Una discusión técnica sobre los comandos de C ++

Nota

This article assumes that you have a fair understanding of advanced C++ concepts, including templates, smart pointers, inheritance, rvalue references, copy semantics, move semantics, and CRTP. You do not need to understand the information within this article to use the command-based framework in your robot code.

This article will help you understand the reasoning behind some of the decisions made in the 2020 command-based framework (such as the use of std::unique_ptr, CRTP in the form of CommandHelper<Base, Derived>, etc.). You do not need to understand the information within this article to use the command-based framework in your robot code.

Nota

The model was further changed in 2023, as described below.

Modelo de propiedad

El antiguo marco basado en comandos empleaba el uso de punteros sin procesar, lo que significa que los usuarios tenían que usar nuevo (lo que resultaba en asignaciones de montón manuales) en su código de robot. Dado que no había una indicación clara sobre quién era el propietario de los comandos (el programador, los grupos de comandos o el usuario mismo), no era evidente quién se suponía que debía encargarse de liberar la memoria.

Varios ejemplos en el antiguo marco basado en comandos involucraban código como este:

#include "PlaceSoda.h"
#include "Elevator.h"
#include "Wrist.h"

PlaceSoda::PlaceSoda() {
  AddSequential(new SetElevatorSetpoint(Elevator::TABLE_HEIGHT));
  AddSequential(new SetWristSetpoint(Wrist::PICKUP));
  AddSequential(new OpenClaw());
}

In the command-group above, the component commands of the command group were being heap allocated and passed into AddSequential all in the same line. This meant that user had no reference to that object in memory and therefore had no means of freeing the allocated memory once the command group ended. The command group itself never freed the memory and neither did the command scheduler. This led to memory leaks in robot programs (i.e. memory was allocated on the heap but never freed).

Este problema evidente fue una de las razones de la reescritura del marco. Se introdujo un modelo de propiedad integral con esta reescritura, junto con el uso de punteros inteligentes que liberarán memoria automáticamente cuando se salgan del alcance.

Default commands are owned by the command scheduler whereas component commands of command compositions are owned by the command composition. Other commands are owned by whatever the user decides they should be owned by (e.g. a subsystem instance or a RobotContainer instance). This means that the ownership of the memory allocated by any commands or command compositions is clearly defined.

std::unique_ptr vs. std::shared_ptr

El uso de std::unique_ptr nos permite determinar claramente quién es el propietario del objeto. Debido a que un std::unique_ptr no se puede copiar, nunca habrá más de una instancia de un `` td::unique_ptr`` que apunte al mismo bloque de memoria en el montón. Por ejemplo, un constructor para SequentialCommandGroup toma un std::vector<std::unique_ptr<Command>> &&. Esto significa que requiere una referencia rvalue a un vector de std::unique_ptr <Command>. Repasemos un código de ejemplo paso a paso para comprender esto mejor:

// Let's create a vector to store our commands that we want to run sequentially.
std::vector<std::unique_ptr<Command>> commands;

// Add an instant command that prints to the console.
commands.emplace_back(std::make_unique<InstantCommand>([]{ std::cout << "Hello"; }, requirements));

// Add some other command: this can be something that a user has created.
commands.emplace_back(std::make_unique<MyCommand>(args, needed, for, this, command));

// Now the vector "owns" all of these commands. In its current state, when the vector is destroyed (i.e.
// it goes out of scope), it will destroy all of the commands we just added.

// Let's create a SequentialCommandGroup that will run these two commands sequentially.
auto group = SequentialCommandGroup(std::move(commands));

// Note that we MOVED the vector of commands into the sequential command group, meaning that the
// command group now has ownership of our commands. When we call std::move on the vector, all of its
// contents (i.e. the unique_ptr instances) are moved into the command group.

// Even if the vector were to be destroyed while the command group was running, everything would be OK
// since the vector does not own our commands anymore.

Con std::shared_ptr, no hay un modelo de propiedad claro porque puede haber varias instancias de un std::shared_ptr que apuntan al mismo bloque de memoria. Si los comandos estuvieran en instancias std::shared_ptr, un grupo de comandos o el programador de comandos no puede tomar posesión y liberar la memoria una vez que el comando ha terminado de ejecutarse porque el usuario, sin saberlo, todavía podría tener un std::shared_ptr instancia que apunta a ese bloque de memoria en algún lugar del alcance.

Uso de CRTP

You may have noticed that in order to create a new command, you must extend CommandHelper, providing the base class (usually frc2::Command) and the class that you just created. Let’s take a look at the reasoning behind this:

Decoradores de comando

El nuevo marco basado en comandos incluye una función conocida como «decoradores de comandos», que permite al usuario hacer algo como esto:

auto task = MyCommand().AndThen([] { std::cout << "This printed after my command ended."; },
  requirements);

Cuando se programa la tarea , primero ejecutará MyCommand() y una vez que ese comando haya terminado de ejecutarse, imprimirá el mensaje en la consola. La forma en que esto se logra internamente es mediante el uso de un grupo de comando secuencial.

Recuerde de la sección anterior que para construir un grupo de comando secuencial, necesitamos un vector de punteros únicos para cada comando. Crear el puntero único para la función de impresión es bastante trivial:

temp.emplace_back(
   std::make_unique<InstantCommand>(std::move(toRun), requirements));

Aquí, temp almacena el vector de comandos que necesitamos pasar al constructor SequentialCommandGroup . Pero antes de agregar ese InstantCommand,necesitamos agregar MyCommand() al``SequentialCommandGroup`` . ¿Como hacemos eso?

temp.emplace_back(std::make_unique<MyCommand>(std::move(*this));

You might think it would be this straightforward, but that is not the case. Because this decorator code is in the Command class, *this refers to the Command in the subclass that you are calling the decorator from and has the type of Command. Effectively, you will be trying to move a Command instead of MyCommand. We could cast the this pointer to a MyCommand* and then dereference it but we have no information about the subclass to cast to at compile-time.

Soluciones al problema

Nuestra solución inicial a esto fue crear un método virtual en Command llamado TransferOwnership() que cada subclase de Command tenía que anular. Tal anulación se habría visto así:

std::unique_ptr<Command> TransferOwnership() && override {
  return std::make_unique<MyCommand>(std::move(*this));
}

Debido a que el código estaría en la subclase derivada, *this realmente apuntaría a la instancia de subclase deseada y el usuario tiene la información de tipo de la clase derivada para hacer el puntero único.

Después de unos días de deliberación, se propuso un método CRTP. Aquí, una clase derivada intermedia de Command llamada CommandHelper  esxistiría. CommandHelper tendría dos argumentos de plantilla, la clase base original y la subclase derivada deseada. Echemos un vistazo a una implementación básica de CommandHelper para entender esto:

// In the real implementation, we use SFINAE to check that Base is actually a
// Command or a subclass of Command.
template<typename Base, typename Derived>
class CommandHelper : public Base {
  // Here, we are just inheriting all of the superclass (base class) constructors.
  using Base::Base;

  // Here, we will override the TransferOwnership() method mentioned above.
  std::unique_ptr<Command> TransferOwnership() && override {
    // Previously, we mentioned that we had no information about the derived class
    // to cast to at compile-time, but because of CRTP we do! It's one of our template
    // arguments!
    return std::make_unique<Derived>(std::move(*static_cast<Derived*>(this)));
  }
};

Así, haciendo que tus comandos personalizados extiendan CommandHelper en lugar de Command implementará automáticamente esta plantilla para ti y este es el razonamiento detrás de pedir a los equipos que usen lo que puede parecer una forma bastante oscura de hacer las cosas.

Volviendo a nuestro AndThen() ejemplo, ahora podemos hacer lo siguiente:

// Because of how inheritance works, we will call the TransferOwnership()
// of the subclass. We are moving *this because TransferOwnership() can only
// be called on rvalue references.
temp.emplace_back(std::move(*this).TransferOwnership());

Falta de decoradores avanzados

La mayoría de los decoradores de C++ toman std::function<void()> en lugar de comandos reales ellos mismos. La idea de tomar comandos reales en decoradores como AndThen(), BeforeStarting(), etc. fue considerada pero luego abandonada debido a una variedad de razones.

Decoradores de plantillas

Debido a que necesitamos saber los tipos de los comandos que estamos añadiendo a un grupo de comandos en tiempo de compilación, necesitaremos usar plantillas (variadic para múltiples comandos). Sin embargo, esto podría no parecer un gran problema. Los constructores de grupos de comandos lo hacen de todas formas:

template <class... Types,
         typename = std::enable_if_t<std::conjunction_v<
             std::is_base_of<Command, std::remove_reference_t<Types>>...>>>
explicit SequentialCommandGroup(Types&&... commands) {
  AddCommands(std::forward<Types>(commands)...);
}

template <class... Types,
         typename = std::enable_if_t<std::conjunction_v<
             std::is_base_of<Command, std::remove_reference_t<Types>>...>>>
void AddCommands(Types&&... commands) {
  std::vector<std::unique_ptr<Command>> foo;
  ((void)foo.emplace_back(std::make_unique<std::remove_reference_t<Types>>(
       std::forward<Types>(commands))),
   ...);
  AddCommands(std::move(foo));
}

Nota

Este es un constructor secundario para el SequentialCommandGroup, además del constructor de vectores que describimos anteriormente.

Sin embargo, cuando realizamos una función de plantilla, su definición debe ser declarada en línea. Esto significa que tendremos que instanciar el SequentialCommandGroup en el encabezado «Command.h», lo que plantea un problema. SequentialCommandGroup incluye «Command.h». Si incluimos SequentialCommandGroup dentro de «Command.h», tenemos una dependencia circular. ¿Cómo lo hacemos ahora entonces?

Usamos una declaración hacia adelante en la parte superior de Command.h:

class SequentialCommandGroup;

class Command { ... };

Y luego incluimos SequentialCommandGroup.h en «Command.cpp». Sin embargo, si estas funciones de decorador se templaron, no podemos escribir definiciones en los archivos .cpp, resultando en una dependencia circular.

Sintaxis de Java vs Sintaxis de C++

Estos decoradores suelen ahorrar más verbosidad en Java (porque Java requiere llamadas nuevas sin procesar) que en C++, así que en general, no hay mucha diferencia sintáctica en C++ si se crea el grupo de comandos manualmente en código de usuario.

2023 Updates

After a few years in the new command-based framework, the recommended way to create commands increasingly shifted towards inline commands, decorators, and factory methods. With this paradigm shift, it became evident that the C++ commands model introduced in 2020 and described above has some pain points when used according to the new recommendations.

A significant root cause of most pain points was commands being passed by value in a non-polymorphic way. This made object slicing mistakes rather easy, and changes in composition structure could propagate type changes throughout the codebase: for example, if a ParallelRaceGroup were changed to a ParallelDeadlineGroup, those type changes would propagate through the codebase. Passing around the object as a Command (as done in Java) would result in object slicing.

Additionally, various decorators weren’t supported in C++ due to reasons described above. As long as decorators were rarely used and were mainly to reduce verbosity (where Java was more verbose than C++), this was less of a problem. Once heavy usage of decorators was recommended, this became more of an issue.

CommandPtr

Let’s recall the mention of std::unique_ptr far above: a value type with only move semantics. This is the ownership model we want!

However, plainly using std::unique_ptr<Command> had some drawbacks. Primarily, implementing decorators would be impossible: unique_ptr is defined in the standard library so we can’t define methods on it, and any methods defined on Command wouldn’t have access to the owning unique_ptr.

The solution is CommandPtr: a move-only value class wrapping unique_ptr, that we can define methods on.

Commands should be passed around as CommandPtr, using std::move. All decorators, including those not supported in C++ before, are defined on CommandPtr with rvalue-this. The use of rvalues, move-only semantics, and clear ownership makes it very easy to avoid mistakes such as adding the same command instance to more than one command composition.

In addition to decorators, CommandPtr instances also define utility methods such as Schedule(), IsScheduled(). CommandPtr instances can be used in nearly almost every way command objects can be used in Java: they can be moved into trigger bindings, default commands, and so on. For the few things that require a Command* (such as non-owning trigger bindings), a raw pointer to the owned command can be retrieved using get().

There are multiple ways to get a CommandPtr instance:

  • CommandPtr-returning factories are present in the frc2::cmd namespace in the Commands.h header for almost all command types. For multi-command compositions, there is a vector-taking overload as well as a variadic-templated overload for multiple CommandPtr instances.

  • All decorators, including those defined on Command, return CommandPtr. This has allowed defining almost all decorators on Command, so a decorator chain can start from a Command.

  • A ToPtr() method has been added to the CRTP, akin to TransferOwnership. This is useful especially for user-defined command classes, as well as other command classes that don’t have factories.

For instance, consider the following from the HatchbotInlined example project <https://github.com/wpilibsuite/allwpilib/blob/v2023.2.1/wpilibcExamples/src/main/cpp/examples/HatchbotInlined/>:

33frc2::CommandPtr autos::ComplexAuto(DriveSubsystem* drive,
34                                    HatchSubsystem* hatch) {
35  return frc2::cmd::Sequence(
36      // Drive forward the specified distance
37      frc2::FunctionalCommand(
38          // Reset encoders on command start
39          [drive] { drive->ResetEncoders(); },
40          // Drive forward while the command is executing
41          [drive] { drive->ArcadeDrive(kAutoDriveSpeed, 0); },
42          // Stop driving at the end of the command
43          [drive](bool interrupted) { drive->ArcadeDrive(0, 0); },
44          // End the command when the robot's driven distance exceeds the
45          // desired value
46          [drive] {
47            return drive->GetAverageEncoderDistance() >=
48                   kAutoDriveDistanceInches;
49          },
50          // Requires the drive subsystem
51          {drive})
52          .ToPtr(),
53      // Release the hatch
54      hatch->ReleaseHatchCommand(),
55      // Drive backward the specified distance
56      // Drive forward the specified distance
57      frc2::FunctionalCommand(
58          // Reset encoders on command start
59          [drive] { drive->ResetEncoders(); },
60          // Drive backward while the command is executing
61          [drive] { drive->ArcadeDrive(-kAutoDriveSpeed, 0); },
62          // Stop driving at the end of the command
63          [drive](bool interrupted) { drive->ArcadeDrive(0, 0); },
64          // End the command when the robot's driven distance exceeds the
65          // desired value
66          [drive] {
67            return drive->GetAverageEncoderDistance() <=
68                   kAutoBackupDistanceInches;
69          },
70          // Requires the drive subsystem
71          {drive})
72          .ToPtr());
73}

To avoid breakage, command compositions still use unique_ptr<Command>, so CommandPtr instances can be destructured into a unique_ptr<Command> using the Unwrap() rvalue-this method. For vectors, the static CommandPtr::UnwrapVector(vector<CommandPtr>) function exists.