CODEWITHSUNDEEP
c++stlset
karven
karven

Reputation: 65

C++ std::copy from std::deque to std:;set

I have a class with "array of array" private member represented as:

std::deque<std::deque<SomeClass> > someArray_;

Also this class have a public method which allows to receive all unique SomeClass instances, containing in someArray_. Unique for SomeClass instances means different by at least one of several class members. I decided to use std::set for that purpose. This method has prototype like following: 

std::set<SomeClass> getAllUniqueInstances() const;

In this method implementation I use following construction to populate std::set:

std::set<SomeClass> allUniqueInstances;
for(auto it = std::begin(someArray_); it != std::end(someArray_); ++it){
    std::copy((*it).begin(), 
              (*it).end(),
              std::inserter(allUniqueInstances, allUniqueInstances.end()));
}

operator<() is defined for SomeClass class. As a result my std::set is populated, but huge amount of instances is missed. Modyfing operator<() for SomeClass class, alters situation, but breaks desirable sorting order. How in this case std::copy determines whether considerable instance is unique?

UPD: source code for SomeClass

class SomeClass{
    private:
        uint32_t from_;
        uint32_t to_;
        double capacity_;
        double flow_;
    public:        
        ...
        bool operator<(const SomeClass& rhs) const;
        ...
    };

I want SomeClass instances to be ordered in set by from_ member:

bool SomeClass::operator<( const SomeClass& rhs ) const{
    if(this->from_ < rhs.from_)
        return true;    
    return false;    
}

Upvotes: 2

Views: 1692

Answers (1)

juanchopanza
juanchopanza

Reputation: 227468

It is not std::copy who decides whether instances are unique, but std::set. The logic is something like

(A < B is false) and (B < A is false)

So the criterion that defines the ordering also defines the "uniqueness". It seems like std::set is the wrong data structure for this problem, or your ordering criteria are either incorrect (as in not implementing strict weak ordering), or too broad to suit the problem (as in, you order based on a small number of attributes when you could use more).

Here is an example of a lexicographical comparison using more attributes than you currently have:

#include <tuple> // for std::tie

bool SomeClass::operator<( const SomeClass& rhs ) const
{
  return std::tie(from_, to_, capacity_, flow_) < std::tie(rhs.from_, rhs.to_, rhs.capacity_, rhs.flow_);
}

Upvotes: 5

Related Questions