Variadic Templates and Fold Expressions
Parameter packs, pack expansion, and the folds that replaced recursive boilerplate — write sum, min, and counting utilities in one line.
Parameter packs
template <class... A> void f(A... args); captures any number of arguments of any types. sizeof...(args) is the count. Expansion args... unfolds the pack at each use site.
Unary folds — the one-line algorithms
A fold expression applies an operator across a pack:
template <class... A> constexpr auto sum(A... a) { return (a + ... + 0); } // binary right fold, 0 = empty-pack value
template <class... A> constexpr auto product(A... a) { return (a * ... * 1); }
template <class... A> constexpr int truthy(A... a) { return (0 + ... + (a ? 1 : 0)); }
Forms: (pack op ...) (unary right), (... op pack) (unary left), (pack op ... op init) (binary). The binary forms matter: they define the result for an empty pack — without one, sum() is ill-formed, which is exactly the bug your tests should catch.
if constexpr — compile-time branching
Inside the fold body you can branch on each element's type:
template <class... A> std::string describe(A... a);
// per element: if constexpr (std::is_integral_v<decltype(v)>) ... else ...
The discarded branch is not even compiled — which is what makes "serialize this only if it has that member" clean without SFINAE.