Shall we always includes definition into head file when define template class?

[肥猫]

{ The basics are covered in FAQ item 35.12 "Why can't I separate the definition
of my templates class from it's declaration and put it inside a .cpp file?"
<url: http://www.parashift.com/c++-faq-lite/templates.html#faq-35.12>. -mod }

When I define a template class,I found that i can't separate the
template class head and the class definition into two files like
defineing a common class. when i trying to do this ,it report error
LNK2019..we can play the probrom back using the code following:
the code divides into three files.
//LinkedList.h
template<typename T>
class LinkedList
{
public:
LinkedList();
~LinkedList();
void clearall();
private:
int*begin;
int listcounter;
};
//////////////////////////
//LinkedList.cpp
#include "LinkedList.h"
template<typename T>
void LinkedList<T>::clearall()
{
}
template <typename T>
LinkedList<T>::~LinkedList()
{
clearall();
}

template<typename T>
LinkedList<T>::LinkedList()
{
begin=0;
listcounter=0;
}
/////////////////////
#define CONSTRUCTING

#include<iostream>
using namespace std;

#include "LinkedList.h"

int main()
{
#ifdef CONSTRUCTING
LinkedList<int> test1;
//LinkedList<int> test2(test1);
#endif
}

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[Louis Lavery]

?? wrote:
> { The basics are covered in FAQ item 35.12 "Why can't I separate the definition
> of my templates class from it's declaration and put it inside a .cpp file?"
> <url: http://www.parashift.com/c++-faq-lite/templates.html#faq-35.12>. -mod }
>
> When I define a template class,I found that i can't separate the
> template class head and the class definition into two files like
> defineing a common class.

But you can, so long as you include the definition in the header.

when i trying to do this ,it report error
> LNK2019..we can play the probrom back using the code following:
> the code divides into three files.
> //LinkedList.h

#ifndef LINKEDLIST_H // Add guard
#define LINKEDLIST_H

> template<typename T>
> class LinkedList
> {
> public:
> LinkedList();
> ~LinkedList();
> void clearall();
> private:
> int*begin;
> int listcounter;
> };

#include "LinkedList.cpp" // Include definition.
#endif

> //////////////////////////
> //LinkedList.cpp
#ifndef LINKEDLIST_CPP // Add guard
#define LINKEDLIST_CPP

> #include "LinkedList.h"
> template<typename T>
> void LinkedList<T>::clearall()
> {
> }
> template <typename T>
> LinkedList<T>::~LinkedList()
> {
> clearall();
> }
>
>
> template<typename T>
> LinkedList<T>::LinkedList()
> {
> begin=0;
> listcounter=0;
> }

#endif

HTH, Louis.

[Martin B.]

?? wrote:
> Re: Shall we always includes definition into head file when define
template class?

In the general case: Yes you have to. Fopr specific (large) classes,
please see my reply below.

> ...

If you have a ((very) large) templated class and you know beforehand
which types you are going to use with it - or you can just augment the
cpp file if a new type is needed - then you can resolve the unresolved
external linker warnings by adding an explicit template instantiation
for all required types: (LinkedList is rather something where you won't
do this)

// in LinkedList.cpp
template LinkedList<int>;
template LinkedList<unsigned int>;
template LinkedList<std::string>;
// ...

br,
Martin