\begin{frame}[fragile]

\frametitlecpp[11]{Containers: \texttt{std::unordered\_map}}

\begin{block}{}

Conceptually a container of \cppinline{std::pair<Key const, Value>}

\end{block}

\begin{cppcode*}{gobble=4}

#include <unordered_map>

std::unordered_map<std::string, int> m;

m["hello"] = 1; // inserts new key, def. constr. value

m["hello"] = 2; // finds existing key

auto [it, isNewKey] = m.insert("hello");

int val = m["world"]; // inserts new key (val == 0)

int val = m.at("monde"); // throws std::out_of_range

if (auto it = m.find("hello"); it != m.end()) // C++17

m.erase(it); // remove by iterator (fast)

if (m.contains("hello")) // C++20

m.erase("hello"); // remove by key, 2. lookup, bad

for (auto const& [k, v] : m) // iterate k/v pairs (C++17)

std::cout << k << ": " << v << '\n';

\end{cppcode*}

\end{frame}

\begin{frame}[fragile]

\frametitlecpp[11]{\texttt{std::hash}}

\begin{block}{}

\begin{itemize}

\item The standard utility to create hash codes

\item Used by \cppinline{std::unordered_map} and others

\item Can be customized for your types via template specialization

\end{itemize}

\end{block}

\begin{cppcode*}{gobble=4}

#include <functional>

std::hash<std::string> h;

std::cout << h("hello"); // 2762169579135187400

std::cout << h("world"); // 8751027807033337960

class MyClass { int a, b; ... };

template<> struct std::hash<MyClass> {

std::size_t operator()(MyClass const& c) {

std::hash<int> h;

return h(c.a) ^ h(c.b.); // xor to combine hashes

}

};

\end{cppcode*}

\end{frame}