Re: basic question on c++ linking

From:

Peskov Dmitry <vamsi.komati@gmail.com>

Newsgroups:

comp.lang.c++

Date:

Sun, 29 Jun 2008 04:10:27 -0700 (PDT)

Message-ID:

<d66a31b7-ab20-4c2c-b775-f8bb1c245ddc@u12g2000prd.googlegroups.com>

On Jun 29, 3:04 pm, James Kanze <james.ka...@gmail.com> wrote:

On Jun 29, 10:26 am, Rolf Magnus <ramag...@t-online.de> wrote:

Peskov Dmitry wrote:

It is a very basic question.Surely i got something wrong in my basic
understanding.
//Contents of file1.cpp
using namespace std;
#include <iostream>
template <typename T>
class my_stack;
int main(){
my_stack<int> st1;
int top_element;
top_element = st1.pop();
cout << "Hello World !! Top = " << top_element << endl;
return 0;
}
The class my_stack is defined in another file.

It's not a class. It's a class template.

say (template.cpp)
file.o template.o -> project
why does it give linker error, since i provided the class
definition while linking too !!!

A class template is not a class. It's a description for the
compiler that tells it how to build the class once it knows
the template arguments. But for that, the compiler must know
the template's definition, not just a declaration.

But that doesn't really change anything here. The statement
template< typename T > class my_stack ;
is a declaration, not a definition. The same would be true
without the "template< typename T >" (although it would declare
a class, rather than a class template).

Certain uses require a definition; a simple declaration is not
sufficient. Instantiating a template is one, and defining a
variable with a given type is another. Since he does both in
main, a definition must be available, and he's not made one
available.

--
James Kanze (GABI Software) email:james.ka...@gma=

il.com

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Let me put my question differently ? which one below

a)
class new_class;

or

b)
class new_class
{
    int a;
    public:
    new_class();
};

is a class declaration ?

Because if i include (b) in my code as this one it works.

using namespace std;
#include <iostream>

class new_class
{
    int a;
    public:
    new_class();

};

int main(){
    new_class st1;
    return 0;
}

I assume class definition includes the function that you are defining
something like
class new_class
{
    int a;
    public:
    new_class();
};

new_class::new_class
{
     a = 10;
     }

is a class definition ?

Please let me know if my understanding is incorrect ?