C++: Placeholder _

C++ is on its way to get a placeholder variable _ similar to other languages, e.g., Python, rust.

This is useful in multiple scenarios:

• Variables used just for their side effects do not require a unique name anymore, e.g. locks, and other RAII objects.

  std::lock_guard someUniqueName(mutex); // old: needed to find unique name
  std::lock_guard _(mutex); // new: no name needed anymore
  

• When unpacking objects via structured binding some parts might not be of interest. This can now be represented directly in code.

  [[maybe_unused]] auto [x, y, zUnused] = f(); // old
  auto [x, y, _] = f(); // new
  

• Supposedly useful for pattern matching.

Implementation available in gcc14 and clang18 (both unreleased trunk versions).

Resources:

• Short and precise explanation by Jason Turner’s cppweekly on youtube.
• Original paper: Placeholder variables with no name
• cppreference has no separate page as of this writing but the feature is listed on its C++26 overview page.