Question

I have two classes

class PopulationMember
{
public:
    void operationOnThisMember1();
    void operationOnThisMember2();
    ...

private:

    Population* populaltion_;
}

class Population
{
public:
    void operationOnAllMembers1();
    void operationOnAllMembers2();
    ...
    void operationOnAllMembers100();

    void sortAllMembersCriterium1();
    void sortAllMembersCriterium2();
    ...
    void sortAllMembersCriterium100();

private:
    QVector<PopulationMember*> members_;
}

I would like to implement a SELECT-like functionality to my framework. That is be able to perform operations only on those members which share a certain combination of properties.

So far I have thought out two approaches:

1. Implement a method that would return a new Population object composed of Members that satisfy a certain condition.

   Popuation Popuation::select(bool (predicate*) (PopulationMember*));
   

2. Add a

   bool selected_; 
   

   flag to each PopulationMember.

If I do 1. There is no way to implement sorting of selected data and deletion. If I do 2. There is overhead with checking for selectedness and I would have to reimplement sorting and other algorithms to operate only on selected members.

Is there a third, better way?

Answer 1

The approach I would take is to expose an iterator interface to the entire collection. To implement some sort of selection I would then use iterator adapters, e.g. one taking a unary predicate, which provide a new view of the range. This way there is neither an impact on the stored object nor any overhead in creating a separate collection. If you look at Boost’s iterator adapters you may already get pretty much what is needed.