CODEWITHSUNDEEP
c++for-loopc++11templates
romain.bqt4
romain.bqt4

Reputation: 159

C++ for range loop for template class

I have a template class below that depends on 2 classes U and T and have implemented the iterator function for both classes (see the end of this post).

I can iterate over the respective vectors of the class using iterators,
but I was wondering if it was possible to do the same using the for range loop syntax instead of using iterators. With something like

for (auto x : myclass  <double>)
{
  std::cout << x << std::endl ; 
}

I know it does not work but I could not manage to find the syntax if any, Thanks in advance for your answer

#include<iostream> 
#include<string> 
#include<vector> 

template<class U,class T>
class MyClass
{
  public:
  MyClass(
    const std::vector<U> & vect_u ,
    const std::vector<T> & vect_t    )
  {
    m_vect_u = vect_u; 
    m_vect_t = vect_t; 
  }

  ~MyClass(){}
  
  // begin()
  template<class Z>
  typename std::enable_if<std::is_same<Z,T>::value, typename std::vector<Z>::iterator>::type
  begin() noexcept { return m_vect_t.begin(); }
  template<class Z>
  typename std::enable_if<std::is_same<Z,U>::value, typename std::vector<Z>::iterator>::type
  begin() noexcept { return m_vect_u.begin(); }
  // end()
  template<class Z>
  typename std::enable_if<std::is_same<Z,T>::value, typename std::vector<Z>::iterator>::type
  end() noexcept { return m_vect_t.end(); }
  template<class Z>
  typename std::enable_if<std::is_same<Z,U>::value, typename std::vector<Z>::iterator>::type
  end() noexcept { return m_vect_u.end(); }
  // cbegin()
  template<class Z>
  typename std::enable_if<std::is_same<Z,T>::value, typename std::vector<Z>::const_iterator>::type
  cbegin() const noexcept { return m_vect_t.cbegin(); }
  template<class Z>
  typename std::enable_if<std::is_same<Z,U>::value, typename std::vector<Z>::const_iterator>::type
  cbegin() const noexcept { return m_vect_u.cbegin(); }
  // cend() 
  template<class Z>
  typename std::enable_if<std::is_same<Z,T>::value, typename std::vector<Z>::const_iterator>::type
  cend() const noexcept { return m_vect_t.cend(); }
  template<class Z>
  typename std::enable_if<std::is_same<Z,U>::value, typename std::vector<Z>::const_iterator>::type
  cend() const noexcept { return m_vect_u.cend(); }

  private: 
    std::vector<U>  m_vect_u ; 
    std::vector<T>  m_vect_t    ; 

}; 


int main()
{ 
  std::vector<double> vect_double = {1.5,2.5,3.5} ; 
  std::vector<int>    vect_int    = {-1,-2,-3} ; 
  MyClass<double,int> myclass( vect_double, vect_int); 
 
  std::cout << "iteration over int" << std::endl ; 
  for(auto itr = myclass.begin<int>(); itr < myclass.end<int>() ; ++itr)
  {
    std::cout << *itr << std::endl ; 
  }

  std::cout << "iteration over double" << std::endl ; 
  for(auto itr = myclass.begin<double>(); itr < myclass.end<double>() ; ++itr)
  {
    std::cout << *itr << std::endl ; 
  }

  return 0 ; 
}

Upvotes: 2

Views: 965

Answers (2)

Jarod42
Jarod42

Reputation: 218323

You might provide function to return "range" (directly std::vector or a wrapper):

template <typename Z> // Or SFINAE or if constexpr or any other implementation
auto& getVector() { return std::get<std::vector<Z>&>(std::tie(m_vect_u, m_vect_t)); }

template <typename Z>
const auto& getVector() const { return std::get<const std::vector<Z>&>(std::tie(m_vect_u, m_vect_t)); }

and then

std::cout << "iteration over int" << std::endl ; 
for (auto e : myclass.getVector<int>())
{
    std::cout << e << std::endl ; 
}

Demo

Upvotes: 4

463035818_is_not_an_ai
463035818_is_not_an_ai

Reputation: 123568

You need an object that has non-templated begin and end. For example a class template that wraps MyClass:

template <typename Z,typename U,typename T>
struct MyClassWrapper {
    MyClass<U,T>* parent;
    auto begin() { return parent-> template begin<Z>(); }
    auto end() { return parent-> template end<Z>();}    
};

template <typename Z,typename U,typename T>
MyClassWrapper<Z,U,T> make_wrapper(MyClass<U,T>* p){
    return {p};
}

Then this works:

for (auto& x : make_wrapper<int>(&myclass)){
    std::cout << x << "\n";
}
for (auto& x : make_wrapper<double>(&myclass)){
    std::cout << x << "\n";
}

Live Demo

Upvotes: 4

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