std::chrono::locate_zone returns null pointer on some platforms

Answer to the Core Question

On any platform, std::chrono::locate_zone will return nullptr when the requested zone name cannot be found in the time‑zone database that the implementation has loaded. This includes:

• Missing or incomplete IANA tzdata on the host.
• An implementation that bundles no database (e.g., many Windows builds of libstdc++).
• A misspelled or unsupported zone name.
• Failure to load the database due to permission or configuration errors.

The C++20 standard allows either a nullptr or a thrown std::runtime_error for database load failures, so the observed behavior can vary between libstdc++, libc++, and other libraries.

Confirmed Facts

• std::chrono::locate_zone returns a std::chrono::time_zone const* or nullptr if the zone is not found.
• The standard does not mandate an embedded zone database; implementations typically read from /usr/share/zoneinfo on POSIX or require the user to supply a path on Windows.
• libstdc++ supports the environment variable TZDATA_PATH to point to a custom tzdata tree.
• libc++ follows the same rule but uses TZDATA_PATH or falls back to the system default.
• There is no standard‑conforming API to enumerate all available zone names.

Likely Explanation for Your Observation

On the platforms where you saw nullptr, the binary was either executed on a system without a tzdata tree or the library could not locate the requested zone due to a mis‑configured path. For example, a Windows build of libstdc++ will return nullptr for any zone unless you install tzdata and set TZDATA_PATH.

Portable Detection & Handling

1. Compile a small test:

   #include <chrono>
   #include <iostream>
   int main(){
       if(auto p = std::chrono::locate_zone("America/New_York"))
           std::cout << "found";
       else
           std::cout << "nullptr";
   }
   

2. Run on each target OS: If the output is nullptr, the zone is not available.
3. Supply tzdata on Windows: Place the IANA files under C:\\tzdata and set the environment variable before launching:

   set TZDATA_PATH=C:\\tzdata
   

4. On POSIX, ensure /usr/share/zoneinfo is present and readable. If you ship a custom database, set TZDATA_PATH to its root.
5. Always check the returned pointer before dereferencing:

   auto tz = std::chrono::locate_zone(name);
   if(!tz) {
       // Fallback: use std::chrono::current_zone() or a user‑supplied default
   }
   

Runtime Fallback Strategy

When nullptr is returned, you can:

• Fall back to std::chrono::current_zone() to use the host’s local zone.
• Use a third‑party library (e.g., date::tzdb) that bundles its own database.
• Prompt the user to install tzdata or provide a path.

Enumerating Available Zones

The C++20 standard does not expose an enumeration API. Some libraries provide non‑standard extensions, such as std::chrono::tzdb::names() in date.h, but relying on them reduces portability. If enumeration is essential, ship your own tzdata set or use a library that offers it.

Missing Diagnostic Detail

To refine the recommendation, could you confirm which standard library (libstdc++, libc++, etc.) and compiler version you are using on each platform?