Summary - Variadic templates

Combine fold expressions and perfect forwarding to write a print function.

Summary - Variadic templates

In this group you met five tools of variadic templates: type packs with typename... Args, value packs with Args... args, pack size with sizeof..., fold expressions with (args + ...) and heterogeneous packing with std::tuple. Perfect forwarding ties these tools together in wrapper functions.

Together they form the foundation of modern APIs like std::make_unique and std::emplace_back. Now you will write a small task that combines what you learned.

Exercise Summary with tuple + fold

Write template <typename... Args> auto sum_all(Args... args) with body return (args + ...);. In main create auto t = std::make_tuple(1, 2, 3);, then call sum_all(std::get<0>(t), std::get<1>(t), std::get<2>(t)) and print the result via std::cout.

Show example solution

#include <iostream>
#include <tuple>

template <typename... Args>
auto sum_all(Args... args) {
    return (args + ...);
}

int main() {
    auto t = std::make_tuple(1, 2, 3);
    std::cout << sum_all(std::get<0>(t), std::get<1>(t), std::get<2>(t));
    return 0;
}

Test yourself

Which tool reduces a pack with one operator?
- sizeof...
- std::forward
- std::tuple
- fold expression
Show answer

fold expression — A fold expression (args + ...) reduces a pack with one operator.
Match each tool to its purpose

typename... Args

Type pack definition
sizeof...(Args)

Pack size
(args + ...)

Fold reduction
std::tuple<T1, T2>

Heterogeneous pack

Complete the fold line

template <typename... Args>
auto sum_all(Args... args) {
    return (args + ____);
}

Show answer

template <typename... Args>
auto sum_all(Args... args) {
    return (args + ...);
}

Next lesson: What are type traits?

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