Hi there,


Before creating the subject, I was able to do research from the official documentation, forums but unfortunately, I could not or very well had an answer.

Context :

This is the code:

Code:
#include <boost/multi_index_container.hpp>
#include <boost/multi_index/sequenced_index.hpp>
#include <boost/multi_index/hashed_index.hpp>
#include <boost/multi_index/key.hpp>
 
template<typename Tkey, typename Tval>
class CacheWeb{
  private:
  using value_type = std::pair<Tkey, Tval>;
 
  unsigned int capacity;
  boost::multi_index_container<
    value_type,
    boost::multi_index::indexed_by<
      boost::multi_index::sequenced<>,
      boost::multi_index::hashed_unique<boost::multi_index::key<&value_type::first>>
    >
  > container;
 
  CacheWeb(const CacheWeb&) = delete;
  CacheWeb& operator=(const CacheWeb&) = delete;
 
  int capacityOut(){
      if( capacity == 0 || container.size() < capacity ) {
        return 0;
      }
      int cnt = 0;
      while(container.size() > capacity) {
        container.pop_back();
        ++cnt;
      }
      return cnt;     
  };
 
  public:
  CacheWeb(int icapacity) : capacity(icapacity){};
  virtual ~CacheWeb() = default;
 
 
  int size(){
    return container.size();
  };
  bool empty(){
    return container.empty();
  };
  void clear(){
    container.clear();
  };
 
  bool contains(const Tkey& key){
    const auto& lookup = container.template get<1>();
    return lookup.find(key) != container.template get<1>().end();
  };
 
  void remove(const Tkey& key){
    container.erase(key);
  };
 
  void put(const Tkey& key, const Tval& val){
    auto& lookup = container.template get<1>();
    auto it = lookup.find(key);
    if( it != lookup.end() ) {
      lookup.modify(it,[&](value_type& x){ x.second = val; });
    }
    else{
      it=lookup.emplace(key, val).first;
    }
    container.relocate(container.begin(),container.template project<0>(it));
    capacityOut();
  };
 
  std::list<std::pair<Tkey, Tval>>getItems(){
    return {container.begin(), container.end()};
  };
 
  const Tval& get(const Tkey& key){
    const auto& lookup = container.template get<1>();
    const auto it = lookup.find(key);
    if( it == lookup.end() ) {
      throw std::invalid_argument("Key does not exist");
    }
    return it->second;
  }
};
 
#include <iostream>
 
int main()
{
  CacheWeb<int,int> c(10);
  for(int i=0;i<11;++i)c.put(i,i);
 
  for(const auto& x:c.getItems()){
    std::cout<<"("<<x.first<<","<<x.second<<")";
  }
  std::cout<<"\n";
 
  for(int i=1;i<11;++i){
    std::cout<<i<<"->"<<c.get(i)<<" ";
  }
  std::cout<<"\n";
}
Unfortunately, several questions come to my mind and also some problems:

1- At this line:

Code:
boost::multi_index_container<value_type,boost::multi_index::indexed_by<boost::multi_index::sequenced<>,boost::multi_index::hashed_unique<boost::multi_index::key<&value_type::first>>>> container;
I don't really well understand the concept behind this? It is a "container" variable of generic type "boost::multi_index_container" where each element is of type "value_type" and with 2 indexes ?

2 Compared to the previous line of code of the container, so this one:

Code:
boost::multi_index_container<value_type,boost::multi_index::indexed_by<boost::multi_index::sequenced<>,boost::multi_index::hashed_unique<boost::multi_index::key<&value_type::first>>>> container;

I builde the code to better understand what it was doing, but I get a few errors:

2.1: Too few template arguments for class template 'sequenced'clang(template_arg_list_different_arity )


2.2: No member named 'key' in namespace 'boost::multi_index'clang(no_member)

I tried to solve the problem by modifying the container

Code:
boost::multi_index::key<&value_type::first>
By this:

Code:
boost::multi_index::member<value_type,Tkey,&value_type::first>
Unfortunately, I get this error: error: template argument 2 is invalid


3 Last question, I misunderstand some functions related to boost:

3.1 templateget<1>();
3.2 location

I fumbled in the boost documents, but either I missed the page or I don't know.

Well, thank you for reading my post! Of course, I want to move forward with this project and my knowledge, and I have arrived at the time when I am looking for help.


I wish you a good day or evening.