Inheritance and template in C++: Why doesn't the following piece of code compile?

Question

I have a simple c++ program , which I am not able to compile , although i tried to search in google and try to read about template , inheritance and vector , but i didn't got any clue that what mistake I am doing, Can anyone please help me!! following is the code:

template <class T>
class Base
{
  public:
  int entries;
};
template <class T>
class Derive : public Base<T *>
{
  public:
  int j;
  void pankaj(){j = entries;}
  void clear();

};
template <class T> void Derive<T>::clear()
{
  int i;
  int j=entries;
};
int main()
{
  Derive b1;
}

And I am getting following error: pankajkk> g++ sample.cpp

sample.cpp: In member function 'void Derive<T>::pankaj()':
sample.cpp:14: error: 'entries' was not declared in this scope
sample.cpp: In member function 'void Derive<T>::clear()':
sample.cpp:22: error: 'entries' was not declared in this scope
sample.cpp: In function 'int main()':
sample.cpp:26: error: missing template arguments before 'b1'
sample.cpp:26: error: expected `;' before 'b1'

Thanks!!

@KarolyHorvath That does work, but why doesn't it work without `this->`? FWIW `Base::entries` works as well. — Emil Laine, Apr 26 '15 at 10:48
http://stackoverflow.com/questions/17772960/c-template-inheritance-scheme — Karoly Horvath, Apr 26 '15 at 10:50
@KarolyHorvath it worked after changing entries to this->entries. Thanks alot :) — pankaj kushwaha, Apr 26 '15 at 11:16

score 2 · Answer 1 · edited May 23 '17 at 12:30

You must use this->foo to access member variable foo in template base classes. You may ask why.

Also, as Old Fox explains, you must specify the type T when you declare your variable b1.

template <class T>
class Base
{
  public:
  int entries;
};

template <class T>
class Derive : public Base<T *>
{
  public:
  int j;
  void pankaj(){j = this->entries;}
  void clear();
};

template <class T> void Derive<T>::clear()
{
  int i;
  int j=this->entries;
};

int main()
{
  Derive<int> b1;
}

live demo here

Old Fox · Answer 2 · 2015-04-26T10:50:25.213

1

your main method has syntax error, change to:

int main()
{
    Derive<int> b1;
}

you can put other type rather than integer...

in c++ templates are compile time

edited Apr 26 '15 at 10:50

answered Apr 26 '15 at 10:46

Old Fox

8,629
4
34
52

@cmbasnett , I took his code into visual studio, after my change it was compiled without any error – Old Fox Apr 26 '15 at 10:51
unfortunately, that doesn't mean the code is correct... blame VS. – Karoly Horvath Apr 26 '15 at 10:52
@Old Fox - Fair enough, I use VS too, but it looks like VS doesn't conform to the C++ standard, as per the link in the comments in the OP. – Colin Basnett Apr 26 '15 at 10:53
@OldFox It fixed compiler error at line no. 26 , But I am still getting other error. – pankaj kushwaha Apr 26 '15 at 11:04
@OldFox I am compiling using g++ 4.9. – pankaj kushwaha Apr 26 '15 at 11:09
@pankajkushwaha ,Karoly Horvath was right, the right syntax is this->entries – Old Fox Apr 26 '15 at 11:20
@OldFox yup , it compiled after changing entries to this->entries. Thanks karoly , thanks OldFox – pankaj kushwaha Apr 26 '15 at 11:27

Inheritance and template in C++: Why doesn't the following piece of code compile?

2 Answers2