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

Using std::source_location in Variadic Template Functions

When leveraging the C 20 feature std::source_location to capture context information in variadic template functions, a challenge arises in determining where to place the source_location parameter.

Failed Attempts

Incorrect attempts include:

• Variadic Parameters at the End:

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

This fails because variadic parameters must be at the end.

• Parameter Insertion:

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

This confuses the caller, which would error when passing regular arguments (e.g., debug(42);).

Solution: Deduction Guide

The issue can be resolved by adding a deduction guide to the first form:

<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 deduction guide infers the template parameters based on the function arguments, allowing the source_location parameter to be placed at the end.

Test and Demonstration

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

Live Demo: https://godbolt.org/z/n9Wpo9Wrj

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