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

Resolving Source Location Woes in Variadic Template Functions

In C 20, capturing function calling context details is made possible with std::source_location. However, employing it with variadic template functions has proven challenging due to the positioning of the source_location parameter.

The Position Predicament

Invariably, variadic parameters inhabit the end of the parameter list. This hindered the use of std::source_location due to the following reasons:

• First Attempt:

  <code class="cpp">template <typename... Args>
  void debug(Args&&... args, const std::source_location& loc = std::source_location::current());</code>

  fails because variadic parameters must reside at the end.

• Second Attempt:

  <code class="cpp">template <typename... Args>
  void debug(const std::source_location& loc = std::source_location::current(),
           Args&&... args);</code>

  introduces ambiguity for callers as it interjects an unexpected parameter.

Solution: Embracing Deduction Guides

The initial form can be revitalized by introducing a deduction guide:

<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>

This allows calls like:

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

Conclusion:

Through the utilization of deduction guides, C programmers can seamlessly incorporate std::source_location into variadic template functions to capture function call context information effortlessly.

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