Pairs

The idea of a pair of values often comes handy in programming. C++ allows us to make these pairs.

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With std::pair, you can build pairs of arbitrary types. The class template std::pair needs the header <utility>. std::pair has a default, copy and move constructor. Pair objects can be swapped: std::swap(pair1, pair2).

Pairs will often be used in the C++ library. For example, the function std::minmax returns its result as a pair, the associative container std::map, std::unordered_map, std::multimap and std::unordered_multimap manage their key/value association in pairs.

To get the elements of a pair p, you can either access it directly or via an index. So, with p.first or std::get<0>(p) you get the first, with p.second or std::get<1>(p) you get the second element of the pair.

Pairs support the comparison operators ==, !=, <, >, <= and >=. If you compare two pairs for identity, at first the members pair1.first and pair2.first will be compared and then pair1.second and pair2.second. The same strategy holds for the other comparison operators.

std::make_pair

C++ has the practical help function std::make_pair to generate pairs, without specifying their types. std::make_pair automatically deduces their types.

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// pair.cpp
#include <iostream>
#include <utility>
using namespace std;
int main(){
  pair<const char*, double> charDoub("str", 3.14);
  pair<const char*, double> charDoub2 = make_pair("str", 3.14);
  auto charDoub3 = make_pair("str", 3.14);
  cout << charDoub.first << ", " << charDoub.second << "\n";    // str, 3.14
  charDoub.first = "Str";
  get<1>(charDoub) = 4.14;
  cout << charDoub.first << ", " << charDoub.second << "\n";    // Str, 4.14
  
  return 0;
}

In the next lesson, we will talk about tuples in C++.