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:

  template 
  void debug(Args&&... args, const std::source_location& loc = std::source_location::current());

  fails because variadic parameters must reside at the end.

• Second Attempt:

  template 
  void debug(const std::source_location& loc = std::source_location::current(),
           Args&&... args);

  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:

template 
struct debug
{    
    debug(Ts&&... ts, const std::source_location& loc = std::source_location::current());
};

template 
debug(Ts&&...) -> debug;

This allows calls like:

int main()
{
    debug(5, 'A', 3.14f, "foo");
}

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.