Idiomatic way to store two related values of the same type

Question

This will be a pretty simple question: in C++03, I would store two values of the same type with std::pair. However, having to repeat the type twice is somehow a bother when I want both of my values to be of the same type. Now, with C++11, we have std::array. Would it be more idiomatic to write this:

std::array<int, 2> foo;

...instead of that:

std::pair<int, int> foo;

...when the aim is to store two related data (picture for example the result of a function solving a quadratic equation)?

Answer 1

I don't think there's any data type suitable for this in either the standard library or in Boost.

pair<T, T> is definitely wrong:
make_pair(1, 2) == make_pair(2, 1) should be true if these represent the roots of a polynomial!

For the same reason, an array/vector won't work either, unless you change the comparison behavior.

So I'd say make a bag<T, N> data type that represents a multiset<T> with a fixed size, kind of like how array<T, N> represents a vector<T> with a fixed size.

Since the size is small, you can just do everything by brute force (comparison, equality checking, etc.).

Answer 2

I'd still use pair to indicate that one value is related to the other. Array does not convey that meaning to me.

Answer 3

std::pair makes sense if you have relation between two items, and you want to describe that relation.

However, in your case I would use std::array, as two solutions of quadratic equation don't have any relationship between each other (I can see the relationship between the solutions and the equatiation, but not between two solutions).

Answer 4

Depends on the larger context.

In the example you mentioned I'd go with std::array, because then you can also template the function on the degree of the equation:

template <int N>
std::array<double, N> solveEquation(const Polynomial<N>& poly);

Answer 5

I'd say the corresponding C++11 abstraction of std::pair<> is std::tuple<>, since a pair is just a special case of tuple.

std::pair<> doesn't hold equal types necessarily, BTW.

Answer 6

Precisely for idiomatic reasons, I would suggest using pair a denotes that both values are related while array is more related to a list of values (in my humble opinion).

For example, in your suggested usage, the relation between both values would be the quadratic function itself.