How to Use `std::source_location` with Variadic Templates in C 20?

Utilizing std::source_location in Variadic Templates: A Comprehensive Guide

Introduction:

The std::source_location, introduced in C 20, provides valuable insights into the execution context of functions. However, its integration with variadic template functions can pose challenges. This article unravels the complexities of using std::source_location in variadic template functions and presents practical solutions.

Problem:

Attempts to incorporate std::source_location in variadic template functions face obstacles due to its placement within the parameter list. Placing it at the beginning of the list conflicts with the variadic parameter requirement, which must be the last parameter. On the other hand, inserting it between the variadic parameters can disrupt the calling convention.

Solution 1: Deduction Guide Rescue

The first solution involves employing a deduction guide to deduce the proper function template based on the parameters provided. By adding a deduction guide that includes std::source_location as an optional parameter, we can resolve the placement issue.

<code class="cpp">template <typename... Ts>
struct debug
{
    debug(Ts&&... ts, const std::source_location& loc = std::source_location::current());
};

template <typename... Ts>
debug(Ts&&...) -> debug<Ts...>;</code>

Solution 2: Out-of-Parameter Placement

Alternatively, we can declare the std::source_location parameter outside the variadic template function, bypassing the placement limitations.

<code class="cpp">auto debug_caller(const std::source_location& loc = std::source_location::current())
{
    return [=](auto&&... args)
    {
        // Utilize location information within the lambda expression
    };
}</code>

This approach provides more flexibility and prevents disruption of the calling convention.

Example Demonstration:

<code class="cpp">int main()
{
    debug(5, 'A', 3.14f, "foo");
}</code>

In this example, the location information is captured and can be utilized within the debug function.

Conclusion:

While using std::source_location in variadic template functions requires careful consideration, the solutions presented in this article offer practical ways to leverage its capabilities. Whether through deduction guides or parameter placement modifications, developers can effectively capture execution context information, enriching their code with additional insights.

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