostream>
#include <vector>
#include <iterator>
using std::istream;
using std::vector;
template <class T>
class CTest
{
template<T>
friend istream& operator>>(istream& in, const CTest<T>& test);
};
template <class T>
istream& operator>>(istream& in, const CTest<T>& test)
{
vector<vector<T> > rows;
//does not compile
vector<vector<T> >::iterator it=rows.begin();
return in;
}
#endif /*CTEST_H_*/
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nevergone
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| Posted: Sat Nov 18, 2006 10:10 am Post subject: Re: Problem with templates and iterators |
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template <class T>
class CTest
{
template<T>
friend istream& operator>>(istream& in, const CTest<T>& test);
};
template <class T>
istream& operator>>(istream& in, const CTest<T>& test)
{
vector<vector<T> > rows;
vector<vector<T> >::iterator it=rows.begin();
return in;
}
my answer:
template <class T>
class CTest
{
template <U>
friend istream&operator>>(istream &in,const CTest<U>&test);
};
template <class T>
istream& operator>>(istream& in, const CTest<T>& test)
{
vector<vector<T> > rows;
vector<vector<T> >::iterator it=rows.begin();
return in;
}
or:
template <class T>
class CTest;
template <class T>
istream &operator>>(istream &in,const CTest<T>&test);
template<class T>
class CTest
{
friend istream& operator>> <> (istream& in, const CTest<T>& test);
};
template <class T>
istream& operator>>(istream& in, const CTest<T>& test)
{
vector<vector<T> > rows;
vector<vector<T> >::iterator it=rows.begin();
return in;
}
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Ivan Novick
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| Posted: Sat Nov 18, 2006 10:10 am Post subject: Re: Problem with templates and iterators |
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| Quote: | can you help me out with the question why the construct in main.cpp does
compile and the one in CTest.h does not (sources below).
template <class T
istream& operator>>(istream& in, const CTest<T>& test)
{
vector<vector<T> > rows;
//does not compile
vector<vector<T> >::iterator it=rows.begin();
return in;
}
|
The reason is because on CTest.h this code:
vector<vector<T> >::iterator
Is the type of variable "it".
Because it is a type inside a template you must precede it with the
keyword typename.
Try changing that line to:
typename vector<vector<T> >::iterator it=rows.begin();
Ivan
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Bob
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| Posted: Sat Nov 18, 2006 10:10 am Post subject: Re: Problem with templates and iterators |
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|
Bart Decuypere wrote:
| Quote: | Hi,
can you help me out with the question why the construct in main.cpp
does compile and the one in CTest.h does not (sources below).
error from g++ on windows/MinGW:
In file included from ../main.cpp:1:
.../CTest.h: In function `std::istream& operator>>(std::istream&,
const CTest<T>&)':
.../CTest.h:26: error: expected `;' before "it"
|
[Code snipped].
The short answer is because the compiler needs a bit of help when
working with templates.
In your header file ....
| Quote: |
template <class T
istream& operator>>(istream& in, const CTest<T>& test)
{
vector<vector<T> > rows;
//does not compile
vector<vector<T> >::iterator it=rows.begin();
|
the last line needs to be replaced with
typename vector<vector<T> >::iterator it=rows.begin();
Unrelated to your problem, but it is usually a VERY good idea
to avoid having "using namespace" directives in any header files.
While that means you have to prefix things in the header with
the "std::" prefix (if you're using namespace std) it prevents
all sorts of ambiguity problems in code that #include's your header
and employs multiple namespaces.
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Chris Uzdavinis
Guest
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| Posted: Sat Nov 18, 2006 10:10 am Post subject: Re: Problem with templates and iterators |
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|
Bart Decuypere wrote:
| Quote: | Hi,
can you help me out with the question why the construct in main.cpp does
compile and the one in CTest.h does not (sources below).
template <class T
istream& operator>>(istream& in, const CTest<T>& test)
{
vector<vector<T> > rows;
//does not compile
vector<vector<T> >::iterator it=rows.begin();
return in;
}
|
Since T is a template argument to your function, and the actual type of
your vector depends on that T, to access its type members you must
prefix it with "typename". Example:
typename vector<vector<T> >::iterator it=rows.begin();
Hope this helps.
Chris
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Salt_Peter
Guest
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| Posted: Sat Nov 18, 2006 10:10 am Post subject: Re: Problem with templates and iterators |
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|
Bart Decuypere wrote:
| Quote: | Hi,
can you help me out with the question why the construct in main.cpp does
compile and the one in CTest.h does not (sources below).
error from g++ on windows/MinGW:
In file included from ../main.cpp:1:
.../CTest.h: In function `std::istream& operator>>(std::istream&, const
CTest<T>&)':
.../CTest.h:26: error: expected `;' before "it"
main.cpp
----------
#include "CTest.h"
#include <vector
using std::vector;
int main(int argc, char **argv) {
vector<vector<int> > metType;
|
here, std::vector is *not* a dependant type.
| Quote: |
//compiles OK
vector<vector<int> >::iterator it=metType.begin();
}
CTest.h
---------
#ifndef CTEST_H_
#define CTEST_H_
#include <iostream
#include <vector
#include <iterator
using std::istream;
using std::vector;
template <class T
class CTest
{
template<T
|
You are declaring a friend with the same template parameter as the
CTest class.
Either
a) you remove the above template< T > and implemet the friend function
here.
b) or change typename T to something that won't clash with the Class's
template parameter.
| Quote: |
friend istream& operator>>(istream& in, const CTest<T>& test);
};
template <class T
istream& operator>>(istream& in, const CTest<T>& test)
|
That does not match the signature given above.
| Quote: |
{
vector<vector<T> > rows;
|
The std::vector is a dependant type.
It depends on the template parameter provided.
typedef typename std::vector< std::vector< T > > V2Type;
V2Type rows;
the same goes for iterator. It too is a dependant type.
| Quote: |
//does not compile
vector<vector<T> >::iterator it=rows.begin();
return in;
}
#endif /*CTEST_H_*/
|
#include <iostream>
#include <vector>
template < typename T >
class Test
{
template< typename P >
friend std::istream&
operator>>(std::istream& in, const Test< P >& r_test);
};
template < typename P >
std::istream&
operator>>(std::istream& in, const Test< P >& r_test)
{
typedef typename std::vector< P > VColsType;
typedef typename std::vector< std::vector< P > > VRowsType;
typedef typename VRowsType::iterator VRIter;
VRowsType rows;
VRIter it = rows.begin(); // r_test.rows.begin()
return in;
}
int main()
{
Test< double > test;
}
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[ See http://www.gotw.ca/resources/clcm.htm for info about ]
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