value_producer.h

#pragma once

#include <functional>
#include <memory>
#include <typeinfo>
#include <utility>

#include "drake/common/drake_copyable.h"
#include "drake/common/value.h"
#include "drake/systems/framework/abstract_value_cloner.h"
#include "drake/systems/framework/context_base.h"

namespace drake {
namespace systems {

/** %ValueProducer computes an AbstractValue output based on a ContextBase
input. This is commonly used for declaring output ports and cache entries.

It provides two functions for that purpose:
- Allocate() returns new storage that is suitably typed to hold the output.
- Calc() takes a context as input and writes to an output pointer.

For example, given this example calculator lambda:

<!-- The below is repeated in the unit test as DocumentationExample1; keep
it in sync between here and the unit test. -->

@code
  std::function calc = [](const Context<T>& context, std::string* output) {
    *output = std::to_string(context.get_time());
  };
@endcode

We can capture it into a producer and then call it:

@code
  ValueProducer producer(calc);
  std::unique_ptr<AbstractValue> storage = producer.Allocate();
  const LeafContext<T> context;
  producer.Calc(context, storage.get());
  EXPECT_THAT(storage->get_value<std::string>(), ::testing::StartsWith("0.0"));
@endcode

Sugar is provided to create %ValueProducer objects from function pointers that
operate on un-erased types, so that the user can ignore the details of type
erasure and Context<T> downcasting. Refer to the
@ref ValueProducer_constructors "Constructor overloads" for details. */
class ValueProducer final {
 public:
  DRAKE_DEFAULT_COPY_AND_MOVE_AND_ASSIGN(ValueProducer)

  /** Signature of a function suitable for allocating an object that can hold
  a value compatible with our Calc function. The result is always returned as
  an AbstractValue but must contain the correct concrete type. */
  using AllocateCallback = std::function<std::unique_ptr<AbstractValue>()>;

  /** Signature of a function suitable for calculating a context-dependent
  value, given a place to put the value. The function may presume that the
  storage pointed to by the second argument will be of the proper type (as
  returned by an AllocateCallback), but should not presume that the storage
  has been initialized with any particular value; the function should always
  fully overwrite the output storage with a new value. */
  using CalcCallback = std::function<void(const ContextBase&, AbstractValue*)>;

  /** Creates an invalid object; calls to Allocate or Calc will throw. */
  ValueProducer();

  /** @name Constructor overloads
  @anchor ValueProducer_constructors

  Create a ValueProducer by providing it a with calculation callback and
  (if necessary) a way to allocate storage, in cases where the storage cannot
  be default constructed.

  <!-- The below is repeated in the unit test as DocumentationExample2; keep
  it in sync between here and the unit test. -->

  In many cases, your calculator function would be a class member function
  like this:
  @code
    class MyClass {
      void MyCalc(const Context<T>& context, std::string* output) const {
        *output = std::to_string(context.get_time());
      }
    };
  @endcode
  and wrapping it would look like this:
  @code
    MyClass my_class;
    ValueProducer foo = ValueProducer(&my_class, &MyClass::MyCalc);
  @endcode

  <!-- The below is repeated in the unit test as DocumentationExample3; keep
  it in sync between here and the unit test. -->

  If the type of the output value is cheap to copy, then the function may
  return it by-value, instead of as an output argument:
  @code
    class MyClass {
      double MyCalc(const Context<double>& context) const {
        return context.get_time();
      }
    };

    MyClass my_class;
    ValueProducer foo = ValueProducer(&my_class, &MyClass::MyCalc);
  @endcode

  <!-- The below is repeated in the unit test as DocumentationExample4; keep
  it in sync between here and the unit test. -->

  If the type of the output is not default constructible, then you must provide
  ValueProducer with an example value to use for pre-allocation:
  @code
    class MyClass {
      void MyCalc(const Context<T>& context, BasicVector<T>* output) const {
        output->get_mutable_value()[0] = context.get_time();
      }
    };

    MyClass my_class;
    BasicVector<T> model_value(1);
    ValueProducer foo = ValueProducer(
        &my_class, model_value, &MyClass::MyCalc);
  @endcode

  In the rare case that you cannot provide an example value when creating the
  ValueProducer, you may instead provide an allocation callback. Refer to
  more specific documentation below for an example.

  For ease of use, the constructor offers overloads to specify the allocate and
  calc functions.

  The permitted argument types to specify Calc are:

  - (1) `calc` is a member function pointer with an output argument.
  - (2) `calc` is a member function pointer with a return value †.
  - (3) `calc` is a std::function with an output argument.
  - (4) `calc` is a std::function with a return value †.
  - (5) `calc` is a generic CalcCallback.

  † Do not use (2) nor (4) unless the return value is cheap to copy.

  The permitted argument types to specify Allocate are:

  - (a) `allocate` is via the default constructor.
  - (b) `allocate` is via user-supplied model_value.
  - (c) `allocate` is a member function pointer.
  - (d) `allocate` is a generic AllocateCallback.

  All combinations of (1..5) x (a..d) are permitted, except for (5a) because
  the output type cannot be inferred from a generic CalcCallback.

  All member function pointers must refer to member functions declared *const*.

  For `calc` types (3) and (4), to pass bare non-member function pointer instead
  of a lambda, you must explicitly convert the pointer to a `std::function`:

  @code
  void MyCalc(const Context<T>& context, std::string* output) { ... }

  ValueProducer producer{std::function(&MyCalc)};
  @endcode

  The constructors take the following arguments, in order:

  - The `instance` pointer (iff member function callback(s) are used).
  - The `model_value` example or `allocate` callback (may be omitted).
  - The `calc` callback (required).

  If either `allocate` or `calc` is a member function pointer, then the class
  `instance` to bind must be the first argument. This `instance` pointer is
  aliased by the ValueProducer, so must outlive this object. (In practice,
  this happens automatically because the SomeClass `instance` will typically
  own all ValueProducer objects that call back into it.)

  The `model_value` and `allocate` callback may be omitted when the OutputType
  is default constructible; this is the most common case. The need for a custom
  `allocate` function (i.e., options c or d, above) is extremely rare; prefer
  to use the default constructor (option a) or model_value (option b) in almost
  all cases.

  @tparam SomeContext the subclass of ContextBase required by the calc callback
  @tparam SomeOutput the output type of the calc callback
  @tparam SomeClass the static type of the class to receive the callback(s)
  @tparam SomeInstance the type of the instance to receive the callback(s);
     must be castable into SomeClass

  @throws std::exception if any argument is null. */
  //@{

  /** Overload (1a). This is the best choice. Refer to the
  @ref ValueProducer_constructors "Constructor overloads" for details.
  @exclude_from_pydrake_mkdoc{Not bound} */
  template <class SomeInstance, typename SomeClass, typename SomeContext,
            typename SomeOutput>
  ValueProducer(
      const SomeInstance* instance,
      void (SomeClass::*calc)(const SomeContext&, SomeOutput*) const)
      : ValueProducer(make_allocate_mode_a<SomeOutput>(),
                      make_calc_mode_1(instance, calc)) {}

  /** Overload (1b). This is the second-best choice. Refer to the
  @ref ValueProducer_constructors "Constructor overloads" for details.
  @exclude_from_pydrake_mkdoc{Not bound} */
  template <class SomeInstance, typename SomeClass, typename SomeContext,
            typename SomeOutput>
  ValueProducer(
      const SomeInstance* instance,
      const SomeOutput& model_value,
      void (SomeClass::*calc)(const SomeContext&, SomeOutput*) const)
      : ValueProducer(make_allocate_mode_b(model_value),
                      make_calc_mode_1(instance, calc)) {}

  /** Overload (1c). Refer to the
  @ref ValueProducer_constructors "Constructor overloads" for details.
  @exclude_from_pydrake_mkdoc{Not bound} */
  template <class SomeInstance, typename SomeClass, typename SomeContext,
            typename SomeOutput>
  ValueProducer(
      const SomeInstance* instance,
      std::unique_ptr<SomeOutput> (SomeClass::*allocate)() const,
      void (SomeClass::*calc)(const SomeContext&, SomeOutput*) const)
      : ValueProducer(make_allocate_mode_c(instance, allocate),
                      make_calc_mode_1(instance, calc)) {}

  /* Overload (1d). Refer to the
  @ref ValueProducer_constructors "Constructor overloads" for details.
  @exclude_from_pydrake_mkdoc{Not bound} */
  template <class SomeInstance, typename SomeClass, typename SomeContext,
            typename SomeOutput>
  ValueProducer(
      const SomeInstance* instance,
      AllocateCallback allocate,
      void (SomeClass::*calc)(const SomeContext&, SomeOutput*) const)
      : ValueProducer(std::move(allocate),
                      make_calc_mode_1(instance, calc)) {}

  /** Overload (2a). Refer to the
  @ref ValueProducer_constructors "Constructor overloads" for details.
  @exclude_from_pydrake_mkdoc{Not bound} */
  template <class SomeInstance, typename SomeClass, typename SomeContext,
            typename SomeOutput>
  ValueProducer(
      const SomeInstance* instance,
      SomeOutput (SomeClass::*calc)(const SomeContext&) const)
      : ValueProducer(make_allocate_mode_a<SomeOutput>(),
                      make_calc_mode_2(instance, calc)) {}

  /** Overload (2b). Refer to the
  @ref ValueProducer_constructors "Constructor overloads" for details.
  @exclude_from_pydrake_mkdoc{Not bound} */
  template <class SomeInstance, typename SomeClass, typename SomeContext,
            typename SomeOutput>
  ValueProducer(
      const SomeInstance* instance,
      const SomeOutput& model_value,
      SomeOutput (SomeClass::*calc)(const SomeContext&) const)
      : ValueProducer(make_allocate_mode_b(model_value),
                      make_calc_mode_2(instance, calc)) {}

  /** Overload (2c). Refer to the
  @ref ValueProducer_constructors "Constructor overloads" for details.
  @exclude_from_pydrake_mkdoc{Not bound} */
  template <class SomeInstance, typename SomeClass, typename SomeContext,
            typename SomeOutput>
  ValueProducer(
      const SomeInstance* instance,
      std::unique_ptr<SomeOutput> (SomeClass::*allocate)() const,
      SomeOutput (SomeClass::*calc)(const SomeContext&) const)
      : ValueProducer(make_allocate_mode_c(instance, allocate),
                      make_calc_mode_2(instance, calc)) {}

  /** Overload (2d). Refer to the
  @ref ValueProducer_constructors "Constructor overloads" for details.
  @exclude_from_pydrake_mkdoc{Not bound} */
  template <class SomeInstance, typename SomeClass, typename SomeContext,
            typename SomeOutput>
  ValueProducer(
      const SomeInstance* instance,
      AllocateCallback allocate,
      SomeOutput (SomeClass::*calc)(const SomeContext&) const)
      : ValueProducer(std::move(allocate),
                      make_calc_mode_2(instance, calc)) {}

  /** Overload (3a). Refer to the
  @ref ValueProducer_constructors "Constructor overloads" for details.
  @exclude_from_pydrake_mkdoc{Not bound} */
  template <typename SomeContext, typename SomeOutput>
  explicit ValueProducer(
      std::function<void(const SomeContext&, SomeOutput*)> calc)
      : ValueProducer(make_allocate_mode_a<SomeOutput>(),
                      make_calc_mode_3(std::move(calc))) {}

  /** Overload (3b). Refer to the
  @ref ValueProducer_constructors "Constructor overloads" for details.
  @exclude_from_pydrake_mkdoc{Not bound} */
  template <typename SomeContext, typename SomeOutput>
  ValueProducer(
      const SomeOutput& model_value,
      std::function<void(const SomeContext&, SomeOutput*)> calc)
      : ValueProducer(make_allocate_mode_b(model_value),
                      make_calc_mode_3(std::move(calc))) {}

  /** Overload (3c). Refer to the
  @ref ValueProducer_constructors "Constructor overloads" for details.
  @exclude_from_pydrake_mkdoc{Not bound} */
  template <class SomeInstance, typename SomeClass, typename SomeContext,
            typename SomeOutput>
  ValueProducer(
      const SomeInstance* instance,
      std::unique_ptr<SomeOutput> (SomeClass::*allocate)() const,
      std::function<void(const SomeContext&, SomeOutput*)> calc)
      : ValueProducer(make_allocate_mode_c(instance, allocate),
                      make_calc_mode_3(std::move(calc))) {}

  /** Overload (3d). Refer to the
  @ref ValueProducer_constructors "Constructor overloads" for details.
  @exclude_from_pydrake_mkdoc{Not bound} */
  template <typename SomeContext, typename SomeOutput>
  ValueProducer(
      AllocateCallback allocate,
      std::function<void(const SomeContext&, SomeOutput*)> calc)
      : ValueProducer(std::move(allocate),
                      make_calc_mode_3(std::move(calc))) {}

  /** Overload (4a). Refer to the
  @ref ValueProducer_constructors "Constructor overloads" for details.
  @exclude_from_pydrake_mkdoc{Not bound} */
  template <typename SomeContext, typename SomeOutput>
  explicit ValueProducer(
      std::function<SomeOutput(const SomeContext&)> calc)
      : ValueProducer(make_allocate_mode_a<SomeOutput>(),
                      make_calc_mode_4(std::move(calc))) {}

  /** Overload (4b). Refer to the
  @ref ValueProducer_constructors "Constructor overloads" for details.
  @exclude_from_pydrake_mkdoc{Not bound} */
  template <typename SomeContext, typename SomeOutput>
  ValueProducer(
      const SomeOutput& model_value,
      std::function<SomeOutput(const SomeContext&)> calc)
      : ValueProducer(make_allocate_mode_b(model_value),
                      make_calc_mode_4(std::move(calc))) {}

  /** Overload (4c). Refer to the
  @ref ValueProducer_constructors "Constructor overloads" for details.
  @exclude_from_pydrake_mkdoc{Not bound} */
  template <class SomeInstance, typename SomeClass, typename SomeContext,
            typename SomeOutput>
  ValueProducer(
      const SomeInstance* instance,
      std::unique_ptr<SomeOutput> (SomeClass::*allocate)() const,
      std::function<SomeOutput(const SomeContext&)> calc)
      : ValueProducer(make_allocate_mode_c(instance, allocate),
                      make_calc_mode_4(std::move(calc))) {}

  /** Overload (4d). Refer to the
  @ref ValueProducer_constructors "Constructor overloads" for details.
  @exclude_from_pydrake_mkdoc{Not bound} */
  template <typename SomeContext, typename SomeOutput>
  ValueProducer(
      AllocateCallback allocate,
      std::function<SomeOutput(const SomeContext&)> calc)
      : ValueProducer(std::move(allocate),
                      make_calc_mode_4(std::move(calc))) {}

  // Overload (5a) is omitted because we cannot infer the type of SomeOutput
  // from a generic CalcCallback.

  /** Overload (5b). Refer to the
  @ref ValueProducer_constructors "Constructor overloads" for details.
  @exclude_from_pydrake_mkdoc{Not bound} */
  template <typename SomeOutput,
      typename = std::enable_if_t<!std::is_convertible_v<
          SomeOutput, AllocateCallback>>>
  ValueProducer(
      const SomeOutput& model_value,
      CalcCallback calc)
      : ValueProducer(make_allocate_mode_b(model_value),
                      std::move(calc)) {}

  /** Overload (5c). Refer to the
  @ref ValueProducer_constructors "Constructor overloads" for details.
  @exclude_from_pydrake_mkdoc{Not bound} */
  template <class SomeInstance, typename SomeClass, typename SomeOutput>
  ValueProducer(
      const SomeInstance* instance,
      std::unique_ptr<SomeOutput> (SomeClass::*allocate)() const,
      CalcCallback calc)
      : ValueProducer(make_allocate_mode_c(instance, allocate),
                      std::move(calc)) {}

  /** Overload (5d). Refer to the
  @ref ValueProducer_constructors "Constructor overloads" for details.
  @pydrake_mkdoc_identifier{overload_5d} */
  ValueProducer(AllocateCallback allocate, CalcCallback calc);

  //@}

  ~ValueProducer();

  /** Returns true iff the allocate and calc callbacks are both non-null.
  (The only way they can be null is if the ValueProducer was default constructed
  or moved from.) */
  bool is_valid() const;

  /** This static function is provided for users who need an empty CalcCallback.
  Passing `&ValueProducer::NoopCalc` as ValueProducer's last constructor
  argument will create a function that does not compute anything, but can still
  allocate. */
  static void NoopCalc(const ContextBase&, AbstractValue*);

  /** Invokes the allocate function provided to the constructor.
  @throws std::exception if is_valid() is false. */
  std::unique_ptr<AbstractValue> Allocate() const;

  /** Invokes the calc function provided to the constructor.
  @throws std::exception if is_valid() is false. */
  void Calc(const ContextBase& context, AbstractValue* output) const;

 private:
  /** Reports that a callback pointer was null. */
  [[noreturn]] static void ThrowBadNull();

  /** Reports that a dynamic_cast failed. */
  [[noreturn]] static void ThrowBadCast(
      const std::type_info& actual_type,
      const std::type_info& desired_type);

  template <typename SomeClass, class SomeInstance>
  static const SomeClass* instance_cast(const SomeInstance* instance) {
    if (instance == nullptr) {
      ThrowBadNull();
    }
    const auto* typed_instance = dynamic_cast<const SomeClass*>(instance);
    if (typed_instance == nullptr) {
      ThrowBadCast(typeid(*instance), typeid(SomeInstance));
    }
    return typed_instance;
  }

  template <class SomeContext>
  static const SomeContext& context_cast(const ContextBase& context) {
    const auto* typed_context = dynamic_cast<const SomeContext*>(&context);
    if (typed_context == nullptr) {
      ThrowBadCast(typeid(context), typeid(SomeContext));
    }
    return *typed_context;
  }

  // For overload series (1).
  template <class SomeInstance, typename SomeClass, typename SomeContext,
            typename SomeOutput>
  static CalcCallback make_calc_mode_1(
      const SomeInstance* instance,
      void (SomeClass::*calc)(const SomeContext&, SomeOutput*) const) {
    static_assert(std::is_base_of_v<ContextBase, SomeContext>,
                  "The inferred type of SomeContext was invalid;"
                  " typically it should be Context<T>.");
    const SomeClass* typed_instance = instance_cast<SomeClass>(instance);
    if (calc == nullptr) {
      ThrowBadNull();
    }
    return [typed_instance, calc](const ContextBase& context,
                                  AbstractValue* result) {
      const SomeContext& typed_context = context_cast<SomeContext>(context);
      SomeOutput& typed_result = result->get_mutable_value<SomeOutput>();
      (typed_instance->*calc)(typed_context, &typed_result);
    };
  }

  // For overload series (2).
  template <class SomeInstance, typename SomeClass, typename SomeContext,
            typename SomeOutput>
  static CalcCallback make_calc_mode_2(
      const SomeInstance* instance,
      SomeOutput (SomeClass::*calc)(const SomeContext&) const) {
    static_assert(std::is_base_of_v<ContextBase, SomeContext>,
                  "The inferred type of SomeContext was invalid;"
                  " typically it should be Context<T>.");
    const SomeClass* typed_instance = instance_cast<SomeClass>(instance);
    if (calc == nullptr) {
      ThrowBadNull();
    }
    return [typed_instance, calc](const ContextBase& context,
                                  AbstractValue* result) {
      const SomeContext& typed_context = context_cast<SomeContext>(context);
      SomeOutput& typed_result = result->get_mutable_value<SomeOutput>();
      typed_result = (typed_instance->*calc)(typed_context);
    };
  }

  // For overload series (3).
  template <typename SomeContext, typename SomeOutput>
  static CalcCallback make_calc_mode_3(
      std::function<void(const SomeContext&, SomeOutput*)>&& calc) {
    static_assert(std::is_base_of_v<ContextBase, SomeContext>,
                  "The inferred type of SomeContext was invalid;"
                  " typically it should be Context<T>.");
    if (calc == nullptr) {
      ThrowBadNull();
    }
    return [captured_calc = std::move(calc)](const ContextBase& context,
                                             AbstractValue* result) {
      const SomeContext& typed_context = context_cast<SomeContext>(context);
      SomeOutput& typed_result = result->get_mutable_value<SomeOutput>();
      captured_calc(typed_context, &typed_result);
    };
  }

  // For overload series (4).
  template <typename SomeContext, typename SomeOutput>
  static CalcCallback make_calc_mode_4(
      std::function<SomeOutput(const SomeContext&)>&& calc) {
    static_assert(std::is_base_of_v<ContextBase, SomeContext>,
                  "The inferred type of SomeContext was invalid;"
                  " typically it should be Context<T>.");
    if (calc == nullptr) {
      ThrowBadNull();
    }
    return [captured_calc = std::move(calc)](const ContextBase& context,
                                             AbstractValue* result) {
      const SomeContext& typed_context = context_cast<SomeContext>(context);
      SomeOutput& typed_result = result->get_mutable_value<SomeOutput>();
      typed_result = captured_calc(typed_context);
    };
  }

  // For overload series (a).
  template <typename SomeOutput>
  static AllocateCallback make_allocate_mode_a() {
    static_assert(
        std::is_default_constructible_v<SomeOutput>,
        "When ValueProducer is used with an output type that is not default"
        " constructible, then you must provide either a model_value or an"
        " allocate callback function.");
    return static_cast<std::unique_ptr<drake::AbstractValue>(*)()>(
        &AbstractValue::Make<SomeOutput>);
  }

  // For overload series (b).
  template <typename SomeOutput>
  static AllocateCallback make_allocate_mode_b(const SomeOutput& model_value) {
    return internal::AbstractValueCloner(model_value);
  }

  // For overload series (c).
  template <class SomeInstance, typename SomeClass, typename SomeOutput>
  static AllocateCallback make_allocate_mode_c(
      const SomeInstance* instance,
      std::unique_ptr<SomeOutput> (SomeClass::*allocate)() const) {
    const SomeClass* typed_instance = instance_cast<SomeClass>(instance);
    if (allocate == nullptr) {
      ThrowBadNull();
    }
    return [typed_instance, allocate]() {
      std::unique_ptr<SomeOutput> result = (typed_instance->*allocate)();
      return std::make_unique<Value<SomeOutput>>(std::move(result));
    };
  }

  AllocateCallback allocate_;
  CalcCallback calc_;
};

}  // namespace systems
}  // namespace drake