CODEWITHSUNDEEP
c++templatesc++11inheritanceg++
pankaj kushwaha
pankaj kushwaha

Reputation: 379

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

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!!

Upvotes: 2

Views: 192

Answers (2)

Johan Lundberg
Johan Lundberg

Reputation: 27038

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

Upvotes: 2

Old Fox
Old Fox

Reputation: 8725

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

Upvotes: 1

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