Can we dynamically insert operations to our function?

Question

In this thread link, concerns the discussion on real example of using the strategy design pattern. The second answer which shows an example of dynamically inserting rules to whether approve/decline the assignment of products to people with the RuleAgent. This set of rules is invoked using the function IsApproved.

The example shows what if we wanted to add two more rules, like an intern rule and overtime rule. My question relates to how do we ensure that our polymorphic call to IsApproved would call ALL the added rules. The question has also been asked in the comment to that answer but no replies.

Could you please comment on how to do this on C++ and/or (if possible) Fortran.

You can't magically call all derived versions of a virtual function without instantiating all the derived classes. Create a class derived from (or that implements the interface) `IRule` that contains a set of instances of all the other derived types - since they are polymorphic, it is possible to store them all (using pointers or smart pointers) in a single container, and iterate over all elements of the container, calling each one's `IsApproved()` function as needed. — Peter, Sep 05 '19 at 12:36

George · Accepted Answer · 2019-09-05T10:45:45.967

This example sidesteps polymorphism, where the agent is a vector of function pointers that can be dynamically added to and removed from :

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

class Person
{
public:
    int _timesheet = 50;    
    std::string _title = "Intern";
};

class Assignment
{
public:
    Person _person;
};

namespace OvertimeRule
{
bool IsApproved(Assignment const& assignment)
{
    return assignment._person._timesheet >= 40;
}   
}

namespace InternRule
{
bool IsApproved(Assignment const& assignment)
{
    return assignment._person._title == "Intern";
}   
}

int main()
{
    Assignment testAssignment;

    std::vector<bool(*)(Assignment const&)> assignmentAgent;
    assignmentAgent.push_back(&OvertimeRule::IsApproved);
    assignmentAgent.push_back(&InternRule::IsApproved);

    bool const testSuccess = std::all_of(assignmentAgent.begin(), assignmentAgent.end(), 
        [&testAssignment] (auto const& Func)
    {
        return Func(testAssignment);
    });

    if(testSuccess)
    {
        std::cout << "Requirements Met!";
    }
    else
    {
        std::cout << "Requirements Not Met!";
    }
}

Can we dynamically insert operations to our function?

1 Answers1