map, set, and pair

std::map: key → value

#include <map>
#include <string>

std::map<std::string, int> stock;
stock["pen"] = 120;                 // insert or overwrite
stock["ink"] += 5;                  // reads 0 if absent, then adds — subtle!
int pens = stock.at("pen");         // throws if key missing
if (stock.count("ink")) { /* present */ }
for (const auto& [item, qty] : stock) {   // C++17 structured binding, sorted by key
    std::cout << item << ": " << qty << '\n';
}

operator[] inserts a zero when the key is absent — a legendary source of bugs in counting code. Use [] when you intend to insert/overwrite; use .at() or find/count when merely reading.

std::map keeps keys sorted (a tree underneath). std::unordered_map is the hash-table sibling: faster on average, no ordering. Default to map while learning (predictable order helps debugging); choose unordered_map when profiling says lookup is hot.

std::set: uniqueness

#include <set>
std::set<int> seen;
seen.insert(3);            // returns <iterator, bool inserted>
seen.insert(3);            // second insert does nothing
CHECK(seen.size() == 1);
bool has = seen.count(3);  // 1 = present

A set is a bag of unique, sorted keys — the direct answer to "have I seen this before?" and "which distinct values are there?".

std::pair: a two-slot box

#include <utility>
std::pair<std::string, int> entry{"pen", 120};
entry.first; entry.second;

Pairs show up naturally with maps (each entry is a pair) and as cheap two-value returns (find returns an iterator, insert returns pair<iteratorbool>). The structured binding auto [k, v] unpacks them elegantly.

Coming up

Three containers is a toolbox, not a rulebook. Next: how to actually choose.