Currently in the entire C API of WPILib we have ~8 different ways of handling strings. The C API actually isn't built for pure C callers (We don't actually have any of those). Instead, they're built for interop between languages like LabVIEW and C# which can talk to C API's directly.
For output parameters, the choice was fairly obvious. An output struct containing a const string pointer and a length makes the most sense. Its easy to use these from most other languages, and doesn't require special null termination handling. Freeing these is also easy, as if you ever receive one of these string structures, theres just a single function call to free it.
Input parameters are a bit more complex. To be used from pure C, and from LabVIEW, a null terminated string is the best in most cases. However, null terminated strings in general have a lot of downsides. Additionally, from LabVIEW there are other considerations around encoding that having a wrapper struct helps make a bit easier. From a language like C#, a wrapper struct is by far the easiest, as custom marshalling can make it trivial to marshal both UTF8 and UTF16 strings down.
The final consideration is its nice to have an identical concept for both input and output. It makes the rules fairly easy to understand.
WPILib will not have any APIs that manipulate a string allocated externally. This means WPI_String can be const, as across the boundary it is always const.
If a WPILib API takes a const WPI_String*, WPILib will not manipulate or attempt to free that string, and that string is treated as an input. It is up to the caller to handle that memory, WPILib will never hold onto that memory longer than the call.
If a WPILib API takes a WPI_String*, that string is an output. WPILib will allocate that API with WPI_AllocateString(), fill in the string, and return to the caller. When the caller is done with the string, they must free it with WPI_FreeString().
If an output struct contains a WPI_String member, that member is considered read only, and should not be explicitly freed. The caller should call the free function for that struct.
If an array of WPI_Strings are returned, each individual string is considered read only, and should not be explicitly freed. The free function for that array should be called by the caller.
If an input struct containing a WPI_String, or an input array of WPI_Strings is passed to WPILib, the individual strings will not be manipulated or freed by WPILib, and the caller owns and should free that memory.
Callbacks also follow these rules. The most common is a callback either getting passed a const WPI_String* or a struct containing a WPI_String. In both of these cases, the callback target should consider these strings read only, and not attempt to free them or manipulate them.
DataLog is now a base class, with DataLogBackgroundWriter being the
background thread version and DataLogWriter being a non-threaded version.
Also split the C header into a separate file to make it more wpiformat friendly.
We now use a wrapper (wpi::print) to catch exceptions since we can't patch
std::print() to not throw when we ultimately migrate to it.
fmtlib and std format/print throw the same exceptions and always have. We previously patched fmt::print() to not throw a write failure exception, but we can't do that for std::print(); wpi::print() is the migration plan.
Unit objects now have a reference to the base unit from which they're derived. Constructing a unit object without specifying a base unit implicitly signifies that it's its own base unit, eg new Angle(null, 1, "Radian", "rad") would be the base angle unit of radians, while new Angle(Radians, 2 * PI, "Rotation", "R") would be a new angle unit based on radians.
This fixes much of the hacky code surrounding the derived unit types Velocity, Per, and Mult, but is a breaking change for any user code that defines custom unit classes or uses the anonymous unit type.
C++ doesn't need this because it supports value types, which are much
cheaper to construct. constexpr is also available to make construction
zero-cost.
Flattens parameter from a `std::optional<std::function<...>>` to just a `std::function<...>`. This is a breaking change but a trivial one for teams to fix.
LTVUnicycleController is a drop-in replacement with better tuning knobs.
The RamseteCommand examples were removed instead of retrofitted with
LTVUnicycleController because we're planning on removing the command
controller classes anyway, so it would be wasted effort. The
SimpleDifferentialDriveSimulation example shows direct
LTVUnicycleController usage.
