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C++20 STL Cookbook

Use Fold Expressions for Variadic Tuples

Explore how fold expressions simplify handling variadic tuples in C++20. Learn to create template functions using index sequences and fold expressions to process tuple elements of unknown size and types efficiently. Understand how to apply this technique for tasks like printing or summing tuple contents.

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The std::tuple class is essentially a more complex, and less convenient, struct. The interface for tuple is cumbersome, although class template argument deduction and structured binding have made it somewhat easier.

typically struct is used over tuple , with one significant exception: the one real advantage of tuple is that it can be used with fold expressions in a variadic context.

Fold expressions

Designed to make it easier to expand a variadic parameter pack, fold expressions are a new feature with C++17. Prior to fold expressions, expanding a parameter pack required a recursive function:

template<typename T>
void f(T final) {
    cout << final << '\n';
}
template<typename T, typename... Args>
void f(T first, Args... args) {
    cout << first;
    f(args...);
}
int main() {
    f("hello", ' ', 47, ' ', "world");
}

Output:

hello 47 world

Using a fold expression, this is much simpler:

template<typename... Args>
void f(Args... args) {
    (cout << ... << args);
    cout << '\n';
}

...