C++ – How to design a C++ program to allow for runtime import of functions

Don't expose your guts, guide visitors :

class A { 
    public:

        // we assume you want read-only versions, if not you can add non-const versions
        template< class Func >
        void for_each_primary( Func f ) const { for_each_value( f, m_primary ); }

        template< class Func >
        void for_each_secodary( Func f ) const { for_each_value( f, m_secondary ); }

    private:
        std::vector<int>  m_primary;
        std::vector<char> m_secondary;

        template< class Func, class Container >
        void for_each_value( Func f, const Container& c )
        {
             for( auto i : c )
                 f( i );
        }
};



int main()
{
    A a;
    a.for_each_primary( [&]( int value ) 
         { std::cout << "Primary Value : " << value ; } );
    a.for_each_secondary( [&]( int value ) 
         { std::cout << "Secondary Value : " << value ; } )
}

Note that you could use std::function instead of template parameter if you want to put the implementation in a cpp file, making implementation changes less expensive on compilation times in big projects.

Also, I didn't try to compile it now, but I used a lot this pattern in my open-source projects.

This solution is a C++11 enhancement of B I guess.

HOWEVER

This solution have several issues :

1. It requires C++11 to be effective, because it's efficient for the user of you class ONLY if he can use lambda.
2. It relies on the fact that the class implementer really know what algorithms precisely are to be available to users. If the user need to do complex manipulations to the numbers, jumping from index to index in an unpredictable way for example, then exposing iterators, a copy of the values OR the values would be better.

In fact, this kind of choice totally depends on what you intend the user to do with this class.

By default I prefer the solution I gave you because it's the most "isolated" one, making sure the class know how it's values can be manipulated by external code. It's a bit like "extensions points". If it's a map, providing a find function to your class is easy. So I think that's the more sane way to expose data and it's also made available by lambdas.

As said, if you need to make sure the user can manipulate the data as he wish, then providing a copy of the container is the next "isolated" option (maybe with a way to reset the container with the copy after that). If a copy would be expensive, then iterators would be better. If not enough then a reference is acceptable but it's certainly a bad idea.

Now assuming you're using C++11 and don't want to provide algorithms, the most idiomatic way is using iterators this way (only the user code changes) :

class B { 
    private:
        std::vector<int>  m_primary;
        std::vector<char> m_secondary;
    public:

        // your code is read-write enabled... make sure it's not const_iterator you want
        // also I'm using decltypt to allow changing container type without having to manually change functions signatures
        decltype(m_primary)::iterator primary_begin() const;
        decltype(m_primary)::iterator primary_end() const;

        decltype(m_secondary)::iterator secondary_begin() const;
        decltype(m_secondary)::iterator secondary_end() const; 


};

int main()
{
    B b;


    std::for_each( b.primary_begin(), b.primary_end(), []( int& value ) {
        // ...
    });   
    std::for_each( b.secondary_begin(), b.secondary_end(), []( double& value ) {
        // ...
    });   

}

You've overlooked the key characteristic of the logarithm base.

Because i is divided by 2 in each iteration, the running time is logarithmic with base 2. And

log₂(500) ~ 8.9

What you are looking at is

log₁₀(500) ~ 2.7

(logarithm with base 10)

By the way, the reason why the base is often omitted in runtime discussions (and your calculator probably doesn't have a button for log₂) is that due to the mechanisms of logarithmic math, a different base corresponds to a constant factor and thus is not relevant when you're ignoring constant factors anyway. It can be calculated easily:

log_a(x) = log_b(x) / log_b(a)