ardupilot/libraries/AP_Common/AP_MetaClass.h

// -*- tab-width: 4; Mode: C++; c-basic-offset: 4; indent-tabs-mode: nil -*-
//
// This is free software; you can redistribute it and/or modify it under
// the terms of the GNU Lesser General Public License as published by the
// Free Software Foundation; either version 2.1 of the License, or (at
// your option) any later version.
//

/// @file	AP_Meta_class.h
///	@brief	An abstract base class from which other classes can inherit.
///
/// This abstract base class declares and implements functions that are
/// useful to code that wants to know things about a class, or to operate
/// on the class without knowing precisely what it is.
///
/// All classes that inherit from this class can be assumed to have these
/// basic functions.
///

#ifndef AP_META_CLASS_H
#define AP_META_CLASS_H

#include <stddef.h>			// for size_t
#include <inttypes.h>

#include <avr/io.h>			// for RAMEND
/// Basic meta-class from which other AP_* classes can derive.
///
/// Functions that form the public API to the metaclass are prefixed meta_.
///
class AP_Meta_class
{
public:
    /// Default constructor does nothing.
    AP_Meta_class(void);

    /// Default destructor is virtual, to ensure that all subclasses'
    /// destructors are virtual.  This guarantees that all destructors
    /// in the inheritance chain are called at destruction time.
    ///
    virtual ~AP_Meta_class();

    /// Typedef for the ID unique to all instances of a class.
    ///
    /// See ::meta_type_id for a discussion of class type IDs.
    ///
    typedef uint16_t AP_Type_id;

    /// Obtain a value unique to all instances of a specific subclass.
    ///
    /// The value can be used to determine whether two class pointers
    /// refer to the same exact class type.  The value can also be cached
    /// and then used to detect objects of a given type at a later point.
    ///
    /// This is similar to the basic functionality of the C++ typeid
    /// keyword, but does not depend on std::type_info or any compiler-
    /// generated RTTI.
    ///
    /// The value is derived from the vtable address, so it is guaranteed
    /// to be unique but cannot be known until the program has been compiled
    /// and linked.  Thus, the only way to know the type ID of a given
    /// type is to construct an object at runtime.  To cache the type ID
    /// of a class Foo, one would write:
    ///
    /// AP_Meta_class::AP_Type_id	Foo_type_id;
    ///
    /// { Foo a; Foo_type_id = a.meta_type_id(); }
    ///
    /// This will construct a temporary Foo object a and save its type ID.
    ///
    /// @param	p				A pointer to an instance of a subclass of AP_Meta_class.
    /// @return					A type-unique value.
    ///
    AP_Type_id meta_type_id(void) const {
        return *(AP_Type_id *)this;
    }

    /// External handle for an instance of an AP_Meta_class subclass, contains
    /// enough information to construct and validate a pointer to the instance
    /// when passed back from an untrusted source.
    ///
    /// Handles are useful when passing a reference to an object to a client outside
    /// the system, as they can be validated by the system when the client hands
    /// them back.
    ///
    typedef uint32_t Meta_handle;

    /// Return a value that can be used as an external pointer to an instance
    /// of a subclass.
    ///
    /// The value can be passed to an untrusted agent, and validated on its return.
    ///
    /// The value contains the 16-bit type ID of the actual class and
    /// a pointer to the class instance.
    ///
    /// @return					An opaque handle
    ///
    Meta_handle meta_get_handle(void) const {
        return ((Meta_handle)meta_type_id() << 16) | (uint16_t)this;
    }

    /// Validates an AP_Meta_class handle.
    ///
    /// The value of the handle is not required to be valid; in particular the
    /// pointer encoded in the handle is validated before being dereferenced.
    ///
    /// The handle is considered good if the pointer is valid and the object
    /// it points to has a type ID that matches the ID in the handle.
    ///
    /// @param	handle			A possible AP_Meta_class handle
    /// @return					The instance pointer if the handle is good,
    ///							or NULL if it is bad.
    ///
    static AP_Meta_class *meta_validate_handle(Meta_handle handle) {
        AP_Meta_class *candidate = (AP_Meta_class *)(handle & 0xffff); // extract object pointer
        uint16_t id = handle >> 16; // and claimed type

        // Sanity-check the pointer to ensure it lies within the device RAM, so that
        // a bad handle won't cause ::meta_type_id to read outside of SRAM.
        // Assume that RAM (or addressable storage of some sort, at least) starts at zero.
        //
        // Note that this implies that we cannot deal with objects in ROM or EEPROM,
        // but the constructor wouldn't be able to populate a vtable pointer there anyway...
        //
        if ((uint16_t)candidate >= (RAMEND - 2)) { // -2 to account for the type_id
            return NULL;
        }

        // Compare the typeid of the object that candidate points to with the typeid
        // from the handle.  Note that it's safe to call meta_type_id() off the untrusted
        // candidate pointer because meta_type_id is non-virtual (and will in fact be
        // inlined here).
        //
        if (candidate->meta_type_id() == id) {
            return candidate;
        }

        return NULL;
    }

    /// Tests whether two objects are of precisely the same class.
    ///
    /// Note that in the case where p2 inherits from p1, or vice-versa, this will return
    /// false as we cannot detect these inheritance relationships at runtime.
    ///
    /// In the caller's context, p1 and p2 may be pointers to any type, but we require
    /// that they be passed as pointers to AP_Meta_class in order to make it clear that
    /// they should be pointers to classes derived from AP_Meta_class.
    ///
    /// No attempt is made to validate whether p1 and p2 are actually derived from
    /// AP_Meta_class.  If p1 and p2 are equal, or if they point to non-class objects with
    /// similar contents, or to non-AP_Meta_class derived classes with no virtual functions
    /// this function may return true.
    ///
    /// @param	p1				The first object to be compared.
    /// @param	p2				The second object to be compared.
    /// @return					True if the two objects are of the same class, false
    ///							if they are not.
    ///
    static bool meta_type_equivalent(AP_Meta_class *p1, AP_Meta_class *p2) {
        return p1->meta_type_id() == p2->meta_type_id();
    }

    /// Cast a pointer to an expected class type.
    ///
    /// This function is used when a pointer is expected to be a pointer to a
    /// subclass of AP_Meta_class, but the caller is not certain.  It will return the pointer
    /// if it is, or NULL if it is not a pointer to the expected class.
    ///
    /// This should be used with caution, as _typename's default constructor and
    /// destructor will be run, possibly introducing undesired side-effects.
    ///
    /// @todo	Consider whether we should make it difficult to have a default constructor
    ///			with appreciable side-effects.
    ///
    /// @todo	Check whether we need to reinterpret_cast to get the right return type.
    ///
    /// @param	_p				An AP_Meta_class subclass whose type is to be tested.
    /// @param	_typename		The name of a type with which _p is to be compared.
    /// @return					True if _p is of type _typename, false otherwise.
    ///
    template<typename T>
    static T *meta_cast(AP_Meta_class *p) {
        T tmp;
        if (meta_type_equivalent(p, &tmp)) {
            return (T *)p;
        }
        return NULL;
    }

    /// Serialize the class.
    ///
    /// Serialization stores the state of the class in an external buffer in such a
    /// fashion that it can later be restored by unserialization.
    ///
    /// AP_Meta_class subclasses should only implement these functions if saving and
    /// restoring their state makes sense.
    ///
    /// Serialization provides a mechanism for exporting the state of the class to an
    /// external consumer, either for external introspection or for subsequent restoration.
    ///
    /// Classes that wrap variables should define the format of their serialiaed data
    /// so that external consumers can reliably interpret it.
    ///
    /// @param	buf				Buffer into which serialised data should be placed.
    /// @param	bufSize			The size of the buffer provided.
    /// @return					The size of the serialised data, even if that data would
    ///							have overflowed the buffer.  If the return value is zero,
    ///							the class does not support serialization.
    ///
    virtual size_t serialize(void *buf, size_t bufSize) const;

    /// Unserialize the class.
    ///
    /// Unserializing a class from a buffer into which the class previously serialized
    /// itself restores the instance to an identical state, where "identical" is left
    /// up to the class itself to define.
    ///
    /// Classes that wrap variables should define the format of their serialized data so
    /// that external providers can reliably encode it.
    ///
    /// @param	buf				Buffer containing serialized data.
    /// @param	bufSize			The size of the buffer.
    /// @return					The number of bytes from the buffer that would be consumed
    ///							unserializing the data.  If the value is less than or equal
    ///							to bufSize, unserialization was successful.  If the return
    ///							value is zero the class does not support unserialisation or
    ///							the data in the buffer is invalid.
    ///
    virtual size_t unserialize(void *buf, size_t bufSize);
};

#endif // AP_Meta_class_H