c++ - Virtually turn vector of struct into vector of struct members

Question

I have a function that takes a vector-like input. To simplify things, let's use this print_in_order function:

#include <iostream>
#include <vector>

template <typename vectorlike>
void print_in_order(std::vector<int> const & order,
                    vectorlike const & printme) {
    for (int i : order)
        std::cout << printme[i] << std::endl;
}

int main() {
    std::vector<int> printme = {100, 200, 300};
    std::vector<int> order = {2,0,1};
    print_in_order(order, printme);
}

Now I have a vector<Elem> and want to print a single integer member, Elem.a, for each Elem in the vector. I could do this by creating a new vector<int> (copying a for all Elems) and pass this to the print function - however, I feel like there must be a way to pass a "virtual" vector that, when operator[] is used on it, returns this only the member a. Note that I don't want to change the print_in_order function to access the member, it should remain general.

Is this possible, maybe with a lambda expression? Full code below.

#include <iostream>
#include <vector>

struct Elem {
    int a,b;
    Elem(int a, int b) : a(a),b(b) {}
};

template <typename vectorlike>
void print_in_order(std::vector<int> const & order,
                    vectorlike const & printme) {
    for (int i : order)
        std::cout << printme[i] << std::endl;
}

int main() {
    std::vector<Elem> printme = {Elem(1,100), Elem(2,200), Elem(3,300)};
    std::vector<int> order = {2,0,1};

    // how to do this?
    virtual_vector X(printme) // behaves like a std::vector<Elem.a>
    print_in_order(order, X);
}

Answer 1

It's not really possible to directly do what you want. Instead you might want to take a hint from the standard algorithm library, for example std::for_each where you take an extra argument that is a function-like object that you call for each element. Then you could easily pass a lambda function that prints only the wanted element.

Perhaps something like

template<typename vectorlike, typename functionlike>
void print_in_order(std::vector<int> const & order,
                vectorlike const & printme,
                functionlike func) {
    for (int i : order)
        func(printme[i]);
}

Then call it like

print_in_order(order, printme, [](Elem const& elem) {
    std::cout << elem.a;
});

Since C++ have function overloading you can still keep the old print_in_order function for plain vectors.