basic question on c++ linking

Discussion in 'C++' started by Peskov Dmitry, Jun 29, 2008.

  1. 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. say (template.cpp)

    file.o template.o -> project

    why does it give linker error, since i provided the class definition
    while linking too !!!

    ../src/file1.cpp: In function ‘int main()’:
    ../src/file1.cpp:8: error: ‘my_stack’ was not declared in this scope
    Peskov Dmitry, Jun 29, 2008
    #1
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  2. Peskov Dmitry

    Rolf Magnus Guest

    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.
    Rolf Magnus, Jun 29, 2008
    #2
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  3. Ok ...

    It is not working even for a class definition.
    Peskov Dmitry, Jun 29, 2008
    #3
  4. Peskov Dmitry

    Rolf Magnus Guest

    Peskov Dmitry wrote:

    > Ok ...
    >
    > It is not working even for a class definition.


    What does not work, and how does it not work? Show some code and the error
    you get.
    Rolf Magnus, Jun 29, 2008
    #4
  5. On 2008-06-29 10:47, Peskov Dmitry wrote:
    > Ok ...
    >
    > It is not working even for a class definition.


    Please quote the text you are replying to.

    You do not have a link-error but a compile-error because you are trying
    to use an undeclared and undefined type (my_stack). To solve the problem
    you need to include the header-file of my_stack (which should contain
    the class definition) in file1.cpp.

    Notice also that if you put the template definition in the header-file
    and include it (i.e. rename template.cpp to template.h) you can use the
    template class in file1.cpp.

    --
    Erik Wikström
    Erik Wikström, Jun 29, 2008
    #5
  6. Peskov Dmitry

    James Kanze Guest

    On Jun 29, 10:26 am, Rolf Magnus <> 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:
    Conseils en informatique orientée objet/
    Beratung in objektorientierter Datenverarbeitung
    9 place Sémard, 78210 St.-Cyr-l'École, France, +33 (0)1 30 23 00 34
    James Kanze, Jun 29, 2008
    #6
  7. On Jun 29, 3:04 pm, James Kanze <> wrote:
    > On Jun 29, 10:26 am, Rolf Magnus <> 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:
    > Conseils en informatique orientée objet/
    >                    Beratung in objektorientierter Datenverarbeitung
    > 9 place Sémard, 78210 St.-Cyr-l'École, France, +33 (0)1 30 23 00 34


    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 ?
    Peskov Dmitry, Jun 29, 2008
    #7
  8. On 2008-06-29 13:10, Peskov Dmitry wrote:
    > On Jun 29, 3:04 pm, James Kanze <> wrote:
    >> On Jun 29, 10:26 am, Rolf Magnus <> 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.


    Please do not quota signatures.

    > Let me put my question differently ? which one below
    >
    > a)
    > class new_class;


    This is a declaration, it tells the compiler that there exists a class
    called new_class, but nothing else. If all you need is a pointer to the
    class a declaration is enough, for just about anything else you need the
    definition too.

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


    This is the definition, it tells the compiler what the new_class is (how
    much memory an instance needs, what members it has etc.). Notice that a
    definition is also a declaration, you can declare a type as many times
    as you want but you can only define it once.

    > 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;
    > }


    You can also put the definition of the class in a header-file and incude
    that in the .cpp-files that needs it.

    > 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;
    > }


    No, that is the definition of the member function (the constructor in
    this case), and it does not have to be included with the class
    definition. Actually it is quite common to separate the class definition
    from the definitions of its member-functions like this:

    some_class.h:
    -------------

    class some_class
    {
    int a;
    double b;
    public:
    some_class();
    int getA();
    double getB()
    };

    some_class.cpp:
    ---------------

    #include "some_class.h"

    some_class::some_class()
    {
    a = 1;
    b = 2.5;
    }

    // Actually we should use an initialisation-list instead of setting the
    // values of the class-members in the constructor, but that is for
    // another day.

    int some_class::getA()
    {
    return a;
    }

    double some_class::getB()
    {
    return b;
    }

    Any file that needs to use some_class can then include the some_class.h
    file like this:

    main.cpp:
    ---------

    #include <iostream>

    #include "some_class.h"

    int main()
    {
    some_class c;
    std::cout << c.getA() << "\n" << c.getB() << std::endl;
    return 0;
    }

    --
    Erik Wikström
    Erik Wikström, Jun 29, 2008
    #8
  9. Peskov Dmitry

    James Kanze Guest

    On Jun 29, 1:10 pm, Peskov Dmitry <> wrote:
    > On Jun 29, 3:04 pm, James Kanze <> wrote:


    [...]
    > 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 ?


    Both. The second is also a definition. (A definition is a
    declaration, but the reverse is not necessarily true.)

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


    That's because the way you used the class required a definition.

    > 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();
    > };


    A class definition includes the contents (members) of the class.

    > new_class::new_class
    > {
    > a = 10;
    > }


    > is a class definition ?


    No, that's a syntax error.

    A class definition is:
    class <name> <baseclass-declarations>[opt] { ... } ;
    A class declaration (which isn't also a definition) is just:
    class <name> ;

    --
    James Kanze (GABI Software) email:
    Conseils en informatique orientée objet/
    Beratung in objektorientierter Datenverarbeitung
    9 place Sémard, 78210 St.-Cyr-l'École, France, +33 (0)1 30 23 00 34
    James Kanze, Jun 29, 2008
    #9
  10. Peskov Dmitry

    James Kanze Guest

    On Jun 29, 2:03 pm, Erik Wikström <> wrote:
    > On 2008-06-29 13:10, Peskov Dmitry wrote:
    > > Let me put my question differently ? which one below


    > > a)
    > > class new_class;


    > This is a declaration, it tells the compiler that there exists
    > a class called new_class, but nothing else. If all you need is
    > a pointer to the class a declaration is enough, for just about
    > anything else you need the definition too.


    And references, of course.

    Also, you can *declare* (but not define) objects of the type.
    Thus, "extern new_class some_object ;" is legal with just a
    class declaration. Or, within a class:

    class Another
    {
    new_class illegal; // a non-static member is a definition
    static new_class legal ;
    // but a static member is just a
    // declaration (which means that it
    // must be defined elsewhere).
    } ;

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


    > This is the definition, it tells the compiler what the
    > new_class is (how much memory an instance needs, what members
    > it has etc.). Notice that a definition is also a declaration,
    > you can declare a type as many times as you want but you can
    > only define it once.


    In general. Classes (and templates and inline functions) are an
    exception. You can only define them once in each translation
    unit, but you can define them in as many translation units as
    you want, as long as the definitions are identical.

    [...]
    > > 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;
    > > }


    > No, that is the definition of the member function (the
    > constructor in this case),


    I don't think so. To be the definition of the constructor, he'd
    need to have parentheses after the second new_class.

    --
    James Kanze (GABI Software) email:
    Conseils en informatique orientée objet/
    Beratung in objektorientierter Datenverarbeitung
    9 place Sémard, 78210 St.-Cyr-l'École, France, +33 (0)1 30 23 00 34
    James Kanze, Jun 29, 2008
    #10
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