How to Effectively Iterate Over Packed Variadic Template Argument Lists in C ?

Iterating Over Packed Variadic Template Argument Lists

Introduction

Variadic templates allow you to create generic functions that can take a variable number of arguments. However, iterating over those arguments can be challenging. This article explores methods for iterating over packed variadic template argument lists, including a comprehensive solution using fold expressions.

Method 1: Iterate Over Homogeneous Type Inputs

If your inputs are all of the same type, you can use a macro or lambda function to loop through them. For example:

<code class="cpp">#include <iostream>

#define FOREACH_INPUT(...) (([](auto &&... inputs) { for (auto input : { __VA_ARGS__ }) { std::cout << input << std::endl; } })(inputs))

Usage:

<code class="cpp">FOREACH_INPUT(1, 2, 3, 4, 5);

Method 2: Fold Expressions (>= C 17)

For mixed-type inputs, fold expressions provide a concise and powerful approach. Consider the following example:

<code class="cpp">#include <iostream>
#include <utility>

template <class ... Ts>
void Foo (Ts &&... inputs)
{
    int i = 0;

    ([&amp;]
    {
        std::cout << "input " << ++i << " = " << inputs << std::endl;
    } (), ...);
}

int main()
{
    Foo(2, 3, 4u, (int64_t) 9, 'a', 2.3);
}</code>

Enhanced Fold Expression with Break/Return

If you need breaks or returns in your loop, consider the following workarounds:

• Try/Throw:

<code class="cpp">#include <iostream>
#include <utility>

template <class ... Ts>
void Foo (Ts &&... inputs)
{
    int i = 0;

    try {
        ([&amp;]
        {
            std::cout << "input " << ++i << " = " << inputs << std::endl;
            if (inputs == 'a') throw 1;
        } (), ...);
    } catch (...) {
    }
}

int main()
{
    Foo(2, 3, 4u, (int64_t) 9, 'a', 2.3);
}</code>

• Variable/If Switches:

<code class="cpp">#include <iostream>
#include <utility>

template <class ... Ts>
void Foo (Ts &&... inputs)
{
    int i = 0;
    bool exit = false;

    auto process_input = [&amp;](auto &amp;&amp;input) mutable {
        if (input == 'a') {
            std::cout << "Found 'a'. Breaking loop." << std::endl;
            exit = true;
            return;
        }
        std::cout << "input " << ++i << " = " << input << std::endl;
    };

    (process_input(inputs), ...);
    if (!exit)
        std::cout << "Finished looping through all inputs." << std::endl;
}

int main()
{
    Foo(2, 3, 4u, (int64_t) 9, 'a', 2.3);
}</code>

The above is the detailed content of How to Effectively Iterate Over Packed Variadic Template Argument Lists in C ?. For more information, please follow other related articles on the PHP Chinese website!