Member function of derived class does not care about template argument

Question

I want to simulate a chain of particles either in the up position or the down position. To this end I made a class that inherits from bitset. It looks like:

#include <bitset>
using namespace std;

template <size_t N>
class State : public bitset<N> {
public:
  State<N>();
  State<N>(size_t b);

  long int E();
private:
  size_t length;
};

template<unsigned long N>
State<N>::State()
  : std::bitset<N>(), length(N)
{}

template<unsigned long N>
State<N>::State(size_t b)
  : std::bitset<N>(b), length(N)
{}

Once such an object is instantiated using a certain length, I would like to find the energy associated to such an object. I want to do this

#include "state.h"

long int State::E(){
  long int e = 0;
  for (size_t i = 1; i != length; ++i)
    e += (test[i] == test[i - 1]) ? 1 : -1;

  return e;
}

I receive the error

state/state.cc:3:10: error: ‘template<long unsigned int N> class State’ used without template parameters
 long int State::E(){
          ^~~~~
state/state.cc: In function ‘long int E()’:
state/state.cc:5:27: error: ‘length’ was not declared in this scope
   for (size_t i = 1; i != length; ++i)
                           ^~~~~~
state/state.cc:6:11: error: ‘test’ was not declared in this scope
     e += (test[i] == test[i - 1]) ? 1 : -1;
           ^~~~

I understand this to mean that the compiler does not recognize that E() is a member function of my class due to a missing template argument. However, I was hoping there is a way that I can call s.E() on a State<20> object for instance. So once the object is instantiated I would like to be able to call E() without having to again specify the size. Is this possible? Thanks in advance!

You need to fix the first error. Look at how you defined your constructors. You need to do the same thing for the member function. — NathanOliver, Sep 22 '17 at 13:01
Also, [this](https://stackoverflow.com/questions/495021/why-can-templates-only-be-implemented-in-the-header-file) might be relevant. — NathanOliver, Sep 22 '17 at 13:02
Ah thanks for the comments! I got it now. Feel free to write as an answer and I will accept — Slugger, Sep 22 '17 at 13:04

TemplateRex · Accepted Answer · 2017-09-22T13:21:02.647

2

There are two issues here: defining the member function E() of the class template State<N>, and accessing the test() member function of the dependent base class bitset<N>.

template<size_t N>
long int State<N>::E(){
  long int e = 0;
  for (size_t i = 1; i != length; ++i)
    e += (this->test(i) == this->test(i - 1)) ? 1 : -1;
}

Note both the template<size_t N> and the State<N> as well as the this-> in front of test. See this Q&A for a detailed explanation.

Final note: also be careful: it's test() (parentheses) and operator[] (brackets) in std::bitset.

edited Sep 22 '17 at 13:21

answered Sep 22 '17 at 13:13

TemplateRex

69,038
19
164
304

1

Thanks for the answer! – Slugger Sep 22 '17 at 14:46

M. Yousfi · Answer 2 · 2017-09-22T13:19:12.223

1

In the definition of member function of tempalte class, you must specify the template parameters as you did for the constructor.

The error "lenght not declared" should be fixed also by this change.

template <size_t N>
long int State<N>::E(){
  long int e = 0;
  for (size_t i = 1; i != length; ++i)
    e += (test(i) == test(i - 1)) ? 1 : -1;

  return e;
}

edited Sep 22 '17 at 13:19

answered Sep 22 '17 at 13:13

M. Yousfi

578
5
24

won't compile, needs `this->` – TemplateRex Sep 22 '17 at 13:14
It compiles (at least with VS 2013) but indeed, test() is a method of bitset and it should be used with () not []. I made the change – M. Yousfi Sep 22 '17 at 13:18
@Actarus VS don't implement two phase name lookup – Guillaume Racicot Sep 22 '17 at 14:30
Thanks for the answer! – Slugger Sep 22 '17 at 14:47

Member function of derived class does not care about template argument

2 Answers2